Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U5: Spin Control in Ferromagnetic Semiconductor Structures |
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Sponsoring Units: GMAG DCMP Chair: Peter Schiffer, Penn State University Room: LACC 502B |
Thursday, March 24, 2005 8:00AM - 8:36AM |
U5.00001: Electrical Control of Magnetization in Semiconductors Invited Speaker: Ferromagnetic III-V semiconductor (Ga,Mn)As is characterized by $p-d$ exchange stabilized ferromagnetism, small magnetization, and strong spin-orbit interaction [1, 2], thus offering a unique combination of physics related to current-induced magnetization reversal. Here we present our study on (1) current driven magnetic domain wall motion in a lithographically defined (Ga,Mn)As structure [3], and (2) current driven magnetization reversal in fully epitaxial (Ga,Mn)As magnetic tunnel junctions (MTJ's) using GaAs as a barrier [4]. In the former, two regimes are found to be present in the velocity - current density characteristics and the estimated spin-transfer efficiency is as high as 10{\%} or even higher. In the latter, current density required for the reversal in MTJ is found to be lower than that expected from scaling of magnetization. [1] H. Ohno, Science, 281, 951 (1998). [2] T. Dietl \textit{et al.}, Science, 287, 1019 (2000). [3] M. Yamanouchi \textit{et al.}, Nature, 428, 539 (2004). [4] D. Chiba \textit{et al.}, accepted for publication in Phys. Rev. Lett. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 9:12AM |
U5.00002: Spin accumulation in forward-biased MnAs/GaAs Schottky diodes Invited Speaker: The injection of electrons from ferromagnetic metals into semiconductors has recently received much attention in the field of spintronics since these systems have the potential to serve as room-temperature sources of spin polarization. To date, most research in this vein has focused on electron currents flowing through a tunnel barrier from the ferromagnet to the semiconductor. For example, spin injection has been observed for tunneling through Schottky and aluminum oxide tunneling barriers as well as in more complicated structures such as magnetic tunnel transistors. All of these schemes share the common feature that spin-polarized electrons are injected from ferromagnet to semiconductor. Here we describe experiments demonstrating a new means for the all-electrical generation of spin polarization in ferromagnet/semiconductor epilayers, in which an electron current flows from the semiconductor to the ferromagnet \footnote{J. Stephens et al. Phys. Rev. Lett. 93, 097602 (2004)}. In contrast to the more conventional route of spin injection, we observe spin accumulation at the metal/semiconductor interface of these forward-biased ferromagnetic Schottky diodes. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the non-equilibrium carrier polarization. A simple model can be used to describe the spin accumulation effect in terms of spin-dependent interface transmission and reflection coefficients and to estimate its magnitude. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:48AM |
U5.00003: Interfacial Control of Ferromagnetism in (Ga,Mn)As-based Hetero- and Nano-structures Invited Speaker: We discuss recent experiments that demonstrate how heterointerfaces impact the magnetic properties of hetero- and nanostructures derived from the ``canonical'' ferromagnetic semiconductor (Ga,Mn)As. In this material, holes created by the Mn acceptors mediate a ferromagnetic interaction between the Mn ions, and the Curie temperature ($T_{\rm{C}}$) is determined by a complex interplay between substitutional magnetic ions, interstitial defects and holes. Although as-grown epilayers of (Ga,Mn)As typically have $T_{\rm{C}} \leq 110$K, post-growth annealing at low temperatures ($180^{\circ}\rm{C}$ - $250^{\circ}\rm{C}$) significantly enhances the ferromagnetic properties, leading to $T_{\rm{C}} \sim 150$K. We first describe experiments that examine the effects of capping ferromagnetic (Ga,Mn)As epilayers with a thin layer of undoped GaAs [Stone {\it et al}, Appl. Phys. Lett. {\bf 83}, 4568 (2003)]. We find that the overgrowth of even a few monolayers of GaAs significantly suppresses the enhancement of the ferromagnetism associated with low temperature annealing, suggesting that heterointerfaces have a direct impact on the migration of interstitial defects during post-growth annealing. We next demonstrate how nanopatterning allows us to provide alternate defect diffusion pathways, hence remove the constraints on $T_{\rm{C}}$ imposed by the presence of heterointerfaces [Eid {\it et al}, submitted]. Finally, we examine the influence of an overgrown antiferromagnet (MnO) on the magnetic properties of (Ga,Mn)As, demonstrating the first example of exchange biasing of this ferromagnetic semiconductor [Eid {\it et al},Appl. Phys. Lett. {\bf 85}, 1556 (2004)]. Detailed studies show that systematic control over the highly reactive MnO/(Ga,Mn)As interface is essential for the routine achievement of exchange bias in this important spintronic material. This work was carried out in collaboration with K. F. Eid, M. B. Stone, O. Maksimov, K. C. Ku, B. L. Sheu, W. Fadgen, P. Schiffer, T. Shih, and C. Palmstrom. Supported by ONR and DARPA. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:24AM |
U5.00004: Ferromagnetic Control of Spin-Dependent Electron Currents in a Semiconductor Invited Speaker: L. J. Sham It is well known that electrons or neutrons scattered against a polarized target become polarized. This talk will show how this principle can be used in variety of ways to generate and to change a spin polarization in a current flowing in a semiconductor interfaced with one or more ferromagnets. In theory it is possible to generate a 100{\%} polarized current or a pure spin current without charge current. The relative merits of the various configurations will be assessed. Experiment tests will be described. Possible device applications provide illustrations of the theory. Work done in collaboration with J.P. McGuire, C. Ciuti, Eric Yang, Yuchang Chen, Thomas Grange, and Ed Yu, and supported by NSF DMR 0099572, DARPA/ONR N0014-99-1-1096 and University of California Campus- Laboratories Cooperation project. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 11:00AM |
U5.00005: Unexpected magnetism in thin film dielectric oxides Invited Speaker: High temperature ferromagnetism in thin films of dilute magnetic oxides is a widespread phenomenon, of which there appear to be two distinct sources. One is the contribution of the 3$d$ dopant ions themselves, the other is related to crystal defects in the interface region. The latter contributes a magnetic moment of 100 -- 400 $\mu _{B}$ per square nanometer of substrate area, which is largely independent of film thickness or dopant concentration. In very dilute films it seems as if there is a giant ionic moment when the film moment is expressed per 3d cation, but this is because the source of the magnetism is misattributed. It is suggested that the magnetic defects are two-electron or two-hole centres which have a spin triplet as ground state or low-lying excited state. In ZnO or SnO$_{2}$, examples of the latter, the magnetic dopant stabilizes the spin triplet by exchange. However HfO$_{2}$, ZrO$_{2}$ and WO$_{3}$, examples of the former, are ferromagnetic even when undoped. They are 'd-zero' ferromagnets. A characteristic sign of this exotic magnetism is strong anisotropy of the saturation magnetization. Possible links to other systems such as defective graphite or gold/thiol will be discussed. [Preview Abstract] |
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