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Zutic, Igor; Erwin, Steven; Petukhov, Andre
The magnetization of a ferromagnetic material normally decays monotonically with increasing temperature. Here we demonstrate theoretically the possibility of quite different behavior: reentrant ferromagnetism in semiconductors . Reentrant magnetism can arise in semiconductors because as the temperature rises, the resulting higher concentration of thermally excited carriers can enhance the exchange coupling between magnetic impurities. This opens the possibility of materials exhibiting a transition from the low-temperature paramagnetic phase, in which carriers are frozen out, to a ferromagnetic phase at higher temperature. Thus, in the absence of other ferromagnetic mechanisms there will be two critical temperatures, Tc1 Zuti'c, A. Petukhov, S. C. Erwin, preprint.  I. Zuti'c, J. Fabian, S. C. Erwin, Phys. Rev. Lett. 97, 026602 (2006).
Lee, Jeongsu; Boeris, Guilhem; Vyborny, Karel; Zutic, Igor
The interplay of spin injection in lasers and their nonlinear response leads to novel spintronic devices . Such spin-lasers can enable desirable properties including threshold reduction, bandwidth enhancement, and low chirp [1-3]. These lasers can also be viewed as spin-amplifiers, since high circular polarization in the output can be achieved even with nearly spin-unpolarized injection [2,3]. In the present work, we study chirp in spin-lasers and suggest new modulation schemes to improve their performance. Supported by NSF-ECCS, U.S. ONR, AFOSR-DCT, and NSF-NEB 2020. [4pt]  M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007); J. Rudolph et al., Appl. Phys. Lett. 87, 241117 (2005). [0pt]  J. Lee, W. Falls, R. Oszwadowski, and I. Zuti'c, Appl. Phys. Lett. 97, 041116 (2010).[0pt]  C. Gøthgen, R. Oszwadowski, A. Petrou, and I. Zuti'c, Appl. Phys. Lett. 93, 042513 (2008).[0pt]  G. Boeris, J. Lee, K. V'yborn'y, and I. Zuti'c, preprint (2011).
Zutic, Igor; Abolfath, Ramin; Hawrylak, Pawel
We study magnetism in magnetically doped quantum dots as a function of particle numbers, temperature, confining potential, and the strength of Coulomb interaction screening. We show that magnetism can be tailored by controlling the electron-electron Coulomb interaction, even without changing the number of particles. The interplay of strong Coulomb interactions and quantum confinement leads to enhanced inhomogeneous magnetization which persists at substantially higher temperatures than in the non-interacting case or in the bulk-like dilute magnetic semiconductors. We predict a series of electronic spin transitions which arise from the competition between the many-body gap and magnetic thermal fluctuations. Cond-mat/0612489.  R. Abolfath, P. Hawrylak, I. Zuti'c, preprint.
Zutic, Igor; Lee, Jeongsu; Vyborny, Karel; Han, Jong; Petukhov, Andre
Tunability of magnetic anisotropy (MA) in nanostructures is a fascinating topic, for both fundamental understanding of nanomagnetism and possible spintronic applications. While there have been preceding efforts to systematically study the MA in bulk , we still lack a fundamental understanding of that in magnetic quantum dots (QDs). We first explore electronic properties of nonmagnetic QDs that can be significantly altered from the bulk-state depending upon the material and geometry. Focusing on II-VI materials forming both cubic and non-cubic QDs, we confirm qualitatively different energy spectra between different materials . These findings can guide the control of MA in magnetic QDs. Supported by DOE-BES, NSF-DMR, AFOSR-DCT, U.S. ONR, and NSF-ECCS. [4pt]  X. Liu, Y. Sasaki and J. K. Furdyna, Phys. Rev. B 67, 205204 (2004). [0pt]  K. V'yborn'y, J.E. Han, R. Oszwadowski, I. Zuti'c, and A. G. Petukhov, preprint (2011).
The existence of spin-flip scattering at the interface between ferromagnetic (F) and nonmagnetic (N) layers of magnetoresistive F/N/F structures can significantly reduce the size of the magnetoresistance, limiting the sensitivity and increasing the power consumption of F/N/F devices such as GMR magnetic field sensors, magnetic read heads, and MRAM's . Detecting and measuring the degree of spin flip scattering in F/N/F structures can allow further optimization in such devices as well as increase the understanding of interfacial spin transport. Our nonlocal spin injection and detection experiments on mesoscopic Co-Al2O3-Cu-Al2O3-Co spin valves provide evidence for the existence of interfacial spin-flip scattering in magnetoresistive devices . By extending the conventional picture of spin-dependent interfacial resistances (R, R) to include two additional spin-flip scattering channels (R,R)  we have shown that the nonlocal resistance contains information about both the degree of spin polarization and the degree of spin-flip scattering at the F/N interface. The magnitudes of R and R depend on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal. We have observed that the difference in spin-flip scattering between up and down channels vanishes at low temperatures, but for T>100K it increases nonlinearly with temperature. Further evidence for the presence of interfacial spin-flip scattering can be obtained from noise measurements, which are extremely sensitive to the microscopic transport details.  Spin Dependent Transport in Magnetic Nanostructures, edited by S. Maekawa and T. Shinjo (Taylor & Francis, New York, 2002).  S. Garzon, I. Zuti'c, and R. A. Webb, Phys. Rev. Lett. 94, 176601 (2005).  E. I. Rashba, Eur. Phys. J. B 29, 513 (2002).