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Photo-induced magnetic solitons and the slow relaxation mechanism in diluted magnetic semiconductors
2009-01-01
Taking into account the correlation effect among photo-induced magnetic solitons, the mechanism of the slow relaxation of the spin dynamics in diluted magnetic semiconductors has been discussed by the long-range p-spin spherical model
Longitudinal wave instability in magnetized high correlation dusty plasma
2003-06-18
Low frequency longitudinal wave instability in magnetized high correlation dusty plasmas is investigated. The dust charging relaxation is taken into account. It is found that the instablity of wave is determined significantly by the frequency of wave, the dust charging relaxation, the shear viscosity and viscoelastic relaxation time, the coupling parameter of high correlation of dust as well the strength of magnetic field.
The role of electron scattering in magnetization relaxation in thin Ni$_{81}$Fe$_{19}$ films
2002-08-12
We observe a strong correlation between magnetization relaxation and electrical resistivity in thin Permalloy (Ni$_{81}$Fe$_{19}$, ``Py'') films. Electron scattering rates in the films were affected by varying film thickness and deposition conditions. This shows that the magnetization relaxation mechanism is analogous to ``bulk'' relaxation, where phonon scattering in bulk is replaced by surface and defect scattering in thin films. Another interesting finding is the increased magnetization damping with Pt layers adjacent to the Py films. This is attributed to the strong spin-orbit coupling in Pt, resulting in spin-flip scattering of electrons that enter from the Py.
Nuclear magnetic resonance of perfused tissue
1986-04-01
The effect of perfusion on the NMR signal observed in NMR imaging is studied in a phantom and in two isolated perfused canine lungs. It is observed that perfusion in tissue has little effect on longitudinal relaxation times. Transverse relaxation rates are observed to correlate linearly with rates of perfusion, in accordance with a model presented.
Broadband Relaxation-Optimized Polarization Transfer in Magnetic Resonance
2004-01-01
Many applications of magnetic resonance are limited by rapid loss of spin coherence caused by large transverse relaxation rates. In nuclear magnetic resonance (NMR) of large proteins, increased relaxation losses lead to poor sensitivity of experiments and increased measurement time. In this paper we develop broadband relaxation optimized pulse sequences (BB-CROP) which approach fundamental limits of coherence transfer efficiency in the presence of very general relaxation mechanisms that include cross-correlated relaxation. These broadband transfer schemes use new techniques of chemical shift refocusing (STAR echoes) that are tailored to specific trajectories of coupled spin evolution. We present simulations and experimental data indicating significant enhancement in the sensitivity of multi-dimensional NMR experiments of large molecules by use of these methods.
Systematic Dielectric and NMR Study of the Ionic Liquid 1-Alkyl-3-Methyl Imidazolium
2010-01-01
The dynamic behaviors of ionic liquid samples consisting of a series of 1-alkyl-3-methylimidazolium cations and various counteranionic species are investigated systematically over a wide frequency range from 1 MHz to 20 GHz at room temperature using dielectric relaxation (DR) and nuclear magnetic resonance (NMR) spectroscopies. DR spectra for the ionic liquids are reasonably deconvoluted into two or three relaxation modes. The slowest relaxation times are strongly dependent upon sample viscosity and cation size, whereas the relaxation times of other modes are almost independent of these factors. We attribute the two slower relaxation modes to the rotational relaxation modes of the dipolar cations because the correlation times of the cations evaluated using longitudinal relaxation time (T1 ...
Magnetic correlations in La(2-x)Sr(x)CuO4 from NQR relaxation and specific heat
La-139 and Cu-63 Nuclear Quadrupole Resonance (NQR) relaxation measurements in La(2-x)Sr(x)CuO4 for O = to or less than 0.3 and in the temperature range 1.6 450 K are analyzed in terms of Cu() magnetic ...
Magnetic correlations in La(2-x)Sr(x)CuO4 from NQR relaxation and specific heat
La-139 and Cu-63 Nuclear Quadrupole Resonance (NQR) relaxation measurements in La(2-x)Sr(x)CuO4 for x ranging from 0 up to 0.3, with particular emphasis on the effect of doping on the Cu(2) magnetic ...
Non-monotonic Relaxation in Systems with Reentrant Type Interaction
2007-02-15
Recently, interesting non-monotonic time evolution has been pointed out in the experiments by J\\"onsson, {\\it et al.} and Jonsson {\\it et.al.} and also in the numerical simulation by Takayama and Hukushima where the magnetic susceptibility does not monotonically relax to the equilibrium value, but moves to the opposite side. We study mechanism of this puzzling non-monotonic dynamical property in a frustrated Ising model in which the equilibrium correlation exhibits non-monotonic temperature dependence (reentrant type). We study the time evolution of spin correlation function after sudden change of temperature. There, we find that the value of the correlation function shows non-monotonic relaxation, and analyze mechanisms of the non-monotonicity. We also point out that competition between different configurations widely causes non-monotonic relaxation.
Tuning the relaxation behaviour by changing the content of cobalt in CoxFe3-xO4 ferrofluids
2005-01-01
The frequency-dependent magnetic relaxation of CoxFe3-xO4 nanoparticles stabilized by N-methyloleamidoacetic acid is investigated in three apolar suspending media (toluene, cyclohexane and decane). Hereby, the dependence of the crossover from a Brownian to a Neelian relaxation mechanism on the cobalt content is observed. The experimental results obtained from the complex magnetic susceptibility confirm the theoretically expected crossover between both relaxation mechanisms. The topological characterization is performed by means of x-ray diffraction, photon correlation spectroscopy and-in the Brownian regime-by magnetic relaxometry, allowing the comparison of crystallographic and hydrodynamic particle sizes
2007-01-01
Ferrimagnetic iron oxide particles were inhaled by 17 healthy volunteers (9 non-smokers, 8 smokers), and the retained particles were magnetized and detected by a SQUID. Stochastic particle transport due to cytoskeletal reorganizations within macrophages (relaxation) and directed particle motion in a weak magnetic twisting field were investigated with respect to viscous and elastic properties of the cytoskeleton. Relaxation and cytoskeletal stiffness were not influenced by cigarette smoking. Relaxation and particle twisting revealed a non-Newtonian viscosity with a pure viscous and a viscoelastic compartment. Viscous and elastic data obtained from relaxation correlated with particle twisting, indicating that the proposed simple model is a reasonable approximation of cytoskeletal mechanical properties
Saturation of Magnetorotational Instability through Magnetic Field Generation
2009-04-21
The saturation mechanism of Magneto-Rotational Instability (MRI) is examined through analytical quasilinear theory and through nonlinear computation of a single mode in a rotating disk. We find that large-scale magnetic field is generated through the alpha effect (the correlated product of velocity and magnetic field fluctuations) and causes the MRI mode to saturate. If the large-scale plasma flow is allowed to evolve, the mode can also saturate through its flow relaxation. In astrophysical plasmas, for which the flow cannot relax because of gravitational constraints, the mode saturates through field generation only.
2004-01-01
The aim of the current study was to perform T2 relaxation time measurements in multiple sclerosis (MS) patients and correlate them with magnetization transfer ratio (MTR) measurements, in order to investigate in more detail the various histopathological changes that occur in lesions and normal-appearing white matter (NAWM). A total number of 291 measurements of MTR and T2 relaxation times were performed in 13 MS patients and 10 age-matched healthy volunteers. Measurements concerned MS plaques (105), NAWM (80), and ''dirty'' white matter (DWM. 30), evenly divided between the MS patients, and normal white matter (NWM. 76) in the healthy volunteers. Biexponential T2 relaxation-time analysis was performed, and also possible linearity between MTR and mean T2 relaxation times was evaluated using linear regression analysis in all subgroups. Biexponential relaxation was ...
Proton magnetic relaxation in aromatic polyamides during water vapor sorption
2009-01-01
The state of the components in the aromatic polyamide-water system was studied by NMR and sorption. A comparative analysis of spin-lattice and spin-spin relaxation in aromatic para-polyamide (para-aramid) technical fibers Rusar, Kevlar, and Technora was performed depending on the sorption value. The NMR results correlated with the supramolecular structure of polymers and quasi-chemical equation parameters for water vapor sorption.
1997-01-01
The backbone dynamics of uniformly 13C/15N-enriched ribonuclease T1 have been investigated using carbonyl carbon relaxation times recorded at three different spectrometer frequencies. Pulse sequences for the determination of the longitudinal (T1) and transverse (T2)relaxation times are presented. The relaxation behaviour was analysed in terms of a multispinsystem. Although the chemical shift anisotropy relaxation mechanism dominates at high magnetic field strength, the contributions of the dipole-dipole interactions and the cross-correlation between these two relaxation mechanisms have also been considered.Information about internal motions has been extracted from the relaxation data using the model-free approach of Lipari and Szabo in order to determine order parameters (S2) and effective internal correlation times (taui). Using a ...
2010-01-01
The spin-lattice relaxation rate of nuclear magnetic resonance (NMR) in a superconductor without inversion symmetry by considering a Cooper pairing model with a two-component order parameter composed of spin-singlet and spin-triplet pairing components in the presence of impurity effect is investigated. By using the Green's function method and linear response theory we calculate the appropriate correlation function to evaluate the spin-lattice relaxation rate. For a noncentrosymmetric and clean superconductor such as CePt3Si a power law temperature dependence (T^3) of the nuclear magnetic relaxation rate at low temperatures, suggests that the gap function has line nodes, consistent with this gap model. In a dirty superconductor the linear temperature dependence of the spin-lattice relaxatio...
Evaluation of brain edema using magnetic resonance proton relaxation times
1990-01-01
Experimental and clinical studies on the evaluation of water content in cases of brain edema were performed in vivo, using MR proton relaxation times (longitudinal relaxation time, T1; transverse relaxation time, T2). Brain edema was produced in the white matter of cats by the direct infusion method. The correlations between proton relaxation times obtained from MR images and the water content of white matter were studied both in autoserum-infused cats and in saline-infused cats. The correlations between T1 as well as T2 and the water content in human vasogenic brain edema were also examined and compared with the data obtained from the serum group. T1 and T2 showed good correlations with the water content of white matter not only in the experimental animals but also in the clinical cases. The quality of the edema fluid did not influence relaxation time and T1 seemed to represent almost solely the water content of the tissue. T2, however, was affected by the nature of existence of water and was more sensitive than T1 in detecting extravasated edema fluid. It seems feasible therefore to evaluate the water content of brain edema on the basis of T1 values.
Effect of Nuclear Quadrupole Interaction on the Relaxation in Amorphous Solids
2005-02-25
Recently it has been experimentally demonstrated that certain glasses display an unexpected magnetic field dependence of the dielectric constant. In particular, the echo technique experiments have shown that the echo amplitude depends on the magnetic field. The analysis of these experiments results in the conclusion that the effect seems to be related to the nuclear degrees of freedom of tunneling systems. The interactions of a nuclear quadrupole electrical moment with the crystal field and of a nuclear magnetic moment with magnetic field transform the two-level tunneling systems inherent in amorphous dielectrics into many-level tunneling systems. The fact that these features show up at temperatures $T<100mK$, where the properties of amorphous materials are governed by the long-range $R^{-3}$ interaction between tunneling systems, suggests that this interaction is responsible for the magnetic field dependent relaxation. We have developed a theory of many-body relaxation in an ensemble of interacting many-level tunneling systems and show that the relaxation rate is controlled by the magnetic field. The results obtained correlate with the available experimental data. Our approach strongly supports the idea that the nuclear quadrupole interaction is just the key for understanding the unusual behavior of glasses in a magnetic field.
2005-01-01
The purpose of this paper is to perform quantitative measurements of the magnetization transfer rate (Kfor) and native T1 relaxation time (T1free) in the brain tissue of normal individuals and patients with multiple sclerosis (MS) by means of multiple gradient echo acquisitions, and to correlate these measurements with the magnetization transfer ratio (MTR). Quantitative magnetization transfer imaging was performed in five normal volunteers and 12 patients with relapsing-remitting MS on a 1.5 T magnetic resonance (MR) scanner. The T1 relaxation time under magnetization transfer irradiation (T1sat) was calculated by means of fitting the signal intensity over the flip angle in several 3D spoiled gradient echo acquisitions (3 , 15 , 30 , and 60 ), while a single acquisition without MT irradiation (flip angle of 3 ) was utilized to calculate the MTR. The Kfor and T1free ...
Magnetic relaxation in La0.250Pr0.375Ca0.375MnO3 with varying phase separation
2002-03-05
We have studied the magnetic relaxation properties of the phase-separated manganite compound La0.250Pr0.375Ca0.375MnO3 . A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes - at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and non-ferromagnetic phases.
Spin-glass dynamics determined from muon spin-relaxation and neutron spin-echo measurements
1983-01-01
Muon spin-relaxation (..mu..SR) and neutron spin-echo (NSE) measurements of magnetic-ion correlation times and correlation functions in the spin-glass systems CuMn, AgMn, and AuFe are compared. It is found that the ..mu..SR and NSE measurements are in excellent agreement both above and below the spin-glass freezing temperatures. The experimental results are compared to recent theories of spin-glass dynamics.
Synthesis of superparamagnetic MgFe{sub 2}O{sub 4} nanoparticles by coprecipitation
1999-04-01
MgFe{sub 2}O{sub 4} nanoparticles have been synthesized by coprecipitation method. Magnetic measurements in combination with neutron diffraction have determined the existence of a superparamagnetic state in this metal oxide system. The superparamagnetic relaxation of magnetization in these nanoparticles has been studied by using Moessbauer spectroscopy. The relaxation time has been correlated with the particle size and temperature and is consistent with Neel theory. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)
Spin dynamics on percolating networks
We have used inelastic neutron scattering to measure the order parameter relaxation rate GAMMA in the dilute, two-dimensional Ising antiferromagnet Rb/sub 2/CoMg/sub 1-c/F/sub 4/ with c very close to the magnetic percolation threshold. Where kappa is the inverse magnetic correlation length, GAMMA approx. kappa/sup z/ with z = 2.4/sub -0.1//sup +0.2/. Our results are discussed in terms of current ideas about spin relaxation on fractals. 13 refs., 1 fig.
Role of paramagnetic ions and water proton spin-lattice relaxation time in biological systems
1993-02-01
This paper summarizes the observations of different studies concerning the influence of paramagnetic ions on spin-lattice relaxation times in magnetic resonance imaging. Based on findings that manganese ion content in cancer tissues is decreased in comparison to normal tissues, the results of different papers analysing the influence of tissue manganese concentration on spinlattice relaxation times are collected and compared. Neither the comparison between different organs, different animals nor the comparison between different tissues (normal and malignant) showed correlations of practical consequences between manganese concentrations and spin-lattice relaxation times. These results are consistent with those from studies with copper and iron ions in living systems. (orig.)
NMR Study of Water Distribution inside Tomato Cells: Effects of Water Stress
2010-01-01
Tomato pericarp tissue was studied by low-field nuclear magnetic resonance (NMR) relaxometry. Two kinds of experiments were performed to investigate the correlation between multi-exponential NMR relaxation and the subcellular compartments. The longitudinal (T 1) versus transverse (T 2) relaxation times were first measured on fresh samples and then the transverse relaxation time was measured on samples exposed to water stress. Four signal components were found in all experiments. The results showed that all signal components corresponded to the water in different cell compartments, and that no signal from non-exchangeable protons was present. Moreover, we demonstrated that NMR relaxation is suitable for the continuous monitoring of water rebalancing between subcellular compartments of plant...
2006-11-23
Based on a quantum dissipation theory of open systems, we present a theoretical study of slow dynamics of magnetization for the ordered state of the new molecule-based magnetic complex [Mn(hfac)_{2}BNO_{H}] composed from antiferromagnetically coupled ferrimagnetic (5/2,1) spin chains. Experimental investigations of the magnetization process in pulsed fields have shown that this compound exhibits a metamagnetic AF-FI transition at a critical field in the order of the interchain coupling. A strong frequency dependence for the ac-susceptibility has been revealed in the vicinity of the AF-FI transition and was associated with an AF-FI interface kink motion. We model these processes by a field-driven domain-wall motion along the field-unfavorable chains correlated with a dissipation effect due to a magnetic system-bath coupling. The calculated longitudinal magnetization has a two-step relaxation after the field is switched off and are found in good agreement with the experiment. The relaxation time determined from the imaginary part of the model ac-susceptibility agrees qualitatively with that found from the remanent magnetization data.
Nuclear magnetic relaxation dispersion
1988-01-01
Nuclear magnetic relaxation dispersion (NMRD) is the set of data representing the dependence of nuclear spin-lattice relaxation rates on the strength of the d.c. magnetic field used in NMR imaging. The field dependence of the nuclear spin relaxation provides a clear picture of the several factors that may contribute to or dominate relaxation in paramagnetic systems. Without this information, it is difficult to develop rational strategies for control of nuclear spin relaxation. (U.K.).
Features of low-frequency spin dynamics in manganite LaMnO{sub 3} according to {sup 139}La NMR data
The spin-lattice and spin-spin relaxation times of {sup 139}La are measured in manganite LaMnO{sub 3}. Analysis of the frequency dependence of the spin-lattice relaxation rate in the paramagnetic temperature range shows that this quantity is determined by magnetic fluctuations. The magnitude of the fluctuating field is estimated. It is shown that the correlation time for spin fluctuations varies with temperature in accordance with the Arrhenius law. The high value of the spin-spin relaxation rate in the paramagnetic region can be due to strong anisotropy of fluctuating magnetic fields at La nuclei.
1995-12-31
The analysis of the NMR spectra for two non-equivalent spins (I{sub 1},I{sub 2} 1/2)interacting with a external paramagnetic center in the presence of spin exchange is presented. The paramagnetic center is treated as a randomly changed source of magnetic fields influenced the relaxation transitions. 5 refs.
Information dynamics of a particle in a magnetic field
2010-01-01
We discuss the time evolution of information entropy (S) of a harmonic oscillator driven by thermal noise in the presence of a magnetic field. Our analysis is based on the Fokker-Planck description of the stochastic process. It shows that the relaxation time of a given non equilibrium state increases with increase of strength of the applied magnetic field. It further increases if the thermal noise becomes colored. The dependence of the time derivative of the entropy, its upper bound and related quantity on the strength of magnetic field, damping, noise correlation time and temperature is studied in detail.
2004-01-01
The aim of the current study was to perform T2 relaxation time measurements in multiple sclerosis (MS) patients and correlate them with magnetization transfer ratio (MTR) measurements, in order to investigate in more detail the various histopathological changes that occur in lesions and normal-appearing white matter (NAWM). A total number of 291 measurements of MTR and T2 relaxation times were performed in 13 MS patients and 10 age-matched healthy volunteers. Measurements concerned MS plaques (105), NAWM (80), and ''dirty'' white matter (DWM; 30), evenly divided between the MS patients, and normal white matter (NWM; 76) in the healthy volunteers. Biexponential T2 relaxation-time analysis was performed, and also possible linearity between MTR and mean T2 relaxation times was evaluated using linear regression analysis in all subgroups. Biexponential relaxation was more pronounced in ''black-hole'' lesions (16.6%) and homogeneous enhancing plaques (10%), whereas DWM, NAWM, and mildly hypointense lesions presented biexponential behavior with a lower frequency(6.6, 5, and 3.1%, respectively). Non-enhancing isointense lesions and normal white matter did not reveal any biexponentional behavior. Linear regression analysis between monoexponential T2 relaxation time and MTR measurements demonstrated excellent correlation for DWM(r=-0.78, p<0.0001), very good correlation for black-hole lesions(r=-0.71, p=0.002), good correlation for isointense lesions(r=-0.60, p=0.005), moderate correlation for mildly hypointense lesions(r=-0.34, p=0.007), and non-significant correlation for homogeneous enhancing plaques, NAWM, and NWM. Biexponential T2 relaxation-time behavior is seen in only very few lesions (mainly on plaques with high degree of demyelination and axonal loss). A strong correlation between MTR and monoexponential T2 values was found in regions where either inflammation or demyelination predominates; however, when both pathological conditions coexist, this linear relationship is lost. (orig.)
Dynamics of beta-CH and beta-CH2 Groups of Amino Acid Side Chains in Proteins
1998-01-01
The dynamics of amino acid side chains of uniformly 13C/15N-enriched ribonuclease T1 (RNase T1) have been investigated. Heteronuclear longitudinal relaxation rates, 1H/13C NOEs, and transverse cross-correlated cross-relaxation rates between the Sx and the SxIz1Iz2 operators (SIIS cross relaxation) [Ernst and Ernst (1994) J. Magn. Reson., A110, 202-213] have been determined in this study. New pulse sequences for measuring the longitudinal relaxation time and the heteronuclear NOE of aliphatic side chain carbon nuclei were developed using the CCONH type of magnetization transfer and 1HN detection. In addition, an improved pulse sequence for the determination of the SIIS cross relaxation is presented. For the analysis of the relaxation rates, the model of restricted rotational diffusion around the chi1 dihedral angle has been applied [London and ...
Kinetic studies of thermal relaxation in plasma
1987-01-01
The Lenard-Balescu equation is generalized for a magnetized plasma from the BBGKY hierarchy equation. The random phase approximation and the two time scale approximation are applied along with the neglect of essential 3-particle correlations. The generalized Lenard-Balescu equation is expressed in terms of binary collisions by the screened Coulomb force and the interaction between the individual particle and the collective mode. Weakly unstable cases which can be handled by quasilinear theory are contained in this treatment. The equation is solved numerically for a two-dimensional charged rod plasma with an external uniform magnetic field whose direction is along the charged rod. The thermal relaxation is measured by calculating the entropy of the system. Results are compared and agree qualitatively with ones from particle simulation. The Fokker-Planck equation is also solved numerically for a three-dimensional unmagnetized plasma. The diffusion and the friction coefficients are obtained. A multi-species plasma is studied. For a two-temperature plasma, the lower-speed portion of the hot plasma distribution overshoots, while the cold plasma is still relaxing to a Maxwellian. In both cases the slowing down of the relaxation rate is due to the relaxation times required for the filling of the high-speed tail.
75As NMR relaxation in glassy Ge-As-Se
2010-01-01
We have used high magnetic fields to perform 75As nuclear magnetic resonance (NMR) relaxation measurements in Ge-As-Se glasses. The spin-lattice relaxation time T1 has been studied as a function of average coordination number, r and temperature, T. We show that the relaxation time constant, T1, in this system can be a useful tool to probe rigidity of the glassy matrix. In the two extreme cases, zero-field nuclear quadrupole resonance (NQR) and high-field NMR, T1 exhibits a similar dependence on the average coordination number. Namely, there is a region in this dependence at low r values, where the spin-lattice relaxation time does not change remarkably, followed by an increase in T1 in glasses with r more than 2.54. This transition can be correlated with the onset of rigidity percolation i...
Internal magnetic relaxation in levitation superconductors
Effect of arresting levitation relaxation, appearing during reverse magnetization of YBaCuO superconducting ceramics, was detected. At bipolar magnetization magnetic moment of a sample remains invariable. Internal magnetic relaxation occurs, in the course of which magnetic flux is redistributed inside the sample. As a result the state of filed at the sample boundary does not change and full force acting on the system of closed currents remains constant. A formula for calculating the time of internal relaxation is provided
Magnetocaloric Study of Spin Relaxation in `Frozen' Dipolar Spin Ice Dy2Ti2O7
2006-09-21
The magnetocaloric effect of polycrystalline samples of pure and Y-doped dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 K to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic interest, it is proposed that the magnetocaloric effect may be used as an appropriate tool for the qualitative study of slow relaxation processes in the spin ice regime. In the high temperature regime the temperature change on adiabatic demagnetization was found to be consistent with previously published entropy versus temperature curves. At low temperatures (T < 0.4 K) cooling by adiabatic demagnetization was followed by an irreversible rise in temperature that persisted after the removal of the applied field. The relaxation time derived from this temperature rise was found to increase rapidly down to 0.3 K. The data near to 0.3 K indicated a transition into a metastable state with much slower relaxation, supporting recent neutron scattering results. In addition, magnetic dilution of 50 % concentration was found to significantly prolong the dynamical response in the milikelvin temperature range, in contrast with results reported for higher temperatures at which the spin correlations are suppressed. These observations are discussed in terms of defects and loop correlations in the spin ice state.
Theory of nuclear magnetic relaxation
A theory of nuclear magnetic interaction is based on the study of the stochastic rotation operator. The theory is applied explicitly to relaxation by anisotropic chemical shift and to spin-rotational ...
Magnetic monolayers on semiconducting substrates. An in situ FMR study of Fe-based heterostructures
2007-10-18
The growth, magnetic anisotropy, g-factor, and magnetization of Fe monolayers grown on GaAs(001), InAs(001), and InP(001) are investigated by a combination of in situ ferromagnetic resonance and SQUID magnetometry as a function of temperature and film thickness. The effect of stress caused by the lattice mismatch and the surface reconstruction on the magnetic anisotropy is quantified. An in-plane spin reorientation transition as a function of film thickness is observed at room temperature for all systems. A magneto-elastic model is used to explain the direction of the easy axis, the spin reorientation transition, and the contributions to the magnetic anisotropy terms using the stress components measured directly by in situ IV-low-energy electron diffraction. While the model gives a quantitative explanation of the out-of-plane magnetic anisotropy, changes of the electronic interface structure have to be taken into account for the in-plane magnetic anisotropy. The influence of Ag and Au buffer and cap layers on the magnetic anisotropy terms are determined. The temperature dependence of the total magnetic anisotropy, as well as the surface-interface and volume contribution to the magnetic anisotropy are determined for Fe monolayers on GaAs(001). It is demonstrated that the temperature dependence of the magnetic anisotropy is correlated with the temperature dependence of the magnetization according to the Callen-Callen model. The temperature dependence of the volume contribution to the perpendicular magnetic anisotropy is fully explained by the temperature dependence of the magneto-elastic anisotropy. A temperature-driven morphological transformation occurring at a temperature higher than 550 K depending on the film thickness is observed. The thin Fe3Si binary Heusler structure epitaxially grown on MgO(001) is investigated. In addition to the structural properties, magnetic anisotropy, magnetization, g-factor, spin, and orbital magnetism, the magnetic relaxation mechanisms are determined as a function of Si concentration, sample treatment, and film thickness. The Arias and Mills model is confirmed yielding a quantitative explanation for the magnetic relaxation parameters from the ferromagnetic resonance linewidth. Two relaxation channels, i.e. dissipative isotropic Gilbert damping as well as anisotropic two-magnon scattering are simultaneously identified. It is demonstrated that changing the film thickness from 8 to 40 nm and slightly modifying the Fe concentration influences the relaxation channels and can be used to tune the relaxation rates. (orig.)
Unconventional superconductivity in Formula Not Shown
2006-01-01
We report nuclear magnetic resonance (NMR) data in the new heavy fermion superconductor Formula Not Shown ( Formula Not Shown ) in the normal and superconducting states. Measurements of the Knight shift and the nuclear spin lattice relaxation rate in the superconducting state show a spin-singlet with lines of nodes in the gap function, and our calculations indicate that the data are consistent with a strong-coupling d-wave gap function with Formula Not Shown . In the normal state, the temperature dependence of the spin lattice relaxation rate suggests the presence of antiferromagnetic correlations, and scales with Formula Not Shown in the same manner as in other heavy fermion superconductors, as well as the high-temperature superconductor, Formula Not Shown .
A cognitive behavioural approach to preventing anxiety during magnetic resonance imaging
1997-01-01
This study evaluated a relaxation intervention designed to prevent anxiety during magnetic resonance imaging (MRI), and assessed the development of fears in patients who felt anxious during the procedure. Patients were assigned to a control condition (no intervention; n = 52), relaxation before the scan (n = 44), or relaxation before and during the scan (n = 43). Compared to the control group, patients who practised relaxation showed reduced anxiety during the scan. Seven months or more after undergoing MRI, there was a positive correlation between anxiety experienced during the scan and the development of MRI-related fears. The intervention did not prevent the development of MRI-related fears at follow-up. Publisher: Elsevier Language: eng Source: Lukins, R., Davan, I.G.P. and Drummond, P.D. (1997) A cognitive behavioural approach to preventing anxiety during magnetic resonance imaging. Journal of Behavior Therapy and Experimental Psychiatry, 28 (2). pp. 97-104. Rights: Elsevier
2008-01-01
We have reported that the magnetic power losses per magnetizing cycle change with the magnetization pausing time in a constant rate of magnetization change and some relaxation phenomena exist in the magnetization pausing period. We discuss the influence of the relaxation phenomena on the dynamic magnetic power losses and suggest that the relaxation phenomena seems to be causes of the frequency dependence of the magnetic losses and of the variation of losses that cannot be expressed only by magnetization speed dependence of losses
Novel dynamical effects and glassy response in strongly correlated electronic system
2007-03-25
We find an unconventional nucleation of low temperature paramagnetic metal (PMM) phase with monoclinic structure from the matrix of high-temperature antiferromagnetic insulator (AFI) phase with tetragonal structure in strongly correlated electronic system $BaCo_{0.9}Ni_{0.1}S_{1.97}$. Such unconventional nucleation leads to a decease in resistivity by several orders with relaxation at a fixed temperature without external perturbation. The novel dynamical process could arise from the competition of strain fields, Coulomb interactions, magnetic correlations and disorders. Such competition may frustrate the nucleation, giving rise to a slow, nonexponential relaxation and "physical aging" behavior.
Study of correlations in molecular motion by multiple quantum NMR
1981-11-01
Nuclear magnetic resonance is a very useful tool for characterizing molecular configurations through the measurement of transition frequencies and dipolar couplings. The measurement of spectral lineshapes, spin-lattice relaxation times, and transverse relaxation times also provide us with valuable information about correlations in molecular motion. The new technique of multiple quantum nuclear magnetic resonance has numerous advantages over the conventional single quantum NMR techniques in obtaining information about static and dynamic interactions of coupled spin systems. In the first two chapters, the theoretical background of spin Hamiltonians and the density matrix formalism of multiple quantum NMR is discussed. The creation and detection of multiple quantum coherence by multiple pulse sequence are discussed. Prototype multiple quantum spectra of oriented benzene are presented. Redfield relaxation theory and the application of multiple quantum NMR to the study of correlations in fluctuations are presented. A specific example of an oriented methyl group relaxed by paramagnetic impurities is studied in detail. The study of possible correlated motion between two coupled methyl groups by multiple quantum NMR is presented. For a six spin system it is shown that the four-quantum spectrum is sensitive to two-body correlations, and serves a ready test of correlated motion. The study of the spin-lattice dynamics of orienting or tunneling methyl groups (CH/sub 3/ and CD/sub 3/) at low temperatures is presented. The anisotropic spin-lattice relaxation of deuterated hexamethylbenzene, caused by the sixfold reorientation of the molecules, is investigated, and the NMR spectrometers and other experimental details are discussed.
Study of the Ground State Properties of LiHoxY1-xF4 Using Muon Spin Relaxation
LiHoxY1-xF4 is an insulator where the magnetic Ho3+ ions have an Ising character and interact mainly through magnetic dipolar fields. We used the muon spin relaxation technique to study the nature of its ground state for samples with x?0.25. In contrast with some previous works, we did not find canonical spin glass behavior down to ?15mK. Instead, below ?300mK we observed temperature-independent dynamic magnetism characterized by a single correlation time. The 300 mK energy scale corresponds to the Ho3+ hyperfine interaction strength, suggesting that this interaction may be involved in the dynamic behavior of the system.
Magnetic resonance imaging and spectroscopy with special reference to tissue characterization
1991-01-01
The presentation focuses on the question whether quantitative MR measurements of T1-and T2 relaxation times may improve the diagnostic specificity of MRI. Furthermore the potential of MR spectroscopy in tissue characterization by MR is considered. The basic conclusion are that quantitative measurements of average T1 and T2 show great biological scatter with great overlap between different pathologies. In special cases an interesting correlation between relaxation rates and therapy effects has been observed. Recent spectroscopy studies are very promising. Thus tissue characterization by MR-relaxation measurements, together with metabolic studies by means of MR-spectroscopy may be clinically useful in the future, especially for non-invasive follow up of treatment.
Nuclear magnetic relaxation studies of semiconductor nanocrystals and solids
Semiconductor nanocrystals, small biomolecules, and {sup 13}C enriched solids were studied through the relaxation in NMR spectra. Surface structure of semiconductor nanocrystals (CdS) was deduced from high resolution {sup 1}H and {sup 13}C liquid state spectra of thiophenol ligands on the nanocrystal surfaces. The surface coverage by thiophenol was found to be low, being 5.6 and 26% for nanocrystal radii of 11.8 and 19.2 {angstrom}. Internal motion is estimated to be slow with a correlation time > 10{sup {minus}8} s{sup {minus}1}. The surface thiophenol ligands react to form a dithiophenol when the nanocrystals were subjected to O{sub 2} and ultraviolet. A method for measuring {sup 14}N-{sup 1}H J-couplings is demonstrated on pyridine and the peptide oxytocin; selective 2D T{sub 1} and T{sub 2} experiments are presented for measuring relaxation times in crowded spectra with overlapping peaks in 1D, but relaxation effects interfere. Possibility of carbon-carbon cross relaxation in {sup 13}C enriched solids is demonstrated by experiments on zinc acetate and L-alanine.
Nuclear magnetic relaxation studies of semiconductor nanocrystals and solids
1993-09-01
Semiconductor nanocrystals, small biomolecules, and {sup 13}C enriched solids were studied through the relaxation in NMR spectra. Surface structure of semiconductor nanocrystals (CdS) was deduced from high resolution {sup 1}H and {sup 13}C liquid state spectra of thiophenol ligands on the nanocrystal surfaces. The surface coverage by thiophenol was found to be low, being 5.6 and 26% for nanocrystal radii of 11.8 and 19.2 {angstrom}. Internal motion is estimated to be slow with a correlation time > 10{sup {minus}8} s{sup {minus}1}. The surface thiophenol ligands react to form a dithiophenol when the nanocrystals were subjected to O{sub 2} and ultraviolet. A method for measuring {sup 14}N-{sup 1}H J-couplings is demonstrated on pyridine and the peptide oxytocin; selective 2D T{sub 1} and T{sub 2} experiments are presented for measuring relaxation times in crowded spectra with overlapping peaks in 1D, but relaxation effects interfere. Possibility of carbon-carbon cross relaxation in {sup 13}C enriched solids is demonstrated by experiments on zinc acetate and L-alanine.
2002-01-01
Three methods for calculating nuclear magnetic resonance cross-relaxation rates from molecular dynamics simulations of small flexible molecules have been compared in terms of their ability to reproduce relaxation data obtained experimentally and to produce consistent descriptions of the system. The importance of the accuracy of the simulation versus the amount of sampling of phase space has also been assessed by comparing different length simulations performed with different time step schemes. A nine-residue peptide from the protein HPr of index. E. Coli was used as a test system. The work shows that, in this case, single conformations or a limited ensemble of configurations are insufficient to properly describe the behavior of the peptide and that different approaches to incorporate molecular motions lead to significant differences in the cross-relaxation rates calculated. The correlation between the cross-relaxation rates calculated from simulations performed with different time step schemes was high and increased with increasing simulation length indicating that the extent of sampling is more important than the details of the atomic motion. Publisher: Kluwer Academic Publ Coverage: 2002-01-01T00:00:00Z
2009-01-01
Contrast-enhanced perfusion studies of the brain by means magnetic resonance imaging (MRI) are used to estimate a number of important brain tissue parameters, including cerebral blood flow and volume. In order to calculate these parameters, the contrast agent (CA) concentration must first be estimated. This is usually accomplished by measurement of a nuclear magnetic resonance (NMR) relaxation rate with the assumption of a linear relationship between the rate and the CA concentration. However, such a linear relationship does not necessarily hold in biological tissues due to compartmentalization of the CA in either the intravascular or extracellular spaces. Here we propose an alternative MRI method of CA quantification based on measurement of the magnetic field correlation (MFC), which is t...
Dynamics of braided coronal loops II: Cascade to multiple small-scale reconnection events
2010-03-31
Aims: Our aim is to investigate the resistive relaxation of a magnetic loop that contains braided magnetic flux but no net current or helicity. The loop is subject to line-tied boundary conditions. We investigate the dynamical processes that occur during this relaxation, in particular the magnetic reconnection that occurs, and discuss the nature of the final force-free state obtained. Methods: The three-dimensional evolution of a braided magnetic field is followed in a series of resistive MHD simulations. Results: It is found that, following an instability within the loop, a myriad of thin current layers forms, via a cascade-like process. This cascade becomes more developed and continues for a longer period of time for higher magnetic Reynolds number. During the cascade, magnetic flux is reconnected multiple times, with the level of this `multiple reconnection' positively correlated with the magnetic Reynolds number. Eventually the system evolves into a state with no more small-scale current layers. This final state is found to be a non-linear force-free field, consisting of two flux tubes of oppositely-signed twist embedded in an approximately uniform background field.
Anomalous magnetic relaxation in superfluid helium-3
1978-12-05
The change of the character of the relaxation of the magnetization in superfluid helium-3 when the magnetization is inclined at large angles is attributed to a transition from an internal stationary Josephson effect to a nonstationary one. The dependence of the critical angle of inclination on the temperature and on the magnetic field is determined. Comparison is made with experiment.
2002-12-01
The goal of this study was to determine a T1-weighted magnetic resonance (MR) imaging sequence appropriate for evaluating the longitudinal relaxation effect of superparamagnetic iron oxide (ferumoxides) in a phantom study. An agarose phantom that included various concentrations of ferumoxides (0-0.5 mmol/l in 0.05 mmol/l increments) was examined for six types of T1-weighted imaging sequences using a 1.5-T MR unit. Three-dimensional (3D) fast spoiled gradient-echo (SPGR) imaging with a short echo time showed a strong linear correlation between the concentration of ferumoxides and the enhancement ratio. Two-dimensional (2D) fast SPGR imaging showed a high signal-to-noise ratio of the phantom even at low ferumoxides concentrations. These results suggest that 3D fast SPGR imaging is an appropriate technique for the evaluation of the longitudinal relaxation effect of ferumoxides, and that 2D fast SPGR imaging can be useful for evaluating the longitudinal relaxation effect at lower ferumoxides concentrations. (author)
Magnetic resonance studies of the interaction of gadolinium(III) with bovine and human serum albumin
1983-09-01
Proton relaxation enhancement has been used to study the binding of gadolinium(III) to bovine and human serum albumin. The number of metal binding sites on the protein and the metal-protein binding constants have been determined. The pH and the frequency dependence of the characteristic enhancement, epsilon(sub b), were also investigated so as to correlate changes in epsilon(sub b) with structural changes induced in the protein.
Structure and Dynamics of Ribosomal Protein L12: An Ensemble Model Based on SAXS and NMR Relaxation
2010-01-01
Ribosomal protein L12 is a two-domain protein that forms dimers mediated by its N-terminal domains. A 20-residue linker separates the N- and C-terminal domains. This linker results in a three-lobe topology with significant flexibility, known to be critical for efficient translation. Here we present an ensemble model of spatial distributions and correlation times for the domain reorientations of L12 that reconciles experimental data from small-angle x-ray scattering and nuclear magnetic resonance. We generated an ensemble of L12 conformations in which the structure of each domain is fixed but the domain orientations are variable. The ensemble reproduces the small-angle x-ray scattering data and the optimized correlation times of its reorientational eigenmodes fit the ^1^5N relaxation data. ...
2009-01-01
Nuclear magnetic resonance and relaxation measurements of the 59Co nuclei for the well-defined field-aligned powder of the triangular lattice superconductor NaxCoO2 . yH2O with a nearly optimal composition for the transition temperature Tc have been performed. Detailed analyses indicate that the Knight shifts for the directions parallel and perpendicular to the CoO2 plane and the spin-lattice relaxation rates, taken by making an inevitable deterioration of specimen minimal, are significantly smaller than most of the data reported to date, and that this compound is classified into one of the unconventional spin-singlet superconductors as suggested by recent works for single crystals with a lower Tc. A small enhancement of two-dimensional antiferromagnetic spin correlations ...
On shape of NMR absorption spectra and cross-relaxation in hetero nuclear spin system
The dynamic theory of the heteronuclear spin systems in solid bodies at high temperatures is developed. The system of the nonlinear integral equations is obtained for the time spin correlation functions in the approximation of the self-consistent fluctuating local field. The corrections, originating due to the fluctuating local fields correlations, existing in the real lattices, are accounted for thereby. The theory is applied to describing available experimental data for the LiF crystal (with two nuclei kinds). The free precession signals for the Li and F nuclei, as well as, the harmonic cross-relaxation spectra, the sup 6 Li isotope cross-polarization and the sup 8 Li isotope depolarization are calculated by the magnetic field orientations along the basic crystallographic axes. Good agreement between theory and experiment is achieved
Correlation-Driven Charge Order at a Mott Insulator - Band Insulator Digital Interface
2006-08-09
To study digital Mott insulator LaTiO3 and band insulator SrTiO3 interfaces, we apply correlated band theory (LDA+U) to (n,m) multilayers, 1<n,m<9. If the on-site repulsion on Ti is large enough to model the magnetic insulating behavior of cubic bulk LaTiO3, the charge imbalance at the interface is found in all cases to be accommodated by disproportionation (Ti4+ + Ti3+), charge ordering, and Ti3+ d_xy-orbital ordering, with antiferromagnetic exchange coupling between the spins in the interface layer. Lattice relaxation affects the conduction behavior by shifting (slightly but importantly) the lower Hubbard band, but the disproportionation and orbital ordering are robust against relaxation.
Nuclear magnetic resonance water relaxation time changes in bananas during ripening: a new mechanism
2010-01-01
BACKGROUND: Nuclear magnetic resonance studies of banana fragments during ripening show an increase on the water transverse relaxation time (T2) and a decrease in water self-diffusion coefficient (D). As T2 and D are normally directly correlated, we studied these two properties in intact bananas during ripening, in an attempt to rule out the effect of injury on the apparent discrepancies in the behavior of T2 and D. RESULTS: The results show that injury in bananas causes a decrease in T2 of the water in vacuoles (T2vac). They also show that T2vac increased and D decreased during ripening, ruling out the injury effect. To explain the apparent discrepancies, we propose a new hypothesis for the increase in T2 values, based on the reduction of Fe3+ ions to Fe2+ by galacturonic acid, produced b...
Magnetic resonance microscopy for monitoring osteogenesis in tissue-engineered construct in vitro
2006-02-07
Magnetic resonance microscopy (MRM) is used to monitor osteogenesis in tissue-engineered constructs. Measurements of the developing tissue's MR relaxation times (T{sub 1} and T{sub 2}), apparent diffusion coefficient (ADC) and elastic shear modulus were conducted over a 4-week growth period using an 11.74 T Bruker spectrometer with an imaging probe adapted for MR elastography (MRE). Both the relaxation times and the ADC show a statistically significant decrease after only one week of tissue development while the tissue stiffness increases progressively during the first two weeks of in vitro growth. The measured MR parameters are correlated with histologically monitored osteogenic tissue development. This study shows that MRM can provide quantitative data with which to characterize the growth and development of tissue-engineered bone.
Magnetic resonance microscopy for monitoring osteogenesis in tissue-engineered construct in vitro
2006-01-01
Magnetic resonance microscopy (MRM) is used to monitor osteogenesis in tissue-engineered constructs. Measurements of the developing tissue's MR relaxation times (T1 and T2), apparent diffusion coefficient (ADC) and elastic shear modulus were conducted over a 4-week growth period using an 11.74 T Bruker spectrometer with an imaging probe adapted for MR elastography (MRE). Both the relaxation times and the ADC show a statistically significant decrease after only one week of tissue development while the tissue stiffness increases progressively during the first two weeks of in vitro growth. The measured MR parameters are correlated with histologically monitored osteogenic tissue development. This study shows that MRM can provide quantitative data with which to characterize the growth and development of tissue-engineered bone
2004-01-01
Biocompatible magnetic dispersions have been prepared from gamma-Fe2O3 nanoparticles (5 nm) synthesized by continuous laser pyrolysis of Fe(CO)5 vapours. The feasibility of using these dispersions as magnetic resonance imaging (MRI) contrast agents has been analysed in terms of chemical structure, magnetic properties, 1H NMR relaxation times and biokinetics. The magnetic nanoparticles were dispersed in a strong alkaline solution in the presence of dextran, yielding stable colloids in a single step. The dispersions consist of particle-aggregates 25 nm in diameter measured using transmission electron microscope and a hydrodynamic diameter of 42 nm measured using photon correlation spectroscopy. The magnetic and relaxometric properties of the dispersions were of the same order of magnitude as those ...
Non-Markovian Effects in Turbulent Diffusion in Magnetized Plasmas
2009-01-01
The derivation of the kinetic equations for inhomogeneous plasma in an external magnetic field is presented. The Fokker-Planck-type equations with the non-Markovian kinetic coefficients are proposed. In the time-local limit (small correlation times with respect to the distribution function relaxation time) the relations obtained recover the results known from the appropriate quasilinear theory and the Dupree-Weinstock theory of plasma turbulence. The equations proposed are used to describe zonal flow generation and to estimate the diffusion coefficient for saturated turbulence
Muon studies of heavy fermions
Recent muon spin relaxation ({mu}SR) studies have been particularly effective in revealing important properties of the unusual magnetism and superconductivity found in heavy fermion (HF) systems. In this paper {mu}SR experiments elucidating the symmetry of superconducting order parameter in UPt{sub 3} and UBe{sub 13} doped with thorium and reviewed. Also discussed is the correlation between the enhanced superconducting specific heat jump and the reduced Kondo temperature in B-doped UBe{sub 13}, indicating possible direct experimental evidence for a magnetic pairing mechanism in HF superconductors. 23 refs., 3 figs.
Muon studies of heavy fermions
1991-01-01
Recent muon spin relaxation ({mu}SR) studies have been particularly effective in revealing important properties of the unusual magnetism and superconductivity found in heavy fermion (HF) systems. In this paper {mu}SR experiments elucidating the symmetry of superconducting order parameter in UPt{sub 3} and UBe{sub 13} doped with thorium and reviewed. Also discussed is the correlation between the enhanced superconducting specific heat jump and the reduced Kondo temperature in B-doped UBe{sub 13}, indicating possible direct experimental evidence for a magnetic pairing mechanism in HF superconductors. 23 refs., 3 figs.
The magnetic resonance microscopy (MRM) work at Montana State University has extended the imaging of a single biofilm in a 1 mm capillary reactor to correlate T2 magnetic relaxation maps displaying biofilm structure with the corresponding velocity patterns in three dimensions in a Staphylococcus epidermidis biofilm fouled square capillary. A square duct geometry is chosen to provide correlation with existing experiments and simulations, as research bioreactors tend to be of square or rectangular cross section for optical or microelectrode access. The spatially resolved velocity data provide details on the impact of biofilm induced advection on mass transport from the bulk fluid to the biofilm and through the capillary bioreactor.
2005-06-01
The magnetic resonance microscopy (MRM) work at Montana State University has extended the imaging of a single biofilm in a 1 mm capillary reactor to correlate T2 magnetic relaxation maps displaying biofilm structure with the corresponding velocity patterns in three dimensions in a Staphylococcus epidermidis biofilm fouled square capillary. A square duct geometry is chosen to provide correlation with existing experiments and simulations, as research bioreactors tend to be of square or rectangular cross section for optical or microelectrode access. The spatially resolved velocity data provide details on the impact of biofilm induced advection on mass transport from the bulk fluid to the biofilm and through the capillary bioreactor.
Enhanced carrier scattering rates in dilute magnetic semiconductors with correlated impurities
2006-07-07
In III-V dilute magnetic semiconductors (DMSs) such as Ga$_{1-x}$Mn$_x$As, the impurity positions tend to be correlated, which can drastically affect the electronic transport properties of these materials. Within the memory function formalism we have derived a general expression for the current relaxation kernel in spin and charge disordered media and have calculated spin and charge scattering rates in the weak-disorder limit. Using a simple model for magnetic impurity clustering, we find a significant enhancement of the charge scattering. The enhancement is sensitive to cluster parameters and may be controllable through post-growth annealing.
2009-01-01
We discuss the multiple technical choices that have to be made in ab initio density-functional calculations of the magnetic anisotropy of supported nanostructures: (i) choice of the exchange-correlation functional, (ii) degree of optimization of the geometry of the adsorbate/substrate complex, (iii) magnetic anisotropy energy calculated self-consistently or via the 'force theorem', (iv) calculations based on slab models of the substrate or using a Green's function describing a semi-infinite substrate, (v) full potential approach or atomic-sphere approximation. Using isolated Fe and Co atoms on Pt(111) as an example we demonstrate that by using a judicious combination of relatively crude approximations (complete neglect of structural relaxation, local exchange-correlation functional,...) seemingly good agreement with experimental anisotropy energies can ...
NMR (Nuclear Magnetic Resonance) of solid biopolymers
It has been possible through the study of proton relaxation in solid proteins and related molecules to identify and characterize the following molecular motions which contribute to relaxation: (1) methyl group ...
2004-01-14
The effect of the transverse AC magnetic field on relaxation process in YBa$_2$Cu$_3$O$_x$ melt-textured superconductor was studied. A factor of 50 suppression of the relaxation rate could be achieved at the expense of some reduction in the maximum trapped field, with the magnetic-induction gradient being unchanged. This phenomenon is interpreted as a result of an increase of the pinning force after the action of the transverse AC magnetic field.
2009-01-01
We have manufactured more than 250 nominally identical paraffin-coated Cs vapor cells (28mm inner diameter bulbs) for multi-channel atomic magnetometer applications. We describe our dedicated cell characterization apparatus. For each cell we have determined the intrinsic longitudinal, 01, and transverse, 02, relaxation rates. Our best cell shows 01/20.5Hz, and 02/22Hz. We find a strong correlation of both relaxation rates which we explain in terms of reservoir and spin exchange relaxation. For each cell we have determined the optimal combination of rf and laser powers which yield the highest sensitivity to magnetic field changes. Out of all produced cells, 90% are found to have magnetometric sensitivities in the range of 9to 30fT Formula Not Shown . Noise an...
2008-12-24
We have manufactured more than 250 nominally identical paraffin-coated Cs vapor cells (30 mm diameter bulbs) for multi-channel atomic magnetometer applications. We describe our dedicated cell characterization apparatus. For each cell we have determined the intrinsic longitudinal, $\\sGamma{01}$, and transverse, $\\sGamma{02}$, relaxation rates. Our best cell shows $\\sGamma{01}/2\\pi\\approx 0.5 Hz, and $\\sGamma{02}/2\\pi\\approx 2 Hz. We find a strong correlation of both relaxation rates which we explain in terms of reservoir and spin exchange relaxation. For each cell we have determined the optimal combination of rf and laser powers which yield the highest sensitivity to magnetic field changes. Out of all produced cells, 90% are found to have magnetometric sensitivities in the range of 9 to 30 fTHz. Noise analysis shows that the magnetometers operated with such cells have a sensitivity close to the fundamental photon shot noise limit.
Magnetic Resonance Water Proton Relaxation in Protein Solutions and Tissue: T
BackgroundImage contrast in clinical MRI is often determined by differences in tissue water proton relaxation behavior. However, many aspects of water proton relaxation in complex...Full Text Available
A comprehensive analysis of the sup 3 sup 5 Cl nuclear magnetic resonance (NMR) relaxation data in Sr sub 2 CuO sub 2 Cl sub 2 single crystals is presented. Both the spin-lattice relaxation rate, T sub 1 sup - sup 1 (=2W), and the spin-spin relaxation rate, T sub 2 sup - sup 1 , show a crossover of the spin dimensionality well above the Neel temperature T sub N. The crossover is due to easy-plane anisotropy and is apparently signaled by the partial suppression of the Cu sup 2 sup + spin fluctuations along the tetragonal c-axis. By analyzing 2W for H ll c in terms of the critical behavior of the spin correlation length, we estimate the temperature for the crossover of the Cu sup 2 sup + spin correlations from Heisenberg to XY-like behavior to be T approx =290 K.
2001-11-01
Low field NMR relaxation for laboratory or in-situ applications provides critical information for oil recovery such as porosity, saturation, and permeability of rocks. In addition, pore size distribution and wettability can also be obtained in some cases. The technique relies on the measurement of proton longitudinal (T{sub 1}) or transverse (T{sub 2}) nuclear relaxation times. For better predictions, the surface micro-dynamics and the chemical properties of the liquids entrapped in the pore space are important and must be characterized. It is well known that the NMR relaxation is enhanced by the paramagnetic impurities at the pore surface but many other parameters influence the relaxation time distributions. These parameters are used to derive the petrophysical properties of the rocks. We propose here an original method to probe the dynamics of water and oil at the pore surface. In the present study, we used both nuclear relaxation at 2.2 MHz and field cycling Nuclear Magnetic Relaxation Dispersion (NMRD) techniques. We applied these two techniques to different kinds of water or oil saturated macroporous media (grain packings, outcrop and reservoir rocks with SiO{sub 2} or CaCO{sub 3} surfaces). We studied the dependence of NMR relaxation on pore size, magnetic field and temperature. Varying the pore size and the surface density of paramagnetic impurities of water saturated grain packings allowed experimental evidence for the two limiting regimes of the water relaxation in pores (surface- and diffusion-limited regimes). NMRD technique (evolution of 1/T{sub 1} with the magnetic field) allowed us to probe liquid surface dynamics in water or oil fully saturated grain packing, outcrop rocks or reservoir rocks (water- and oil-wet surfaces). We evidenced a two-dimensional molecular surface diffusion and directly estimated important parameters such as correlation times, residence times and molecular self-diffusion on the surface. Finally, we proved that the temperature dependence of T{sub 1} and T{sub 2} is directly related to the surface chemistry of the rocks. Such a dependence is clearly important for oil field in-situ measurements (well logging). (author)
Brownian rotational relaxation and power absorption in magnetite nanoparticles
2007-09-15
We present a study of the power absorption efficiency in several magnetite-based colloids, to asses their potential as magnetic inductive hyperthermia (MIH) agents. Relaxation times {tau} were measured through the imaginary susceptibility component {chi}{sup '}'(T), and analyzed within Debye's theory of dipolar fluid. The results indicated Brownian rotational relaxation and allowed to calculate the hydrodynamic radius close to the values obtained from photon correlation. The study of the colloid performances as power absorbers showed no detectable increase of temperature for dextran-coated Fe{sub 3}O{sub 4} nanoparticles, whereas a second Fe{sub 3}O{sub 4}-based dispersion of similar concentration could be heated up to 12K after 30min under similar experimental conditions. The different power absorption efficiencies are discussed in terms of the magnetic structure of the nanoparticles.
The authors investigate the dispersion of the magnetic susceptibility of a magnetic liquid based on a cobalt ferrite colloid under the influence of a magnetic field. The complex magnetic susceptibility was determined experimentally in relation the strength of the magnetic field. The magnetization relaxation time was also determined as a function of field strength. The physical parameters of the liquid were determined by representing the experimental data in a Cowell-Cowell diagram. It was found that the dependence of the magnetic susceptibility dispersion of a colloid of magnetically rigid particles on field strength can be described mathematically in the rigid dipole approximation if proper allowance is made for the distribution of magnetic relaxation time.
Control of Transport-barrier relaxations by Resonant Magnetic Perturbations
2009-02-17
Transport-barrier relaxation oscillations in the presence of resonant magnetic perturbations are investigated using three-dimensional global fluid turbulence simulations from first principles at the edge of a tokamak. It is shown that resonant magnetic perturbations have a stabilizing effect on these relaxation oscillations and that this effect is due mainly to a modification of the pressure profile linked to the presence of both residual residual magnetic island chains and a stochastic layer.
Influence of carriers on magnetization relaxation in (Cd,Mn)Te quantum wells
2007-07-01
The relaxation of the magnetization after a short pulse of magnetic field in n-doped and p-doped (Cd,Mn)Te quantum wells is determined from the giant Zeeman shift of the photoluminescence and transmission lines. This technique allows us to investigate the magnetization dynamics in zero field. In n-doped samples the magnetization relaxation is found to be faster than 20 ns. In case of p-doped samples two components of relaxation are observed. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Influence of carriers on magnetization relaxation in (Cd,Mn)Te quantum wells
2007-01-01
The relaxation of the magnetization after a short pulse of magnetic field in n-doped and p-doped (Cd,Mn)Te quantum wells is determined from the giant Zeeman shift of the photoluminescence and transmission lines. This technique allows us to investigate the magnetization dynamics in zero field. In n-doped samples the magnetization relaxation is found to be faster than 20 ns. In case of p-doped samples two components of relaxation are observed. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Influence of carriers on magnetization relaxation in (Cd,Mn)Te quantum wells
2007-01-01
The relaxation of the magnetization after a short pulse of magnetic field in n-doped and p-doped (Cd,Mn)Te quantum wells is determined from the giant Zeeman shift of the photoluminescence and transmission lines. This technique allows us to investigate the magnetization dynamics in zero field. In n-doped samples the magnetization relaxation is found to be faster than 20 ns. In case of p-doped samples two components of relaxation are observed. (Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Effective collective barrier for magnetic relaxation in frozen ferrofluids
2003-12-03
Magnetic relaxation and frequency response were measured in frozen ferrimagnetic colloids of different concentrations. A crossover from reversible to irreversible behavior is observed for concentrated colloids. In irreversible state, magnetic relaxation is time-logarithmic over seven orders of magnitude of experimental time windows. A master curve construction within mean field phenomenological model is applied to extract effective collective barrier as a function of the irreversible magnetization. The barrier logarithmically diverges, providing evidence for self-organized critical behavior during magnetic relaxation in frozen ferrofluids.
Why could electron spin resonance be observed in a heavy fermion Kondo lattice?
2009-01-01
We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model success...
2010-01-01
In this article, we use rapid two-dimensional nuclear magnetic resonance T 1T 2 relaxation time correlations to monitor hydration kinetics, microstructural evolution, and surface interactions simultaneously in gypsum plaster pastes of industrial relevance. In particular, we explore the influence of ground gypsum mineral (GGM) as an accelerator and citric acid as a retarding agent in the hydration of the ? form of plaster. Quantitative relative water fractions are determined and the modified hydration processes are observed as expected. These measurements also reveal the evolution of a secondary water population in the presence of the retarding agent when sedimentation of the hemihydrate occurs prior to crystallisation. In these samples, the ratio of relaxation times T 1/T 2 provides an ...
Moisture determination in paddy seeds by proton nuclear magnetic relaxation measurements
1981-01-01
Proton spin-spin relaxation times (T2) have been measured for 4 varieties of paddy seeds at different levels of moisture. It is found that for all the varieties of paddy studied, T2 increases almost identically with increasing moisture content. This suggests that the change in T2 with moisture content can be used as a method for moisture determination. The necessary calibration curve for moisture determination in paddy has been worked out. The moisture percentage values determined by this method for a variety of paddy have been compared with the values obtained by oven drying method and the correlation coefficient works out to be 0.990. It is a rapid and non-destructive method and does not require weighing the sample. This technique of moisture determination has significant applications in post harvest storage, physiological studies and ...
2009-01-01
Purpose To optimize high-resolution MR spectroscopy (MRS) for obtaining neurochemical composition of adult zebrafish brain in vivo. Materials and Methods A flow-through setup for supporting MRS of living zebrafish has been designed. In vivo MR microscopy (MRM) images were obtained using a rapid acquisition with relaxation enhancement (RARE) sequence to select a volume of interest. In vivo MR spectra from zebrafish brain were obtained using an optimized point-resolved spectroscopy (PRESS) sequence preceded by a variable pulse power and optimized relaxation delays (VAPOR) sequence for global water suppression interleaved with outer volume suppression (OVS). In vitro MR spectra in the brain extract were obtained by using correlated spectroscopy (COSY) sequences. Results Optimized high-resolut...
Muon spin relaxation studies of iron(II) spin crossover complexes
2008-01-01
A series of model iron(II) spin crossover complexes have been investigated by temperature dependent muon spin relaxation (mSR) techniques at ISIS, UK. The thermally induced spin crossover in these materials could be monitored by following the initial asymmetry parameter, a0, in zero-field. We established that the behavior of a0 correlates well with the shape of the spin crossover curve derived from magnetic susceptibility measurements, whether hysteretic, smooth, or abrupt. In addition, the longitudinal field dependence of a0 not only provides information on the nature of the muonic species but also on their interactions and respective localization in the crystal lattice. Useful insights to the electronic structure and dynamic phenomena of these model spin crossover complexes can be derive...
Mesodynamics in the SARS nucleocapsid measured by NMR field cycling
2009-01-01
Protein motions on all timescales faster than molecular tumbling are encoded in the spectral density. The dissection of complex protein dynamics is typically performed using relaxation rates determined at high and ultra-high field. Here we expand this range of the spectral density to low fields through field cycling using the nucleocapsid protein of the SARS coronavirus as a model system. The field-cycling approach enables site-specific measurements of R1 at low fields with the sensitivity and resolution of a high-field magnet. These data, together with high-field relaxation and heteronuclear NOE, provide evidence for correlated rigid-body motions of the entire beta-hairpin, and corresponding motions of adjacent loops with a time constant of 0.8 ns (mesodynamics). MD simulations substantiate these findings and provide direct verification ...
Flux dynamics and magnetovoltage measurements in a macroscopic cylindrical hole drilled in BSCCO
2007-10-01
Slow transport relaxation measurements (V-t curves) and magnetovoltage measurements (V-H curves) were carried out in a polycrystalline sample of Bi{sub 1.7}Pb{sub 0.3}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} (BSCCO) with a macroscopic cylindrically drilled hole (CH). The time evolution of quenched state in V-t curves was interpreted in terms of enhancement of the superconducting order parameter and the relaxation of moving entity. Upon cycling of the external magnetic field with different sweep rates, unusual counter clockwise hysteresis effects and asymmetry in V-H curves are observed in BSCCO sample with CH, which can also be correlated to the trapping of the macroscopic flux bundles in CH.
Anomalous transport phenomena in Fermi liquids with strong magnetic fluctuations
2008-01-01
In this paper, we present recent developments in the theory of transport phenomena based on the Fermi liquid theory. In conventional metals, various transport coefficients are scaled according to the quasiparticles relaxation time, tau, which implies that the relaxation time approximation (RTA) holds well. However, such a simple scaling does not hold in many strongly correlated electron systems. The most famous example would be high-Tc superconductors (HTSCs), where almost all the transport coefficients exhibit a significant deviation from the RTA results. This issue has been one of the most significant unresolved problems in HTSCs for a long time. Similar anomalous transport phenomena have been observed in metals near their antiferromagnetic (AF) quantum critical point (QCP). The main goal of this study is to demonstrate whether the ...
Recovery of nuclear magnetization under extreme inhomogeneous broadening
1996-12-01
A quantitative model is presented for the transient recovery of nuclear magnetization under conditions where nuclear spin dipolar relaxation to dilute relaxation centers proceeds without the intermediary of nuclear spin diffusion. The model is developed for rigid arrays in three, two, and one dimensions. Comparison with experimental results yields measures of effective relaxation rates and relaxation center concentrations. {copyright} {ital 1996 The American Physical Society.}
Recovery of nuclear magnetization under extreme inhomogeneous broadening
1996-01-01
A quantitative model is presented for the transient recovery of nuclear magnetization under conditions where nuclear spin dipolar relaxation to dilute relaxation centers proceeds without the intermediary of nuclear spin diffusion. The model is developed for rigid arrays in three, two, and one dimensions. Comparison with experimental results yields measures of effective relaxation rates and relaxation center concentrations. copyright 1996 The American Physical Society
1999-01-01
A method is presented that makes it possible to estimate both the orientation and the magnitude of the chemical shift anisotropy (CSA) tensor in molecules with a pair of spin 1/2 nuclei, typically 13C-1H or 15 N-1H. The method relies on the fact that the longitudinal cross-correlation rate as well as a linear combination of the autorelaxation rates of longitudinal heterospin magnetization, longitudinal two-spin order and longitudinal proton magnetization are proportional to the spectral density at the Larmor frequency of the heterospin. Therefore the ratio between the cross-correlation rate and the above linear combination is independent of the dynamics. From the field dependence of the ratio both the magnitude and the orientation of the CSA tensor can be estimated. The method is applicable to molecules in all motional ...
2008-01-01
Since clinical measures of bone mineral density do not necessarily predict whether a person will fracture a bone without an intervention, there is a need to find supplementary tools for assessing bone quality. Presently, we hypothesized that measures of mobile and bound water by a Nuclear Magnetic Resonance (NMR) technique are correlated with bone strength and toughness, respectively. To test this, bending specimens from the mid-diaphysis of 18 human femurs were collected from 18 male donors and divided into middle aged and elderly groups. After NMR measurements of each hydrated specimen, an inversion technique was used to convert the free induction decay data into a distribution of spinspin (T2) relaxation rates. Then, the distribution resolved into three distinct components that likel...
sup 13 C NMR relaxation times of hepatic glycogen in vitro and in vivo
1990-07-24
The field dependence of relaxation times of the C-1 carbon of glycogen was studied in vitro by natural-abundance {sup 13}C NMR. T{sub 1} is strongly field dependent, while T{sub 2} does not change significantly with magnetic field. T{sub 1} and T{sub 2} were also measured for rat hepatic glycogen enriched with (1-{sup 13}C)glucose in vivo at 4.7 T, and similar relaxation times were observed as those obtained in vitro at the same field. The in vitro values of T{sub 1} were 65 {plus minus} 5 ms at 2.1 T, 142 {plus minus} 10 ms at 4.7 T, and 300 {plus minus} 10 ms at 8.4 T, while T{sub 2} values were 6.7 {plus minus} 1 ms at 2.1 T, 9.4 {plus minus} 1 ms at 4.7 T, and 9.5 {plus minus} 1 ms at 8.4 T. Calculations based on the rigid-rotor nearest-neighbor model give qualitatively good agreement with the T{sub 1} field dependence with a best-fit correlation time of 6.4 {times} 10{sup {minus}9} s, which is significantly smaller than {tau}{sub M}, the estimated overall correlation time for the glycogen molecule (ca. 10{sup {minus}5} s). A more accurate fit of T{sub 1} data using a modified Lipari and Szabo approach indicates that internal fast motions dominate the T{sub 1} relaxation in glycogen. On the other hand, the T{sub 2} relaxation is dominated by the overall correlation time {tau}{sub M} while the internal motions are almost but not completely unrestricted.
Magnetic relaxation in solar and laboratory plasmas
1988-01-01
Taylor's theory of magnetic relaxation is developed to driven systems, in which helicity is injected into the plasma from an external source. Applications are presented to solar plasmas, in the context of the solar coronal heating problem, and laboratory plasmas in a Spheromak system.
Probing semiconductor carrier kinetics with ultrafast nonlinear Faraday rotation
1998-11-01
We demonstrate that in semiconductors with strong Coulomb exchange correlations, transient nonlinear Faraday rotation spectroscopy (FRS) more accurately probes the ultrafast carrier relaxation than the widely used differential transmission spectroscopy (DTS). We calculate nonlinear FRS and DTS by solving the six-band semiconductor Bloch equations for magnetic Cd{sub 1{minus}x}Mn{sub x}Te and CdTe quantum wells. We show that for highly excited carriers, nonlinear FRS rotation spectra closely monitor the ultrafast thermalization of the carrier distribution, unlike DTS signals. We further show that the magnetic band tuning capabilities (giant Zeeman splitting) of dilute magnetic semiconductors allows one to investigate the electron kinetics in selective bands. {copyright} {ital 1998} {ital The American Physical Society}
2007-01-01
The presence of highly concentrated dissolved laser-polarized xenon (1mol/L, polarization up to 0.2) induces numerous effects on proton and xenon NMR spectra. We show that the proton signal enhancements due to 129Xe-1H cross-relaxation (SPINOE) and overall shifts of the proton resonances due to the average dipolar shift created by the intense xenon magnetization are correlated. Protons behave as very useful sensors of the xenon magnetization. Indeed the xenon resonances exhibit many features such as superimposition of narrow lines on the main resonance due to clustering effects, or such as a polarization-dependent line broadening that is tentatively assigned to the effects of temperature fluctuations that decorrelate some distant dipolar field effects from local interactions, transforming ...
Observation of the topological phase transition in the triangular Heisenberg antiferromagnet NiGa2S4
2009-01-01
It has been shown in 2005 by neutron powder diffraction that the triangular layer Heisenberg compound NiGa2S4 is characterized by nano-scale magnetic correlations below approx 18 K. Using the muon-spin rotation and relaxation (muSR) techniques, we have evidenced that the compound displays a phase transition at Tc = 9.2 (2) K below which a coherent oscillation of the muon spins is detected. Remarkably, the magnetic specific heat Cm exhibits a rounded low- temperature maximum at approx 13 K, i.e. at a temperature above Tc. This is consistent with the theory of Kawamura and Miyashita which attributes the transition at Tc to the unbinding of Z2 vortices. NiGa2S4 is characterized by a wide spectrum of magnetic fluctuations as ...
Nuclear magnetic resonance imaging of experimentally induced liver disease. [Rats]
1983-09-01
Experimental animal models of hepatitis, fatty liver, and hepatic iron overload were evaluated using a 3.5-kGauss nuclear magnetic resonance (NMR) imaging system. Increases in image intensity measurements and in T/sub 2/ relaxation times equalled the sensitivity of histologic findings for the detection of early stages of hepatitis. A significant shift in T/sub 1/ relaxation times characterized the early stages of hepatic necrosis. Liver triglyceride content correlated significantly with increases in NMR intensity measurements (p<0.01); however, changes in liver water content had a much greater influence on intensity, T/sub 1/, and T/sub 2/. Thus, it may be possible to distinguish hepatitis from benign fatty liver. Liver iron content correlated with decreases in NMR intensity measurements (p<0.001), and iron levels as low as 1.2 mg/g were detected. NMR may more specifically identify hepatocellular iron overload than do other techniques that do not distinguish hepatocellular from reticuloendothelial iron.
2007-08-10
In this paper we study the relaxation singularities of human motor system at aging. Our purpose is to examine the structure of force output variability as a function of human aging in the time and frequency domains. For analysis of experimental data we have developed here the statistical theory of relaxation of force output fluctuation with taking into account the effects of two relaxation channels. The first of them contains the contribution of short-range correlation whereas other relaxation component reflects the effect of long-range correlation. The analysis of experimental data shows, that the general behavior of relaxation processes at human aging is determined by a complicated combination and nonlinear interactions two above stated relaxation processes as a whole.
The Alpha Dynamo Effects in Laboratory Plasmas
A concise review of observations of the alpha dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This alpha-effect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This drive-and-relax cycle, or the so-called self-organization process, happens in magnetized plasmas in a timescale much shorter than resistive diffusion time, thus it is a fast and unquenched dynamo process. The observed alpha-effect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast and unquenched dynamos are natural consequences of a driven system where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed.
The Alpha Dynamo Effects in Laboratory Plasmas
2001-10-16
A concise review of observations of the alpha dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This alpha-effect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This drive-and-relax cycle, or the so-called self-organization process, happens in magnetized plasmas in a timescale much shorter than resistive diffusion time, thus it is a fast and unquenched dynamo process. The observed alpha-effect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast and unquenched dynamos are natural consequences of a driven system where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed.
Magnetic irreversibility and Verwey transition in nano-crystalline bacterial magnetite
2007-03-12
Magnetic properties of biological magnetite nanocrystals produced by four different magnetotactic bacteria are compared to synthetic nanocrystals and to large high quality single crystals. The magnetic feature at the Verwey temperature, $T_{V}$, is clearly seen in all nanocrystals, but its sharpness depends on the shape of individual nanoparticles and whether or not they are arranged in magnetosome chains. No correlation between the particle size and $T_{V}$ is found. Instead, it is shown that the transition is smeared in the individual superparamagnetic nanoparticles for which $T_{V}>T_{B}$ where $T_{B}$ is the blocking temperature. For the magnetosome chains, measurements of $M(H,T,time) $ suggest that they behave as long magnetic dipoles where local magnetic field is directed along the chain and where $T_{V}<T_{B}$, in which case the Verwey transition is clearly observed. In addition, the results confirm that time-logarithmic magnetic relaxation is due to collective (dipolar) nature of the barrier for magnetic moment reorientation.
Magnetic after-effect in manganite films
2001-05-01
The time dependence of the magnetic and transport properties on La{sub 0.6}Sr{sub 0.4}MnO{sub 3} films and bulk samples has been studied through magnetization and resistivity measurements. A magnetic after-effect has been observed in all samples. At low temperatures, the low-field magnetization, can be described by the function M(t)=M{sub c}+M{sub d} exp(-t/{tau})+S(H,T)ln(t). The resistivity increases logarithmically in the same temperature range, indicating the evolution of the sample to a more disordered state. Above a characteristic temperature, this behaviour is reversed and an increase of the magnetization with time is observed. The relaxation parameters depend on the bulk or films character of the samples. In the latter case, a dependence on the film thickness was found. A direct correlation between the time dependence of the resistivity and magnetization curves in manganite compounds was found.
2007-03-06
We study an entropy-driven ordering process in macroscopically degenerate spin configurations in the magnetically ordered state of Ising-like Heisenberg antiferromagnetic model on the \\kagome lattice. This model has been pointed out to have a finite temperature phase transition accompanying with the uniform spontaneous magnetization. In spite of the symmetry breaking of the magnetization, macroscopic degenerated spin configuration exists in the ordered state. Reflecting these two ordering mechanisms, the relaxation process consists of two steps: fast relaxation of energy and magnetization, and slow relaxation of spin configuration to entropically preferable spin configuration.
Vertex corrections and the Korringa ratio in strongly correlated electron materials
2009-05-13
We show that the Korringa ratio, associated with nuclear magnetic resonance in metals, is unity if vertex corrections to the dynamic spin susceptibility are negligible, the hyperfine coupling is momentum independent, and there exists an energy scale below which the density of states is constant. In the absence of vertex corrections we also find a Korringa behaviour for T{sub 1}, the nuclear spin relaxation rate, i.e., 1/T{sub 1}{proportional_to}T, and a temperature independent Knight shift. These results are independent of the form and magnitude of the self-energy (so far as is consistent with neglecting vertex corrections) and of the dimensionality of the system.
Vertex corrections and the Korringa ratio in strongly correlated electron materials
2009-01-01
We show that the Korringa ratio, associated with nuclear magnetic resonance in metals, is unity if vertex corrections to the dynamic spin susceptibility are negligible, the hyperfine coupling is momentum independent, and there exists an energy scale below which the density of states is constant. In the absence of vertex corrections we also find a Korringa behaviour for T1, the nuclear spin relaxation rate, i.e., 1/T1propor toT, and a temperature independent Knight shift. These results are independent of the form and magnitude of the self-energy (so far as is consistent with neglecting vertex corrections) and of the dimensionality of the system
2009-01-01
Wide varieties of strongly correlated electron phenomena are performed on the stage of a 'filled skutterudite structure'. Especially when one of the players contains a plural number of 4f electrons, the orbital degrees of freedom play a major role as a new type of nonmagnetic and/or weak-magnetic phenomena. Several examples found in Pr- and Sm-based filled skutterudites are introduced in relation to muon spin relaxation (muSR) experiments
Siloxanes have a wide variety of applications throughout the aerospace industry which take advantage of their exceptional insulating and adhesive properties and general resilience. They also offer a wide range of tailorable engineering properties with changes in composition and filler content. They are, however, subject to degradation in radiatively and thermally harsh environments. We are using solid state nuclear magnetic resonance techniques to investigate changes in network and interfacial structure in siloxane elastomers and their correlations to changes in engineering performance in a series of degraded materials. Nuclear magnetic resonance (NMR) parameters such as transverse (T{sub 2}) relaxation times, cross relaxation rates, and residual dipolar coupling constants provide excellent probes of changes crosslink density and motional dynamics of the polymers caused by multi-mechanism degradation. The results of NMR studies on aged siloxanes are being used in conjunction with other mechanical tests to provide insight into component failure and degradation kinetics necessary for preliminary lifetime assessments of these materials as well as into the structure-property relationships of the polymers. NMR and magnetic resonance imaging (MRI) results obtained both from high resolution NMR spectrometers as well as low resolution benchtop NMR screening tools will be presented.
2005-02-03
Siloxanes have a wide variety of applications throughout the aerospace industry which take advantage of their exceptional insulating and adhesive properties and general resilience. They also offer a wide range of tailorable engineering properties with changes in composition and filler content. They are, however, subject to degradation in radiatively and thermally harsh environments. We are using solid state nuclear magnetic resonance techniques to investigate changes in network and interfacial structure in siloxane elastomers and their correlations to changes in engineering performance in a series of degraded materials. Nuclear magnetic resonance (NMR) parameters such as transverse (T{sub 2}) relaxation times, cross relaxation rates, and residual dipolar coupling constants provide excellent probes of changes crosslink density and motional dynamics of the polymers caused by multi-mechanism degradation. The results of NMR studies on aged siloxanes are being used in conjunction with other mechanical tests to provide insight into component failure and degradation kinetics necessary for preliminary lifetime assessments of these materials as well as into the structure-property relationships of the polymers. NMR and magnetic resonance imaging (MRI) results obtained both from high resolution NMR spectrometers as well as low resolution benchtop NMR screening tools will be presented.
Magnetic relaxation in highly anisotropic systems
1985-01-01
Using the Mori formalism, a general theory is developed for the calculation of paramagnetic relaxation effects in a general crystal field doublet. For systems with extreme magnetic anisotropy, the relaxation rates show a strong angular dependence, being severely peaked in a direction perpendicular to the magnetic easy axis. Application to the RRh/sub 4/B/sub 4/ (R = rare-earth) compounds is discussed. 5 refs.
Dynamics of {beta}-CH and {beta}-CH2 Groups of Amino Acid Side Chains in Proteins
1998-02-15
The dynamics of amino acid side chains of uniformly 13C/15N-enriched ribonuclease T1 (RNase T1) have been investigated. Heteronuclear longitudinal relaxation rates, 1H/13C NOEs, and transverse cross-correlated cross-relaxation rates between the Sx and the SxIz1Iz2 operators (SIIS cross relaxation) [Ernst and Ernst (1994) J. Magn. Reson., A110, 202-213] have been determined in this study. New pulse sequences for measuring the longitudinal relaxation time and the heteronuclear NOE of aliphatic side chain carbon nuclei were developed using the CCONH type of magnetization transfer and 1HN detection. In addition, an improved pulse sequence for the determination of the SIIS cross relaxation is presented. For the analysis of the relaxation rates, the model of restricted rotational diffusion around the {chi}1 dihedral angle has been applied [London and Avitabile (1978) J. Am. Chem. Soc., 100, 7159-7165]. These techniques were used in order to describe the side chain dynamics of the small globular protein RNase T1 (104 amino acids, MW about 11 kDa). Qualitative values of microdynamical parameters were obtained for 73 out of 85 amino acid side chains (glycine and alanine residues excepted) whereas more quantitative values were derived for 67 {beta}-CH and {beta}-CH2 groups.
Carrier relaxation dynamics in heavy fermion compounds
2002-01-01
The first femtosecond carrier relaxation dynamics studies in heavy fermion compounds are presented. The carrier relaxation time shows a dramatic hundred-fold increase below the Kondo temperature revealing a dramatic sensitivity to the electronic density of states near the Fermi level. Femtosecond time-resolved optical spectroscopy is an excellent experimental alternative to conventional spectroscopic methods that probe the low energy electronic structure in strongly correlated electron systems. In particular, it has been shown that carrier relaxation dynamics are very sensitive to changes in the low energy density of states (e.g. associated with the formation of a low energy gap or pseudogap) providing new insights into the low energy electronic structure in these materials. In this report we present the first studies of carrier relaxation dynamics in heavy fermion (HF) systems by means of femtosecond time-resolved optical spectroscopy. Our results show that the carrier relaxation dynamics, below the Kondo temperature (T{sub K}), are extremely sensitive to the low energy density of states (DOS) near the Ferini level to which localized f-moments contribute. Specifically, we have performed measurements of the photoinduced reflectivity {Delta}R/R dynamics as a function of temperature and excitation intensity on the series of HF compounds YbXCu{sub 4} (X = Ag, Cd, In) in comparison to their non-magnetic counterparts LuXCu{sub 4}.
Sb nuclear spin-lattice relaxation rate in filled skutterudite superconductor PrOs4Sb12
2007-01-01
Sb121,123 nuclear spin-lattice relaxation rate (1/T1)121,123 has been measured for the filled skutterudite superconductor PrOs4Sb12 with Tc=1.85K in order to probe the multipole fluctuations. Within the conventional analyses for NMR relaxation process, we found that 1/T1 for H parallel [001] is suppressed by magnetic fields. However, the ratio of (1/T1)121 to (1/T1)123 is close to 3.4, implying that the magnetic relaxation process is dominant under the magnetic field. These results cannot be explained by a simple magnetic relaxation mechanism due to the spin-dipolar relaxation process
Magnetic relaxation in analytical, coordination and bioinorganic chemistry
1982-06-01
Nuclear magnetic relaxation is a special type of nuclear magnetic resonance in which the rate is measured of energy transfer between the excited nuclei and their molecular medium (spin-lattice relaxation) or the whole nuclear spin system (spin-spin relaxation). Nuclear magnetic relaxation relates to nuclei with a spin of 1/2, primarily H/sub 1//sup 1/, and is mainly measured in water solutions. It is suitable for (1) analytical chemistry because the relaxation time rapidly reduces in the presence of paramagnetic ions, (2) the study of complex compounds, (3) the study of biochemical reactions in the presence of different metal ions. It is also suitable for testing the composition of a flowing liquid. Its disadvantage is that it requires complex and expensive equipment.
Magnetic relaxation of type II superconductors in a mixed state of entrapped and shielded flux
2004-01-01
The magnetic relaxation has been investigated in type II superconductors when the initial magnetic state is realized with entrapped and shielded flux (ESF) contemporarily. This flux state is produced by an inversion in the magnetic field ramp rate due to for example a magnetic field overshoot. The investigation has been faced both numerically and by measuring the magnetic relaxation in BSCCO tapes. Numerical computations have been performed in the case of an infinite thick strip and of an infinite slab, showing a quickly relaxing magnetization in the first seconds. As verified experimentally, the effects of the overshoot cannot be neglected simply by cutting the first 10-100 seconds in the magnetic relaxation. On the other hand, at very long times, the magnetic states relax toward those corresponding to field profiles with only shielded flux or only entrapped flux, depending on the amplitude of the field change with respect to the full penetration field of the considered superconducting samples. In addition, we have performed numerical simulations in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes; this allowed us to interpret correctly also the first seconds of the $M(t)$ curves.
Nuclear magnetic relaxation measurements on liquid acetonitrile and acetonitrile water mixtures
2000-07-01
Precise temperature dependent H, D, {sup 13}C and {sup 14}N relaxation measurements on acetonitrile in liquid acetonitrile and acetonitrile water mixtures are presented. H and, for the first time, D induced nuclear Overhauser enhancements on {sup 13}C are reported. Isotopic dilution experiments for the acetonitrile deuteron relaxation and relaxation measurements in the two phase region were performed. By a new method, the acetonitrile deuterium quadrupole coupling constant was determined to 162(6) kHz. An isotope effect of about 5% on the acetonitrile correlation time of the reorientation of the symmetry axis is observed. It decreases if the water content is increased. For the effective correlation time of the C-H/D vector the isotope effect is about 25%. This effect was separated into a contribution of 3% for only one molecule which is deuterated and 21% for the deuteration of the remaining molecules. The anisotropy of the reorientation of acetonitrile was calculated in the limit of rotational diffusion. In neat deuterated ammonia the reorientation about the symmetry axis is about ten times faster than the reorientation of the symmetry axis itself. Due to different activation energies the anisotropy increases with decreasing temperature. The isotope effect is also anisotropic and the rotational anisotropy in protonated acetonitrile is about fourteen. The anisotropy is higher in the binary mixtures. At the critical composition it is fourteen for the deuterated and twenty for the protonated sample. In the mixture of critical composition simultaneous relaxation measurements on both phases were possible. This allows a determination of the coexistence curve. No anomaly of the rates was observed on approaching the coexistence curve. (orig.)
On the shape of the gamma resonance spectra of slowly relaxing nanoparticles in a magnetic field
2010-01-01
A nonstandard shape of the gamma resonance spectra of nanoparticles in the form of inverted five-step pedestal has been predicted, observed, and analytically described. This shape corresponds to the limit of high temperatures and slow relaxation of the homogeneous magnetization of single-domain particles with axial magnetic anisotropy. To describe the Mssbauer spectra of the ensemble of chaotically oriented nanoparticles in a magnetic field, a continual magnetic-dynamics model has been developed in the limit of slow relaxation. This model adequately describes the polarization effects observed in the experimental absorption spectra. The revealed features significantly expand the methodical capabilities of Mssbauer spectroscopy for the diagnostics of magnetic nanomaterials.
2008-12-15
Stable colloidal systems composed of a double surfactant water-based magnetic fluid mixed with a cationic isotropic micellar solution of cetyletrymethyleammonium bromide (CTABr) are prepared for different concentrations of magnetic fluid. The structural integrity between nanomagnetic particles of magnetic fluids and soft micelles is investigated using the magneto-optical techniques like magnetic birefringence, transmission and relaxation measurements. Magneto-optically induced retardation and relaxation measurement indicates the existence of magnetically induced interactions in this mixed system. Similar behaviour is also observed in small angle neutron scattering experiments.
2008-01-01
Stable colloidal systems composed of a double surfactant water-based magnetic fluid mixed with a cationic isotropic micellar solution of cetyletrymethyleammonium bromide (CTABr) are prepared for different concentrations of magnetic fluid. The structural integrity between nanomagnetic particles of magnetic fluids and soft micelles is investigated using the magneto-optical techniques like magnetic birefringence, transmission and relaxation measurements. Magneto-optically induced retardation and relaxation measurement indicates the existence of magnetically induced interactions in this mixed system. Similar behaviour is also observed in small angle neutron scattering experiments
Anisotropy dependent magnetization relaxation in (Cd,Mn)Te quantum wells
2006-07-01
An optical study of magnetization relaxation is presented for p-doped quantum wells with magnetic ions. The magnetic anisotropy of the system is controlled by the heavy-light hole splitting, tuned by uniaxial strain. We show that a suppression of the magnetic anisotropy results in an enhancement of the observed relaxation rate by a factor of at least 20. This is explained by the fact that the decrease of the anisotropy results in the lowering of the energy barrier for the domain magnetization flip process. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Anisotropy dependent magnetization relaxation in (Cd,Mn)Te quantum wells
2006-01-01
An optical study of magnetization relaxation is presented for p-doped quantum wells with magnetic ions. The magnetic anisotropy of the system is controlled by the heavy-light hole splitting, tuned by uniaxial strain. We show that a suppression of the magnetic anisotropy results in an enhancement of the observed relaxation rate by a factor of at least 20. This is explained by the fact that the decrease of the anisotropy results in the lowering of the energy barrier for the domain magnetization flip process. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Anisotropy dependent magnetization relaxation in (Cd,Mn)Te quantum wells
2006-01-01
An optical study of magnetization relaxation is presented for p-doped quantum wells with magnetic ions. The magnetic anisotropy of the system is controlled by the heavy-light hole splitting, tuned by uniaxial strain. We show that a suppression of the magnetic anisotropy results in an enhancement of the observed relaxation rate by a factor of at least 20. This is explained by the fact that the decrease of the anisotropy results in the lowering of the energy barrier for the domain magnetization flip process. (Copyright 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Topological Constraints on Magnetic Relaxation
2010-07-29
The final state of turbulent magnetic relaxation in a reversed field pinch is well explained by Taylor's hypothesis. However, recent resistive-magnetohydrodynamic simulations of the relaxation of braided solar coronal loops have led to relaxed fields far from the Taylor state, despite the conservation of helicity. We point out the existence of an additional topological invariant in any flux tube with non-zero field: the topological degree of the field line mapping. We conjecture that this constrains the relaxation, explaining why only one of three example simulations reaches the Taylor state.
Thickness dependence of microstructure in LaCaMnO thin films
2000-03-01
Introducing biaxial strain in doped lanthanum manganite thin films is an excellent tool to modify the electrical and transport properties systematically by controlled changes of the microstructure on an atomic scale. We deposited La{sub 2/3}Ca{sub 1/3}MnO{sub 3} thin films onto SrTiO{sub 3} single-crystal substrates under reproducible conditions in an optimized pulsed laser deposition process. A thickness range of 40 to 500 nm was chosen for the manganite layers to investigate the variation of the average lattice parameters with thickness using X-ray diffraction. We re-investigated the samples after annealing to determine the structural aspects of the relaxation behavior of the layers. A second set of samples was prepared with an analogous in situ annealing process. AFM was applied to examine the morphology and grain size of the layers. Due to a thermally induced strain relaxation process the thicker films show a gradual approach of the out-of-plane lattice constant to the bulk ceramic value. Further relaxation could be achieved with an annealing at 900 deg. C for 1 h. Lattice parameter changes with varying thickness and annealing processes are correlated with transport and magnetic properties.
2009-01-01
The NMR spectrum and spin-lattice relaxation times, T1, of the 1H, 2H, and 7Li nuclei in LiNH4SO4 and LiND4SO4 crystals were obtained, and from these measurements, the rotational correlation times were calculated. In addition, the activation energies for the reorientation of 1H, 2H, and 7Li were obtained for each phase. For both types of crystal, each curve of T1 versus inverse temperature exhibited a minimum value, which is attributed to the effects of reorientational motion. From the 1H, 2H and 7Li T1 curves, we conclude that the relaxation process is affected by molecular motion, as described by the Bloembergen-Purcell-Pound theory. The differences in activation ...
Paramagnetism-Based Restraints for Xplor-NIH
2004-03-15
Modules that use paramagnetism-based NMR restraints have been developed and integrated in the well known program for solution structure determination Xplor-NIH; the complete set of such modules is called PARArestraints for Xplor-NIH. Paramagnetism-based restraints are paramagnetic relaxation enhancements, pseudocontact shifts, residual dipolar couplings due to metal and overall magnetic anisotropy, and cross correlation between Curie relaxation and nuclear-nuclear dipolar relaxation. The complete program has been tested by back-calculating NOEs and paramagnetism-based restraints from the X-ray structure of cytochrome c{sub 553} from B. pasteurii. Furthermore, the same experimental restraints previously used to determine the solution structure of cytochrome c{sub 553} itself, of cytochrome b{sub 5}, and of calbindin D{sub 9k} with the program PARAMAGNETIC DYANA, have been used for structure calculations by using PARArestraints for Xplor-NIH. The agreement between the two programs is quite satisfactory and validates both protocols.
Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems
2006-05-10
We study acoustic-phonon-induced relaxation of charge excitations in single and tunnel-coupled quantum dots containing few confined interacting electrons. The Full Configuration Interaction approach is used to account for the electron-electron repulsion. Electron-phonon interaction is accounted for through both deformation potential and piezoelectric field mechanisms. We show that electronic correlations generally reduce intradot and interdot transition rates with respect to corresponding single-electron transitions, but this effect is lessened by external magnetic fields. On the other hand, piezoelectric field scattering is found to become the dominant relaxation mechanism as the number of confined electrons increases. Previous proposals to strongly suppress electron-phonon coupling in properly designed single-electron quantum dots are shown to hold also in multi-electron devices. Our results indicate that few-electron orbital degrees of freedom are more stable than single-electron ones.
NMR of geophysical drill cores with a mobile Halbach scanner
2007-08-21
This thesis is devoted to a mobile NMR with an improved Halbach scanner. This is a lightweight tube-shaped magnet with sensitive volume larger and a homogeneity of the magnetic field higher than the previous prototype version. The improved Halbach scanner is used for analysis of water-saturated drill cores and plugs with diameters up to 60 mm. To provide the analysis, the standard 1D technique with the CPMG sequence as well as 2D correlation experiments were successfully applied and adapted to study properties of fluid-saturated sediments. Afterwards the Halbach scanner was calibrated to fast non-destructive measurements of porosity, relaxation time distributions, and estimation of permeability. These properties can be calculated directly from the NMR data using the developed methodology. Any independent measurements of these properties with other methods are not needed. One of the main results of this work is the development of a new NMR on-line core scanner for measurements of porosity in long cylindrical and semi cylindrical drill cores. Also dedicated software was written to operate the NMR on-line core scanner. The physical background of this work is the study of the diffusion influence on transverse relaxation. The diffusion effect in the presence of internal gradients in porous media was probed by 1D and 2D experiments. The transverse relaxation time distributions obtained from 1D and from 2D experiments are comparable but different in fine details. Two new methodologies were developed based on the results of this study. First is the methodology quantifying the influence of diffusion in the internal gradients of water-saturated sediments on transverse relaxation from 2D correlation experiments. The second one is the correction of the permeability estimation from the NMR data taking in account the influence of the diffusion. Furthermore, PFG NMR technique was used to study restricted diffusion in the same kind of samples. Preliminary results are reported. (orig.)
Nonclassical correlation in NMR quadrupolar systems
2010-04-02
The existence of quantum correlation (as the one revealed by the quantum discord) other than entanglement and its role in quantum information processing (QIP) is a current subject of discussion. Particularly, it has been suggested that this nonclassical correlation may provide the computational speedup for some quantum algorithms. In this sense bulk nuclear magnetic resonance (NMR) has been successfully used as a test bench for many QIP implementations, although it has also been continuously criticized about not presenting entanglement in most of the systems used so far. In this article we report a theoretical and experimental study on the dynamics of quantum and classical correlations in a NMR quadrupolar system. We present a method for computing the correlations from experimental NMR deviation-density matrices and show that, given the action of the nuclear spin environment, the relaxation produces a monotonic time decay in the correlations. Although the experimental realizations were done in a specific quadrupolar system, the main results presented here can be applied to whichever system using a deviation-density matrix formalism.
muSR studies of magnetic properties of CeRhSb and La0.1Ce0.9RhSb
1997-01-01
Zero, longitudinal, and transverse field muSR data are reported for CeRhSband La0.1Ce0.9RhSbdown to 45 mK. The muon spin relaxation at temperatures above a few Kelvin is caused by nuclear dipoles. Only at lower temperatures the additional depolarizing effect of electronic moments is observable, signalizing the onset of magnetic correlations in the 4f moment system. These findings are compared to the corresponding data on CeNiSn, showing that the ground state properties are basically alike in the two materials, that is, magnetic correlations develop, but remain weakly dynamic and long range antiferromagnetic order does not set in. The results for La0.1Ce0.9RhSbdiffer little from those of pure CeRhSb, indicating that the magnetic ground state properties as seen by muSR have not been altered ...
2001-07-01
Magnetic liposomes containing submicron-sized ferromagnetic particles were prepared encapsulating the muscle relaxant drugs, diadony or diperony, for local anesthesia. Alternatively, metal phthalocyanines (Photosense or Teraphthal), sensitizers for photodynamic or catalytic cancer therapy were loaded into the magnetic liposomes. Animal trials demonstrated successful magnetically guided transport of the drug-loaded liposomes.
2010-01-01
Nuclear magnetic resonance T 1 and T 2 relaxivities (r 1 and r 2) exhibit efficiency of a material to alter the relaxation rates (1/T 1 and 1/T 2), and they are being used for diagnostic purposes. The determination of total relaxivities (r 1t and r 2t) of cystic fluid content and ameloblastoma may therefore be useful for discriminative purposes. In order to determine what makes total relaxivities of hemorrhagic cysts, four sets of tubes containing pooled cyst were doped with increasing concentrations of iron, copper, albumin, and -globulins. These sets were replaced in a phantom together with six individual cysts and one ameloblastoma. The relaxation times were measured by magnetic resonance imaging operating at 1.5T. The relaxivities of individual ions and proteins were determined from ...
Electron-nuclear relaxation times (T(1) sup e's) for (15)N and (13)C in natural abundance are measured for a series of amines of a wide range of pK(a)s using four paramagnetic relaxation reagents that are soluable in organic solutions. Cr(acac)3 and Cr(dpm)3 are seen to affect th...
Introduction: dynamic phenomena in NMR
1994-12-31
An introduction to NMR spectroscopy is given, with a special emphasis on dynamical aspects: general principles of nuclear magnetic resonance (the Zeeman interaction, chemical shift, scalar coupling, the Larmor precession and the rotating frame, thermal equilibrium and longitudinal relaxation, free induction decay and transverse relaxation, Bloch equations); relaxation times as a source of dynamical information (longitudinal and transverse relaxation); molecular rate processes; domains of application. 4 fig., 14 ref.
2005-08-11
We present a theoretical analysis of several aspects of nonequilibirum cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a finite magnetic field in the weak coupling limit. We carry this out by developing a generic quantum Heisenberg-Langevin equation approach leading to a set of Bloch dynamical equations which describe the nonequilibrium cotunneling in a convenient and compact way. These equations describe the time evolution of the spin variables of the QD explicitly in terms of the response and correlation functions of the free reservoir variables. This scheme not only provides analytical expressions for the relaxation and decoherence of the localized spin induced by cotunneling, but it also facilitates evaluations of the nonequilibrium magnetization, the charge current, and the spin current at arbitrary bias-voltage, magnetic field, and temperature. We find that all cotunneling events produce decoherence, but relaxation stems only from {\\em inelastic} spin-flip cotunneling processes. Moreover, our specific calculations show that cotunneling processes involving electron transfer (both spin-flip and non-spin-flip) contribute to charge current, while spin-flip cotunneling processes are required to produce a net spin current in the asymmetric coupling case. We also point out that under the influence of a nonzero magnetic field, spin-flip cotunneling is an energy-consuming process requiring a sufficiently strong external bias-voltage for activation, explaining the behavior of differential conductance at low temperature: in particular, the splitting of the zero-bias anomaly in the charge current and a broad zero-magnitude "window" of differential conductance for the spin current near zero-bias-voltage.
2005-03-01
The purpose of this paper is to perform quantitative measurements of the magnetization transfer rate (Kfor) and native T1 relaxation time (T1free) in the brain tissue of normal individuals and patients with multiple sclerosis (MS) by means of multiple gradient echo acquisitions, and to correlate these measurements with the magnetization transfer ratio (MTR). Quantitative magnetization transfer imaging was performed in five normal volunteers and 12 patients with relapsing-remitting MS on a 1.5 T magnetic resonance (MR) scanner. The T1 relaxation time under magnetization transfer irradiation (T1sat) was calculated by means of fitting the signal intensity over the flip angle in several 3D spoiled gradient echo acquisitions (3 , 15 , 30 , and 60 ), while a single acquisition without MT irradiation (flip angle of 3 ) was utilized to calculate the MTR. The Kfor and T1free constants were quantified on a pixel-by-pixel basis and parametric maps were reconstructed. We performed 226 measurements of Kfor, T1free, and the MTR on normal white matter (NWM) of healthy volunteers (n=50), and normal-appearing white matter (NAWM) and pathological brain areas of MS patients (n=120 and 56, respectively). Correlation coefficients between Kfor-MTR, T1free-MTR, and T1free-Kfor were calculated. Lesions were classified, according to their characteristics on T1-weighted images, into isointense (compared to white matter), mildly hypointense (showing signal intensity lower than white matter and higher than gray matter), and severely hypointense (revealing signal intensity lower than gray matter). ''Dirty'' white matter (DWM) corresponded to areas with diffused high signal, as identified on T2-weighted images. Strong correlation coefficients were obtained between MTR and Kfor for all lesions studied (r{sup 2}=0.9, p<0.0001), for mildly hypointense plaques (r{sup 2}=0.82, p<0.0001), and for DWM (r{sup 2}=0.78, p=0.0007). In contrast, comparison between MTR and T1free values yielded rather low correlation coefficients for all groups assessed. In severely hypointense lesions, an excellent correlation was found between Kfor and T1free measurements (r{sup 2}=0.98, p<0.0001). Strong correlations between Kfor and T1free were found for the rest of the subgroups, except for the NAWM, in which a moderate correlation was obtained (r{sup 2}=0.5, p<0.0001). We conclude that Kfor and T1free measurements are feasible and may improve our understanding of the pathological brain changes that occur in MS patients. (orig.)
2003-06-10
Zero-field muon-spin-relaxation (muSR) and magnetic-susceptibility measurements have been carried out in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with x = 0.10, 0.115 and 0.13 changing the Zn-concentration y finely up to 0.10, with the aim to clarify effects of the nonmagnetic impurity Zn on the Cu-spin dynamics and superconductivity. The muSR measurements have revealed that, in each x, a magnetic order of Cu spins is observed at y ~ 0.0075, while it disappears and Cu spins turn into a fast fluctuating state for y > 0.03. From the magnetic-susceptibility measurements, on the other hand, it has been found that the volume fraction of the superconducting state rapidly decreases through the slight doping of Zn and that its y dependence corresponds to the y dependence of the volume fraction of the fast fluctuating region of Cu spins estimated from the muSR results. Both the rapid decrease of the volume fraction of the superconducting state and the formation of the magnetic order through the slight doping of Zn can be interpreted in terms of the development of the so-called "Swiss cheese" model. That is, it is concluded that Cu spins in a non-superconducting region around Zn exhibit slowing-down of the fluctuations or form an incoherent or coherent static magnetic order. Moreover, the formation of a non-superconducting region around Zn is considered to be due to the pinning of the dynamical spin correlation or dynamical stripe correlations.
2010-01-01
Sludge from a slaughter-house wastewater plant, and mixtures of bulking agent (crushed wood pallet) and sludge were studied by Nuclear Magnetic Resonance (NMR). The NMR spin-spin relaxation (T2) and spin-lattice relaxation (T1) signals for sludge, wet crushed wood pallet and mixtures of sludge and bulking agent were decomposed into three relaxation time components. Each relaxation time component was explained by a non-homogeneous water distribution on a microscopic length scale and by the porosity of the material. For all samples, the T2 relaxation time value of each component was directly related to the dry matter content. The addition of wet crushed wood to sludge induced a decrease in the relaxation time, explained by water transfer between the sludge and the wood. Magnetic Resonance Imaging (MRI) and respirometric measurements ...
Cross-relaxation of 8Li+ in copper
2009-01-01
The 'cross-relaxation' method (also called 'level-crossing resonance' or 'avoided level-crossing') is a powerful technique that can be used to provide detailed structural and site information on impurities in materials. In this paper, we report on the development of the cross-relaxation technique for 8Li+ at the beta-detected nuclear magnetic resonance (beta-NMR) facility located in TRIUMF in Vancouver. The measurements were carried out on a Cu single crystal where the relaxation rate of the 8Li polarization was monitored as a function of the applied longitudinal magnetic field. The occurrence of cross-relaxation, at a particular magnetic field, between the 8Li and the surrounding nuclei is evident as a resonant enhancement of the relaxation rate at the level crossing field. We discuss ...
2005-02-03
The effect of the transverse AC magnetic field on relaxation process in YBa$_2$Cu$_3$O$_x$ melt-textured superconductor was studied. A factor of 50 suppression of the relaxation rate could be achieved at the expense of some reduction in the maximum trapped field, with the magnetic-induction gradient being unchanged. This phenomenon is interpreted as a result of an increase of the pinning force after the action of the transverse AC magnetic field that is confirmed by the measurement of the trapped-induction distribution.
Fast multidimensional NMR by polarization sharing
2007-01-01
The speed of multidimensional NMR spectroscopy can be significantly increased by drastically shortening the customary relaxation delay between scans. The consequent loss of longitudinal magnetization can be retrieved if `new' polarization is transferred from nearby spins. For correlation spectroscopy involving heteronuclei (X = 13C or 15N), protons not directly bound to X can repeatedly transfer polarization to the directly bound protons through Hartmann-Hahn mixing. An order of magnitude increase in speed has been observed for the 600 MHz two-dimensional HMQC spectra of amikacin and strychnine using this technique, and it also reduces the noisy F1 ridges that degrade many heteronuclear correlation spectra recorded with short recovery times. Copyright Copyright 2007 John Wiley & Sons, Ltd.
MRI of estrogen-induced rat pituitary adenoma; Analysis of the proton relaxation time
1992-01-01
Magnetic resonance imaging (MRI) has an advantage for diagnosis of pituitary adenomas because of an avoidance from artifacts from the skull base in comparison with computed tomography. However intensity of pituitary adenomas is variable. Factors or reasons of the variability are unknown. We studied MRI of estrogen-induced rat pituitary adenomas, and analyzed their proton relaxation time comparing with their histological features. Pituitary adenomas were induced in 34 female Wister rats by injection of 2.5 mg estradiol dipropiate every week. The animals were studied by MRI (CXP 20tb, resistive magnet, 0.477 T) with IR and CPMG sequence by midsagittal section at various terms of estrogen administration (2-22 weeks). Proton relaxation times (T{sub 1} and T{sub 2}) were measured at regions of interest on the pituitary glands. After the MR study, the pituitary glands were sampled and evaluated histologically. Weight of all the hypophyses increased from 25 mg to 295 mg, while normal weight was 11{approx}14 mg. Pituitary weight and duration of estrogen administration correlated significantly. On the MR T{sub 1}-weighted images every enlarged pituitary gland was clearly recognized as a high-intensity mass. The calculated T{sub 1} values had negative linear correlation with the terms of administration, and had a higher correlation coefficient with the weight of the adenomas. Though the causes of this shortening of T{sub 1} is not obvious, it is considered that several factors like blood flow, intratumoral bleeding, peptide and organells in the adenoma cells may participate in this fact. (author).
Spin dynamics in 3d electron pyrochlore-like systems
2009-01-01
1H NMR measurements of 3d electron three-dimensional frustrated systems, Co2(OH)3Cl with s = 3/2 and Cu2(OH)3Cl with s = 1/2 have been performed. The relaxation rates T-11 of both compounds suggest the development of spin correlations at much higher temperatures compared with the magnetic transition temperatures. The temperature dependence of T-11 below 6 K of both compounds indicates the magnetic ordering and the existence of magnon excitations. The estimated magnon energy gaps are 13 K and 27 K for Cu2(OH)3Cl and Co2(OH)3Cl, respectively, which are consistent with the inelastic neutron experiments. The intermediate phase between TN1 = 18.1 K and TN2 = 6.4 ...
2000-02-10
In this communication we consider the doping dependence of the strongantiferromagnetic spin fluctuations in the cuprate superconductors. Weinvestigate the effect of an incommensurate magnetic response, as recentlyobserved in inelastic neutron scattering (INS) experiments on severalYBa_2Cu_3O_{6+x} compounds, on the spin-lattice and spin-echo relaxation ratesmeasured in nuclear magnetic resonance (NMR) experiments. We conclude that aconsistent theoretical description of INS and NMR can be reached if one assumesspatially inhomogeneous but locally commensurate spin correlations and that NMRand INS experiments can be described within a single theoretical scenario. Wediscuss a simple scenario of spin and charge inhomogeneities which includes themain physical ingredients required for consistency with experiments.
Nuclear magnetic resonance imaging of induced renal lesions
Nuclear magnetic resonance (NMR) images obtained after unilateral ligation of the ureter, renal artery, or renal vein in the rat were analyzed and compared with NMR images of the normal rat kidney. Anatomic and functional correlation of the induced renal lesions was made by concurrent CT and by gross examination of the excised kidneys. Many normal anatomic structures at the level of the renal hilum can be identified by high resolution NMR imaging. Differentiation of urine from renal parenchyma permits detection of gross changes both in renal function and in the mass of the renal parenchyma. NMR imaging is capable of diagnosing hydronephrosis, acute renal ischemia, and acute venous congestion in this rat model. In addition, a trend toward prolongation of the relaxation times T1 and T2 for abnormal renal parenchyma is demonstrated.
Nuclear magnetic resonance imaging of induced renal lesions
1983-07-01
Nuclear magnetic resonance (NMR) images obtained after unilateral ligation of the ureter, renal artery, or renal vein in the rat were analyzed and compared with NMR images of the normal rat kidney. Anatomic and functional correlation of the induced renal lesions was made by concurrent CT and by gross examination of the excised kidneys. Many normal anatomic structures at the level of the renal hilum can be identified by high resolution NMR imaging. Differentiation of urine from renal parenchyma permits detection of gross changes both in renal function and in the mass of the renal parenchyma. NMR imaging is capable of diagnosing hydronephrosis, acute renal ischemia, and acute venous congestion in this rat model. In addition, a trend toward prolongation of the relaxation times T1 and T2 for abnormal renal parenchyma is demonstrated.
NMR detection of thermal damage in carbon fiber reinforced epoxy resins.
Composite materials of epoxy resins reinforced by carbon fibers are increasingly being used in the construction of aircraft. In these applications, the material may be thermally damaged and weakened by jet blast and accidental fires. The feasibility of using proton NMR relaxation times T1, T1rho, and T2 to detect and quantify the thermal damage is investigated. In conventional spectrometers with homogeneous static magnetic fields, T1rho is readily measured and is found to be well correlated with thermal damage. This suggests that NMR measurements of proton T1rho may be used for non-destructive evaluation of carbon fiber-epoxy composites. Results from T1rho measurements in the inhomogeneous static and RF magnetic fields of an NMR-MOUSE are also discussed.
Magnetic resonance imaging of splenic iron overload
The value of magnetic resonance (MR) imaging in assessing iron overload in the spleen was retrospectively investigated in 40 consecutive patients. MR appearance, mesaure of signal intensity and T1-and T2-relaxation times were correlated with the histologically determined level of iron in the spleen in each patient. Histologic examination revealed no iron overload in 19 patients, mild iron overload in seven, moderate iron overload in six, and severe iron overload in eight. All 19 patients with no splenic iron overload and 11 of the other 21 patients with splenic iron overload were correctly identified by MR imaging (sensitivity 52%, specificity 100%, accuracy 75%). Splenic iron overload was diagnosed when a decrease of signal intensity of the spleen compared with those of adipose tissue and renal cortex was demonstrated. MR images demonstrated all eight cases of severe, three of the six cases of moderate, and none of the seven cases of mild iron overload. Only spleens with severe iron overload had a significant mean decrease in signal intensity and T1- and T2-relaxation times. Although specific, MR imaging is poorly sensitive to splenic iron overload. (author). 15 refs.; 5 figs.; 3 tabs.
1995-12-31
In this work the spin-spin and spin-lattice relaxation processes for the nuclear spin I in the presence of dipole interaction with magnetic balls with spin S are considered. The relaxation time has been calculated by means of computerized simulations. 8 refs, 3 figs.
NMR relaxation in the magnetic balls system; Relaksacja MRJ w ukladzie kul magnetycznych
1994-12-31
The mathematical model of nucleon spin relaxation time in the presence of dipole-dipole magnetic interaction is presented. The relaxation times as a temperature function are calculated using an expansion into spherical harmonics series. Results of calculations are presented. 9 refs, 4 figs.
1994-12-31
The process of water bounding in clam shells has been investigated by nuclear magnetic relaxation of protons measurements. Decomposition of measured relaxation functions have been done using the specially constructed computer program. 4 refs, 4 figs, 3 tabs.
1982-01-01
The renormalization group techniques developed by Wilson for the Kondo problem are applied, for the first time, to the calculation of nuclear spin relaxation rates in dilute magnetic alloys. A procedure that calculates the longitudinal relaxation time T1 over the entire temperature range 0 0. A numerical approach, capable of calculating T1 at finite temperatures, is ...
A Linear Tetranuclear Dysprosium(III) Compound Showing Single-Molecule Magnet Behavior
Although magnetic measurements reveal a single-relaxation time for a linear tetranuclear Dy(III) compound, the wide distribution of the relaxation time observed clearly suggests the presence of two slightly different anisotropic centres, therefore opening new avenues for investigating the relaxation dynamics of lanthanide aggregates.
A Linear Tetranuclear Dysprosium(III) Compound Showing Single-Molecule Magnet Behavior
2010-04-20
Although magnetic measurements reveal a single-relaxation time for a linear tetranuclear Dy(III) compound, the wide distribution of the relaxation time observed clearly suggests the presence of two slightly different anisotropic centres, therefore opening new avenues for investigating the relaxation dynamics of lanthanide aggregates.
Muon-spin relaxation in AuFe and CuMn spin glasses
1985-01-01
Zero- and longitudinal-field muon-spin relaxation (ZF ..mu..SR and LF ..mu..SR) measurements have been applied to dilute-alloy spin glasses AuFe(1.0 and 1.4 at %) and CuMn(1.1, 3, and 5 at. %). A stochastic theory of muon-spin relaxation has been formulated, and an analytic form of zero-field (ZF) muon-spin-relaxation function has been derived to describe the depolarization of muon spins in static, dynamic, and coexisting static and dynamic random local fields from magnetic impurities. The observed experimental results for the muon-spin-relaxation function G/sub z/(t) in all the specimens have been well explained by the model functions based on the present theory. Depolarization of muon spins due to the rapidly fluctuating dynamic local field has been indicated by the ''root-exponential'' shape of G/sub z/(t) observed above the susceptibility-cusp temperature T/sub g/. The spin correlation time tau/sub c/ of Mn (or Fe) moment, deduced from the dynamic depolarization rate of muon spins, shows a rapid change from tau/sub c/approx.10/sup -12/ sec at T> or approx. =2T/sub g/ towards tau/sub c/approx.10/sup -9/ sec at Tapprox.T/sub g/ in all the specimens. Coexistence of static and dynamic random fields at each muon site has been found by ZF and LF ..mu..SR below T/sub g/.
The influence of measurement and relaxation time on flux jumps in high temperature superconductors
2010-01-01
The influence of the magnetization and relaxation time on flux jumps in high temperature superconductors (HTSC) under varying magnetic field is studied using the fundamental electromagnetic field equations and the thermal diffusion equation. Temperature variety corresponding to flux jump is also discussed. We find that for a low sweep rate of the applied magnetic field, the measurement and relaxation times can reduce flux jump and to constrain the number of flux jumps, even stabilizing the HTSC, since much heat produced by the motion of magnetic flux can transfer into coolant during the measurement and relaxation times. As high temperature superconductors are subjected to a high sweep rate or a strong pulsed magnetic field, magnetization undergoes from stability or oscillation to jump for different pause times. And the period of temperature oscillation is equal to the ...
The influence of measurement and relaxation time on flux jumps in high temperature superconductors
2010-01-01
The influence of the magnetization and relaxation time on flux jumps in high temperature superconductors (HTSC) under varying magnetic field is studied using the fundamental electromagnetic field equations and the thermal diffusion equation; temperature variety corresponding to flux jump is also discussed. We find that for a low sweep rate of the applied magnetic field, the measurement and relaxation times can reduce flux jump and to constrain the number of flux jumps, even stabilizing the HTSC, since much heat produced by the motion of magnetic flux can transfer into coolant during the measurement and relaxation times. As high temperature superconductors are subjected to a high sweep rate or a strong pulsed magnetic field, magnetization undergoes from stability or oscillation to jump for ...
Magnetism, spin-wave relaxation and spiral exchange in a trilayer magnetic junction
2006-12-07
We study the non-collinear exchange coupling across a trilayer magnetic junction consisting of two ferromagnets separated by a thin dilute magnetic semiconductor containing itinerant carriers with finite spin relaxation. It is remarkable that, by increasing the spin relaxation, the critical temperature is substantially enhanced and the shape of the magnetization curve becomes more mean-field like. We attribute these interesting changes to the broken time-reversal symmetry which suppresses the oscillatory Ruderman-Kittel-Kasuya-Yosida interaction. Our argument is further strengthened by the emergence of the non-collinear spiral exchange coupling across the trilayer magnetic junction with finite spin relaxation.
Comment on "Magnetoviscosity and relaxation in ferrofluids"
2001-06-20
It is shown and discussed how the conventional system of hydrodynamic equations for ferrofluids was derived. The set consists of the equation of fluid motion, the Maxwell equations, and the magnetization equation. The latter was recently revised by Felderhof [Phys. Rev. E, v.62, p.3848 (2000)]. His phenomenological magnetization equation looks rather like corresponding Shliomis' equation, but leads to wrong consequences for the dependence of ferrofluid viscosity and magnetization relaxation time on magnetic field.
Magnetic correlations in high temperature superconductivity
1994-12-01
Many experimental findings in the materials family of cuprate superconductors are converging towards an electronic picture which involves strong local Coulomb correlations. Characteristic normal state properties appear to be beyond the conventional framework of Landau's Fermi liquid theory. Spin correlations among the charge carriers are discussed as a possible source for the intricate electronic properties. The low energy physics is determined by the charge and spin dynamics in CuO[sub 2] layers which all cuprate superconductors have in common despite their otherwise complex composition. Strong short range Coulomb repulsion between the charge carriers in these planar units imply magnetic ordering phenomena among the Cu spins. Depending on the doping controlled carrier concentration the cuprate compounds are either antiferromagnetic (AF) insulators or correlated metals with short range spin correlations extending also into the superconducting state.The purpose of this article is twofold: In the first part we review results of basic experiments which probe the magnetic correlations in the CuO[sub 2] planes. Particular emphasis is given on neutron and Raman scattering, nuclear magnetic relaxation, and a selected set of experiments which contain information about the nature of the superconducting state. Based on these experiments the evolution of the spin dynamics from the AF insulator into the doped metallic regime will be discussed. On the other hand we give an overview of current phenomenological and microscopic approaches in the context of purely electronic Hubbard type models which focus on the role of AF spin fluctuations. This includes the results for single particle properties as well as for the dynamic spin susceptibility. Special attention is given to the discussion of spin fluctuation exchange as the possible mechanism underlying high temperature superconductivity. ((orig.))
Spin-frustrated antiferromagnets based on BEDT-TTF and manganese dicyanamide complexes
The magnetic properties of new radical cation salts ( BEDT-TTF)2[CuMn( dca)4] ( I) and ( BEDT-TTF)2[Mn( dca)3] ( II) [where BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene and dca = N(CN2)] are investigated using superconducting quantum interference device (SQUID) magnetometry and electron paramagnetic resonance (EPR) spectroscopy. It is established that, at temperatures below 25 K, both salts are characterized by antiferromagnetic deviations from the paramagnetic behavior. The Weiss constants for compounds I and II are determined to be ?5 and ?10 K, respectively. The corresponding correlations in the structure of compound I are short-range correlations and do not lead to a change in the effective spin equal to 5/2. It is found that the widths of the EPR lines attributed to the BEDT-TTF conducting sublattice correlate with the widths of the EPR lines associated with the magnetic sublattice of the Mn( dca){3/?} counterion in the structure of salt II. This correlation suggests that the antiferromagnetic ordering in the magnetic sublattice of compound II affects the spin-lattice relaxation in the BEDT-TTF sublattice. The dependence of the magnetic moment on the magnetic field for compound II at a temperature of 2 K is typical of weakly frustrated uniaxial antiferromagnets and exhibits a kink in a magnetic field of 20 kOe, which corresponds to spin-flop transitions.
Magnetic resonance on correlated semimetals: the case of U2Ru2Sn, CeRu4Sn6 and FeSb2
2009-01-01
A comparative magnetic resonance study on correlated semiconductors is presented. Nuclear spin-lattice relaxation measurements provide an excellent method to gain local information about the gap formation in this new class of materials. In contrast to U2Ru2Sn (DELTA/kB approx 230 K) in CeRu4Sn6 the gap is slightly reduced (DELTA/kB approx 200 K) and correlations form out of a residual density of states in the gap. For FeSb2 there is revived interest after classifying this system as the second Fe containing Kondo insulator beside FeSi. Surprisingly, FeSb2 shows a colossal Seebeck coefficient at low temperatures. Using 121,123Sb nuclei as a local probe, our NMR/NQR investigations strongly support the gap scenario. The spin-lattice ...
Electronic correlations and non-Fermi-liquid behaviour in ACu{sub 3}Ru{sub 4}O{sub 12}-perovskites
2009-07-01
Among the large variety of AC{sub 3}B{sub 4}O{sub 12}-type perovskites, the copper-ruthenates (C=Cu, B=Ru) form a subclass exhibiting strongly correlated electron behaviour. A special feature of these compounds is that the ideal stoichiometry and structure is preserved for a large number of different A-site cations with different valence states. We have systematically studied the electronic properties by specific heat, magnetic susceptibility, transport, NMR/NQR and neutron scattering experiments. The compound A=Ca is a correlated metal showing non-Fermi-liquid behaviour below 2 K, as indicated by a logarithmic increase of the specific heat and deviations from a Korringa behaviour of the spin lattice relaxation rate. In the case of A=Pr, an anomaly in the specific heat occurs at 0.5 K whose maximum shifts to higher temperatures with increasing magnetic field. Along with data acquired from inelastic neutron scattering we obtain strong indications for a pronounced crystal field splitting.
The novel heavy-fermion system Nd{sub 2-x}Ce{sub x}CuO{sub 4}
1997-04-01
Inelastic neutron scattering experiments are reported in the heavy fermion state of Nd{sub 2-x}Ce{sub x}CuO{sub 4}. A complex magnetic response has been observed in zero field that can be divided into contributions from correlated Nd spins (inelastic, q-dependent; reminiscent of the spin waves in Nd{sub 2}CuO{sub 4}) and from independent, slowly relaxing Nd spins (quasi-elastic, q-independent). An applied magnetic field of H > 3 Tesla gives rise to different correlations in Q - {omega} space than in zero field. Field dependent specific heat and {mu}SR experiments can be better understood in the light of these INS results. The experiments were performed on a single crystal at T {<=} 0.1 K with applied magnetic fields of H = 0 - 6 Tesla at the IN14 spectrometer. (author). 6 refs.
Sliding conductivity of a magnetic kink crystal in a chiral helimagnet
We derive a current-driven sliding conductivity of the magnetic kink crystal (MKC) in chiral helimagnet under weak magnetic field applied perpendicular to the helical axis. For this purpose, we discuss the correlated dynamics of quantum-mechanical itinerant spins and the MKC which are coupled via the sd exchange interaction. The itinerant spins are treated as fully quantum-mechanical operators whereas the dynamics of the MKC is considered within classical Lagrangian formalism. By appropriately treating elementary excitations around the MKC state, we construct coupled equations of motion for the collective coordinates (the center-of-mass position and quasi-zero-mode coordinate) associated with the sliding motion of the MKC. By solving them, we demonstrate that the correlated dynamics is understood through a hierarchy of two time scales: Boltzmann relaxation time ?el , when a nonadiabatic spin-transfer torque appears, and Gilbert damping time ?MKC , when adiabatic spin-transfer torque comes up. As a notable consequence, we found that the terminal velocity of the sliding motion reverses its sign depending on the band-filling ratio of the conduction electron system.
NMR (Nuclear Magnetic Resonance) of solid biopolymers
It has been possible through the study of proton relaxation in solid proteins and related molecules to identify and characterize the following molecular motions which contribute to relaxation: (1) methyl group reorientation; ...
Origin of the magnetic-field dependence of the nuclear spin-lattice relaxation in iron
The magnetic-field dependence of the nuclear spin-lattice relaxation at Ir impurities in Fe was measured for fields between 0 and 2 T parallel to the [100] direction. The reliability of the applied technique of nuclear magnetic resonance on oriented nuclei was demonstrated by measurements at different radio-frequency (rf) field strengths. The interpretation of the relaxation curves, which used transition rates to describe the excitation of the nuclear spins by a frequency-modulated rf field, was confirmed by model calculations. The magnetic-field dependence of the so-called enhancement factor for rf fields, which is closely related to the magnetic-field dependence of the spin-lattice relaxation, was also measured. For several magnetic-field-dependent relaxation mechanisms, the form and the magnitude of the field dependence were derived. Only the relaxation via eddy-current damping and Gilbert damping could explain the observed field dependence. Using reasonable values of the damping parameters, the field dependence could perfectly be described. This relaxation mechanism is, therefore, identified as the origin of the magnetic-field dependence of the spin-lattice relaxation in Fe. The detailed theory, as well as an approximate expression, is derived, and the dependences on the wave vector, the resonance frequency, the conductivity, the temperature, and the surface conditions are discussed. The theory is related to previous attempts to understand the field dependence of the relaxation, and it is used to reinterpret previous relaxation experiments in Fe. Moreover, it is predicted that the field dependences of the relaxation in Fe and Co, on one side, and in Ni, on the other side, differ substantially, and it is suggested that the literature values of the high-field limits of the relaxation constants in Fe are slightly too large.
On the correlation between fragility and stretching in glassforming liquids
2006-11-09
We study the pressure and temperature dependences of the dielectric relaxation of two molecular glassforming liquids, dibutyl phtalate and m-toluidine. We focus on two characteristics of the slowing down of relaxation, the fragility associated with the temperature dependence and the stretching characterizing the relaxation function. We combine our data with data from the literature to revisit the proposed correlation between these two quantities. We do this in light of constraints that we suggest to put on the search for empirical correlations among properties of glassformers. In particular, argue that a meaningful correlation is to be looked for between stretching and isochoric fragility, as both seem to be constant under isochronic conditions and thereby reflect the intrinsic effect of temperature.
Intracellular heating of living cells through Nel relaxation of magnetic nanoparticles.
Maghemite and cobalt ferrite anionic magnetic nanoparticles enter tumor cells and can be used as heat sources when exposed to a high-frequency magnetic field. Comparative studies of the two particles enable to unravel the magnetic heating mechanisms (Nel relaxation vs. Brown relaxation) responsible for the cellular temperature rise, and also to establish a simple model, adjusted to the experimental results, allowing to predict the intracellular heating efficiency of iron oxide nanoparticles. Hence, we are able to derive the best nanoparticle design for a given material with a view to intracellular hyperthermia-based applications.
A correlation for miscible flood displacement efficiency in the San Andres with NMR relaxation
1992-01-01
This paper reports that a set of San Andres crystalline dolomite cores was examined for the relationship between pore attributes discernible from NMR relaxation and miscible flooding displacement efficiency. A strong correlation was observed between the inferred abundance of long relaxation time components and laboratory measured residual oil to a multicontact miscible displacement process. NMR relaxation time distribution proved to be a simple, noninvasive, nondestructive indicator of flow performance in the 12 San Andres dolomite samples examined. Spin-lattice relaxation measurements were conducted on clean, brine saturated core end pieces from plugs taken adjacent to those used for flow displacement studies. Sample data were deconvoluted into a distribution of relaxation times, which is ultimately related to surface-to-volume ratio distribution. The ...
Damping of micromechanical structures by paramagnetic relaxation
2003-03-23
We find that the damping of micromechanical cantilevers is sensitive to the relaxation dynamics of paramagnetic ions contained within the levers. We measure cantilevers containing paramagnetic Mn ions as a function of temperature, magnetic field, and the vibrational mode of the lever and find that the levers damping is strongly enhanced by the interplay between the motion of the lever, the ions magnetic anisotropy, and the ratio of the ions longitudinal relaxation rate to the resonance frequency of the cantilever. This enhancement can improve the levers ability to probe the relaxation behavior of paramagnetic or superparamagetic systems; it may also represent a previously unrecognized source of intrinsic dissipation in micromechanical structures.
Bose condensation and relaxation explosion in magnetically trapped atomic hydrogen
1993-11-01
We predict and analyze nontrivial relaxational behavior of magnetically trapped gases near the Bose-condensation temperature [ital T][sub [ital c. Due to strong compression of the condensate by the inhomogeneous trapping field, particularly at low densities, the relaxation rate shows a strong, almost jumplike, increase below [ital T][sub [ital c]]. As a consequence the maximum fraction of condensate particles is limited to a few percent. This phenomenon can be called a relaxation explosion.'' We discuss its implication for the detectability of Bose-Einstein condensation in atomic hydrogen.]]
Collective Spin and Charge Excitations in (Sr,La)_{14-x}Ca_xCu_{24}O_{41} Quantum Spin Ladders
2005-10-07
We study magnetic and electronic properties of two-leg ladder materials. We observed a two-magnon (2M) resonance which we analyze in terms of symmetry, relaxation and resonance properties. Our findings were contrasted to 2M Raman measurements in other magnetic crystals. This comparison made us suggest that the spin-spin correlations in a self-doped two leg ladder may have a modulated component besides the exponential decay characteristic of a spin liquid ground state. We found that the 2M intensity resonates at the Mott gap energy. Interplane Sr substitution for Ca introduces strong disorder leading to inhomogeneous broadening of the 2M resonance. The doped holes in the spin liquid ground state further dilute the magnetic correlations, suppressing the spectral weight of this excitation. At high Ca concentrations are superconducting under pressure and hole pairing was proposed to be a robust feature of doped ladders. The measured dielectric response in the microwave region, the low energy Raman data, the non-linear transport properties along with soft x-ray scattering allowed us to conclude that the ground state for a wide range of Ca concentrations (x < 12) is characterized by charge density wave correlations. This state seems to be driven not by phonons but by Coulomb forces and many-body effects. We highlighted the similarity in the finite frequency Raman response as opposed to the very different behavior of the DC resistivity between undoped and doped ladders. We found that at high Ca concentrations the carrier relaxation is characterized by the same large activation energy (~2000 K) as in the self-doped compound. This observation prompted us to suggest an unconventional metallic transport driven by collective electronic response.
2010-01-01
The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker–Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is de...
Spin dynamics in HTSC cuprates: The singlet--correlated band (or t-J-V) model and its applications
2005-09-05
So far calculations of the spin susceptibility in the superconducting state of cuprates have been performed in the framework of weak-coupling approximations. However, it is known that cuprates belong to Mott-Hubbard doped materials where electron correlations are important. In this paper an analytical expression for the spin susceptibility in the superconducting state of cuprates is derived within the singlet-correlated band model, which takes into account strong correlations. The expression of the spin susceptibility is evaluated using values for the hopping parameters adapted to measurements of the Fermi surface of the materials YBa2Cu3O7 and Bi2Sr2CaCu2O8. We show that the available experimental data which are directly related to the spin susceptibility can be explained consistently within one set of model parameters for each material. These experiments include the magnetic resonance peak observed by inelastic neutron scattering and the temperature dependence of the NMR spin shift, spin-spin and spin-lattice relaxation rates in the superconducting state.
Magnetosensitive conductivity of aluminum and the advantage of Corbino geometry
1997-06-01
The problem of low temperature high density charge transport in disk shaped sample (Corbino geometry) has been investigated experimentally and analytically for polycrystalline aluminum having residual resistance ratio of 10{sup 4}. The values of magnetoresistance and self magnetic field stimulated by Hall drift in an external magnetic field up to 8 T has been estimated for exciting current of the order 10{sup 3} A flowing through the disk samples. The analysis of observed data has been done on the base of equations for connection of current density with electric and magnetic field, the relaxation processes having been taken into account. Definite correlations between experiment and theory for disk shaped sample being inductance were established. These are: magnetoresistance, self magnetic field, its distribution and energy, characteristic transition time. Some possible variants of application of studied phenomena for energy storage and transmission, control systems of cryogenic electric circuits are proposed with account of characteristic inductive and resistive properties of sample organized in such manner.
Magnetism and spin dynamics in the cluster compound [Cr4S(O2CCH3)8(H2O)4](NO3)2centre dotH2O
2000-01-01
The magnetism and spin dynamics of [Cr4S(O2CCH3)8(H2O)4](NO3)2centre dotH2O have been investigated by magnetic susceptibility and proton NMR measurements and by theoretical calculations. The proton-spin lattice relaxation rate 1/T1, as a function of the temperature T and external magnetic field B, provides a very useful probe of the dynamical behavior of the four Cr+3 (spins s=(3/2)) paramagnetic ions which are ferromagnetically coupled via isotropic Heisenberg exchange interaction. From our derived formulas for the two-ion time correlation functions, we find that 1/T1 may be expressed as a function of the single scaling variable muBB/(kBT), where ...
Effect of thickness on magnetic phase coexistence and electrical transport in Nd0.51Sr0.49MnO3 films
2010-01-01
We present the impact of the film thickness on the coexistence of various magnetic phases and its link to the magnetoresistance of Nd0.51Sr0.49MnO3 thin films. These epitaxial films are deposited on LaAlO3 (001) substrates by DC magnetron sputtering. Films with thicknesses of approximately 30 nm are found to be under full compressive strain while those with thicknesses propor to100 nm and beyond exhibit the presence of both strained and relaxed phases, as evidenced from X-ray diffraction studies. Both films exhibit multiple magnetic transitions controlled by strong electron correlations and phase coexistence. These films also display insulator-metal transitions (IMT) and colossal magnetoresistance (CMR) under moderate magnetic fields. Among the two set of films, only the 30-nm films show a weak ...
Effect of thickness on magnetic phase coexistence and electrical transport in Nd0.51Sr0.49MnO3 films
2010-01-01
We present the impact of the film thickness on the coexistence of various magnetic phases and its link to the magnetoresistance of Nd0.51Sr0.49MnO3 thin films. These epitaxial films are deposited on LaAlO3 (001) substrates by DC magnetron sputtering. Films with thicknesses of approximately 30 nm are found to be under full compressive strain while those with thicknesses 100nm and beyond exhibit the presence of both strained and relaxed phases, as evidenced from X-ray diffraction studies. Both films exhibit multiple magnetic transitions controlled by strong electron correlations and phase coexistence. These films also display insulatormetal transitions (IMT) and colossal magnetoresistance (CMR) under moderate magnetic fields. Among the two set of films, only the 30-nm films show a we...
Magnetic state dynamics in itinerant paramagnet UM3B2 (M= Co, Ir) probed by 11B NMR
2006-05-19
We have carried out the $^{11}$B NMR measurement on the itinerant paramagnetic systems U$M_{3}$B$_{2}$ ($M =$ Co, Ir) to investigate the low-dimensional characteristics of the $5f$-electrons due to the structural anisotropy. The recent X-ray analysis suggests that UIr$_3$B$_2$ has a different structure modulated from the ever-known superlattice. The azimuth angle variation of NMR spectrum within the $ab$-plane clarified that B atoms occupy the single site, and a certain ligands arrangement surrounding B atom turns to the same orientation as the another one through the three- or six-fold rotation around the c-axis. These results have been consistent with the X-ray proposition. To evaluate the temperature ($T$) development of general susceptibility ($\\chi_{q,\\omega}$), Knight shift and nuclear spin-lattice relaxation rates measurements were performed and the similar variations of $\\chi_{q,\\omega}$ were identified in both UCo$_{3}$B$_{2}$ and UIr$_{3}$B$_{2}$. Above a crossover point defined as $T^{*}\\simeq50$ K, the evolution of $\\chi_{q,\\omega}$ is dominant at $q=0$, suggesting that ferromagnetic correlations develop in high-$T$ regimes; meanwhile, below $T^{*}$, the $q=0$ part in $\\chi_{q,\\omega}$ shows the saturation tendency, and a different class of dispersion at finite-$q$ suddenly emerges. This particular magnetic correlations are interpreted as the antiferromagnetic correlations, and notable feature of the magnetic state dynamics in low-$T$ regimes is that the antiferromagnetic correlations arise together with the ferromagnetic component at the same time. The unique magnetic correlations obtained from NMR experiment will be discussed by the possible low-dimensionality of U$M_{3}$B$_{2}$ lattice.
Magnetic interactions in transition metal doped ZnO$$bAn abinitio study
2006-05-22
We calculate the nature of magnetic interactions in transition-metal doped ZnO using the local spin density approximation and LSDA+\\textit{U} method of density functional theory. We investigate the following four cases: (i) single transition metal ion types (Cr, Mn, Fe, Co, Ni and Cu) substituted at Zn sites, (ii) substitutional magnetic transition metal ions combined with additional Cu and Li dopants, (iii) substitutional magnetic transition metal ions combined with oxygen vacancies and (iv) pairs of magnetic ion types (Co and Fe, Co and Mn, etc.). Extensive convergence tests indicate that the calculated magnetic ground state is unusually sensitive to the k-point mesh and energy cut-off, the details of the geometry optimizations and the choice of the exchange-correlation functional. We find that ferromagnetic coupling is sometimes favorable for single type substitutional transition metal ions within the local spin density approximation. However, the nature of magnetic interactions changes when correlations on the transition-metal ion are treated within the more realistic LSDA + \\textit{U} method, often disfavoring the ferromagnetic state. The magnetic configuration is sensitive to the detailed arrangement of the ions and the amount of lattice relaxation, except in the case of oxygen vacancies when an antiferromagnetic state is always favored.
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