Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W42: Mostly Half-Metals |
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Sponsoring Units: GMAG Chair: Ching Fong, University of California-Davis Room: LACC 150B |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W42.00001: Electronic structure and magnetic anisotropy of CrO$_2$ Antonina Toropova, Gabriel Kotliar, Sergej Savrasov, Viktor Oudovenko The problem of importance of strong correlations for the electronic structure, transport and magnetic properties of half--metallic ferromagnetic CrO$_{2}$ is addressed by performing density functional based electronic structure calculations in the local spin density approximation (LSDA) as well as using the LSDA+U method. In both schemes we compute electronic structure, optical conductivity and magnetic anisotropy energy for chromium dioxide. It is shown that the corresponding low-- temperature experimental data are best fitted without accounting for the Hubbard $U$ corrections. We conclude that the ordered phase of CrO$_{2}$ is weakly correlated. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W42.00002: Spin polarization in half-metallic ferromagnets M. Biasini, A.P. Mills, Jr. Ferromagnetic contacts for spin injection and analysis are key components determining the performance of spintronic devices. For practical applications the materials for these contacts should have a high electron spin polarization at the Fermi surface (FS) at room temperature. We need to develop suitable new high Curie-temperature ferromagnets from the class of half metallic compounds that are theoretically ideal for spintronics [1]. We point out that a polarized slow positron probe combined with the two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique [2] would allow unambiguous, direct, room-temperature determinations of the spin polarization of the conducting electrons at the FS of important candidate spintronic ferromagnetic thin films and single crystals. The electron spin polarization at the FS may be deduced directly from the amplitudes of the discontinuities in the electron occupation number at the Fermi momentum for two directions of the polarization of a positron probe relative to the saturating magnetic field direction [3]. Work supported in part by NSF grants DMR 0216927 and PHY 0140382 and by DOD/DARPA/DMEA, Award DMEA90-02-2-0216. \newline [1] I. Zutic et al., Rev. Mod. Phys. {\bf 76}, 323 (2004).\newline [2] S. Berko, in Positron Solid-State Physics, Brant and Dupasquier, eds. (North-Holland, 1983) p. 64.\newline [3] K. E. H. M. Hanssen et al., Phys. Rev. B {\bf 42}, 1533 (1990). [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W42.00003: Spin-polarized ballistic transport in a thin superlattice of zincblende half metallic compounds Ching-Yao Fong, Meichun Qian, Warren Pickett We examine theoretically ballistic conduction in a sample made of thin layers of zincblende half metals. The sample is a superlattice consisting of monolayers of GaAs and MnAs, a bilayer of CrAs, and a bilayer of GaAs. The approach is based on density functional theory with the generalized gradient approximation. The superlattice is found to have a half metallic character, and ballistic conduction of electrons within $\sim 0.3$~eV of the Fermi level will give nearly 100\% spin-polarized conductance in the direction of the superlattice. Our calculated results suggest that a thin superlattice involving the MnAs and CrAs will be a good spintronic material and can be grown on GaAs substrate by molecular beam epitaxy. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W42.00004: Strong magnetoelectric effect in a Heusler alloy-based magnetic tunnel junction K. Moon, Taewan Kim, Wanjun Park, Joonwon Rim We study the magnetoelectric property of the magnetic tunnel junction(MTJ) formed between the half-metallic Heusler alloy and $CoFe$ exchange ferromagnet. The MTJ thus fabricated has demonstrated a large magnetoresistance increase for the anti-parallel spin alignment. Remarkably, the system exhibits a zero-bias current for the anti-parallel spin alignment, while it is absent for the parallel alignment. We argue that this manifests the strong magneto-electric coupling present in the MTJ, which is strongly enhanced by the half-metallic nature of Heusler alloy. \\\\ $^*$This work was partially supported by The National Program for Tera-Level Nanodevices of the Korea Ministry of Science and Technology as one of The 21 Century Frontier Programs. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W42.00005: Magnetotransport in Co$_2$TiSn Anke Husmann, Mark Hickey, Stuart N. Holmes, Michael Pepper The Heusler alloy Co$_2$TiSn is a ferromagnet with a Curie temperature of 355K. Our theoretical predictions by CASTEP as well as calculations done by others \footnote{A. Yamasaki et al., Phys. Rev. B {\bf 65}, 104410 (2002). } show a high spin polarisation in the conduction band making it a promising candidate for a metallic spin injector. We study its magnetotransport properties as well as MOKE and magnetisation data on bulk crystals and find a large anomalous Hall coefficient indicating a significant spin-orbit coupling. We compliment our theoretical band structure calculations with point contact probe measurements between the magnetic metal surface and a superconducting tip to obtain the spin polarisation at the Fermi energy. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W42.00006: Anomalous Hall Effect of Co2MnSi thin films on GaAs (001) Will Branford, Fridrik Magnus, Kelly Morrison, Steven Clowes, Yury Bugoslavsky, Lesley Cohen, Laura Singh, Zoe Barber The Heusler alloy Co$_{2}$MnSi is a leading candidate material for spin injection and detection in hybrid spintronic devices. Recent measurements of $\sim $60{\%} transport spin polarization by point contact Andreev reflection\footnote{ L. J. Singh \textit{et al}, APL \textbf{84,} 2367-2369 (2004).} and tunneling magnetoresistance\footnote{ J. Schmalhorst \textit{et al}, PRB \textbf{70,} art. no.-024426 (2004).} have demonstrated that below 10K Co$_{2}$MnSi significantly outperforms the elemental ferromagnets and other high Curie temperature ferromagnets such as NiMnSb. Here we study the anomalous Hall effect in a series of highly textured Co$_{2}$MnSi thin films on lattice-matched GaAs, and contrast with the corresponding measurements of polycrystalline NiMnSb films on silicon. The anomalous Hall effect is a function of the transport spin polarization. The implications of the normal and anomalous Hall signals as a function of temperature and film thickness will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W42.00007: Composition-controlled spin polarization in Co$_{1-x}$Fe$_{x}$S$_{2}$ Lan Wang, Koichiro Umemoto, Renata Wentzcovitch, TingYong Chen, Chia-Ling Chien, Joe Checkelsky, James Eckert, E. Dan Dahberg, Chris Leighton We demonstrate the successful application of a simple scheme to allow for composition control over the spin polarization of the alloy Co$_{1-x}$Fe$_{x}$S$_{2}$. The Fe doping concentration can be used to fine-tune the position of the Fermi level in Co$_{1-x}$Fe$_{x}$S$_{2}$ leading to control over both the sign and magnitude of the spin polarization. We have combined detailed electronic structure calculations, point contact Andreev reflection, conventional magnetometry, calorimetry, and various modes of magnetotransport (including anisotropic magnetoresistance), to assimilate a consistent picture of the evolution of the spin polarization (P) with composition. Spin polarizations in the range --57 {\%} $<$ P $<$ 85 {\%} have been achieved. We suggest that this material could be employed as a tunable source of highly polarized electrons for fundamental studies of spin-electronics. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W42.00008: Anomalous Hall Resistivity and Magnetization of Sm$_{1-x}$Gd$_x$Al$_2$ Y. Y. Xue, F. Chen, Y. Q. Wang, Y. Y. Sun, R. L. Meng, C. W. Chu Both the magnetization $M$ and the anomalous Hall resistivity,
$\rho_{xy}^{an}$, of Sm$_{1-x}$Gd$_x$Al$_2$ with $0 |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W42.00009: Negative Longitudinal Magnetoresistance of silver chalcogenides Jingshi Hu, T.F. Rosenbaum, J.B. Betts The transverse, positive magnetoresistance of doped silver telluride and silver selenide changes linearly with field by thousands of percent, with no sign of saturation up to MegaGauss. The inhomogeneous distribution of 1 part in 10,000 of excess silver ions appears to be essential to this unusual response. We report here a systematic measurement of the longitudinal magnetoresistance up to 60T from sub-Kelvin to room temperatures. We find a non-saturating, but negative longitudinal magnetoresistance over a wide temperature range, becoming most negative and linear where the bands cross and the spatial fluctuations in the current paths reach their maximum. The dependence of the negative longitudinal magnetoresistance on sample thickness was also investigated to search for the length scale of distorted current paths. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W42.00010: Magnetic and Transport Properties of Amorphous GdGe Alloys near the Metal-Insulator Transition Nareg Sinenian, Kevin McCarthy, J.J. Cherry, Li Zeng, Erik Helgren, Frances Hellman, D. J. Smith, Rafiq Islam The temperature and field dependence of magnetization and conductivity of amorphous Ge doped with the rare earth Gd ($a$-Gd$_{x}$Ge$_{1-x})$ has been measured for a range of x near the metal-insulator transition 0$.$08 $<$ x $<$0.25). As in previous work on $a-$Gd-Si, high field magnetization and low field susceptibility per Gd atom in the paramagnetic state above the spin glass freezing temperature are significantly suppressed below that of non-interacting Gd, indicative of strong antiferromagnetic interactions. However, \textit{unlike} $a-$Gd-Si, the low field susceptibility does not fit a Curie-Weiss law, instead showing 1/T$^{\alpha }$ dependence. As in $a-$Gd-Si, Gd causes localization of charge carriers below a characteristic temperature T*, which is also an onset of significant negative magnetoresistance MR. Both T* and the magnitude of MR are however significantly less in $a-$Gd-Ge than in comparable $a-$Gd-Si alloys. It is suggested that the large effects of matrix (Ge vs Si) are due to differences in dielectric constant and bandgap, which cause changes in screening, thereby altering the effect of the magnetic moments of Gd on both localization of carriers and on the indirect mediated Gd-Gd exchange interactions. We thank the NSF for support. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W42.00011: Disorder-induced resistive anomaly at the Curie temperature Carsten Timm, M.E. Raikh, Felix von Oppen The resistive anomaly in disordered itinerant ferromagnets has a long history, dating back to the first observation by Gerlach in 1932. In 1968, Fisher and Langer proposed a theory for this anomaly based on anomalous scaling. We show that the resistivity $\rho(T)$ can exhibit a \emph{stronger} singularity than predicted in that work. Close to the Curie temperature $T_c$ the correlation length becomes large compared to the mean free path. Then, the quenched disorder is probed by \emph{diffusive} carriers, which requires one to go beyond the Boltzmann description used in all previous works. Our approach combines ideas from the theory of random media and from mesoscopic physics. Specifically, we find that $d\rho/dT$ scales as $|T-T_c|^{-1/2}$ assuming Gaussian magnetic fluctuations. Our results are relevant for ferromagnets with low $T_c$, such as SrRuO$_3$ or diluted magnetic semiconductors, whose mobility near $T_c$ is limited by disorder. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W42.00012: Spin current and magneto-electric effect in non-collinear magnets Hosho Katsura, Naoto Nagaosa, Alexander Balatsky A new mechanism of the magneto-electric (ME) effect based on the spin supercurrent is theoretically proposed for non-collinear magnets. The close analogy between the superconductors (charge current) and magnets (spin current) is drawn to derive the distribution of the spin supercurrent and the resultant electric polarization. Applications to various spiral magnets are discussed. [Preview Abstract] |
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