Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J15: Focus Session: Spins in Semiconductors - Spin Currents II |
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Sponsoring Units: DMP GMAG FIAP Chair: Jairo Sinova, Texas A&M University Room: D171 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J15.00001: Spin-Seebeck effect: Local nature of thermally induced spin currents in GaMnAs Invited Speaker: The spin-Seebeck effect refers to a spatial distribution of spins in a ferromagnetic material induced by a thermal gradient. This macroscopic spatial distribution of spins is several orders of magnitude larger than the spin diffusion length [1]. Here we describe measurements of the spin-Seebeck effect in the ferromagnetic semiconductor, GaMnAs [2]. The thermally induced spatial distribution of spins is inferred from the sign and magnitude of the inverse spin Hall voltage generated from local spin currents in platinum bars that are in electrical contact with the ferromagnetic material. From an experimental point of view, GaMnAs provides unique measurement geometries since the magnetic easy axes can be engineered in different directions and the low Curie temperature makes it convenient to perform spin-Seebeck measurements across the magnetic phase transition. Using different experimental configurations we measure either the isolated spin-Seebeck signal, the planar and transverse Nernst effect, or a combination of the spin-Seebeck and Nernst effects. One of the most intriguing aspects of the spin-Seebeck effect is the observation that the spatial distribution of spins is maintained across electrical breaks revealing that the effect does not arise from a longitudinal spin current of charge carriers. \\[4pt] [1] K.~Uchida, S.~Takahashi, K.~Harii, J.~Ieda, W.~Koshibae, K.~Ando, S.~Maekawa, E.~Saitoh, \textit{Nature} \textbf{455}, 778 (2008). \\[0pt] [2] C.~M. Jaworski, J.~Yang, S.~Mack, D.~D. Awschalom, J.~P. Heremans, R.~C. Myers, \textit{Nature Materials} \textbf{9}, 898 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J15.00002: Theory of the Anomalous Hall Effect in the Insulating Regime Xiong-Jun Liu, Xin Liu, Jairo Sinova The anomalous Hall effect (AHE) has been an enigmatic problem that has resisted theoretical and experimental assault for almost one century. The AHE in the metallic regime has been separated into different contributions, i.e. skew scattering side jump and intrinsic contribution. However, the recent experiments on AHE in the insulating regime discover a qualitatively different behavior described by a scaling relation which is different from that in the metallic regime. The new finding cannot be explained by available microscopic theories of metals based on impurity scattering. Here we present a theory to study the anomalous Hall conductivity (AHC) in this regime With this theory we calculate the lower and upper limits for the AHC by taking simple assumptions of the impurity distributions. Our results are quantitatively in agreement with the experimental discoveries, and thus provides the understanding of the AHE in the insulating regime. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J15.00003: Strain manipulation of anomalous Hall response in GaMnAs micromechanical buckled Hall beam structure Chanuk Yang, Hyung Kook Choi, Tai Hoon Kim, Yun Daniel Park The non-magnetic manipulation of magnetic properties of diluted magnetic semiconductors (DMS) has recently received much attention, such as magnetization by electric field [1], and magnetic anisotropy by strain engineering [2]. Especially, in GaMnAs, the spin-orbit interactions (SOIs) are highly strain-dependent and the applied magnetic field plays a crucial role in determining magnetic anisotropies (MA), anisotropic magnetoresistances (AMR), and the intrinsic anomalous Hall effect (AHE) [3]. Here, we present AHE of local-strain-induced GaMnAs micro-Hallbeam by fabricating mechanical suspended structure. We observe a suppression of the AHE that varies symmetrically about the centre of the buckled beam due to strain-related SOIs. \\[4pt] [1] H. Ohno et al., Nature 408, 944 (2000); Y.D. Park et al. Science 295, 651 (2002) \\[0pt] [2] T. Dietl et al. PRB 63, 195205 (2001); M. Glunk et al., PRB 79,195206 (2009); J. Wenisch et al., PRL 99, 077201 (2007); A. W. Rushforth et al., PRB 78, 085314 (2008) \\[0pt] [3] Nagaosa, N. et al., Reviews of Modern Physics 82 (2), 1539 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J15.00004: The spin-Seebeck effect in a GaMnAs/MnAs bilayer Christopher Jaworski, Jing Yang, Shawn Mack, David Awschalom, Joseph Heremans, Roberto Myers The spin-Seebeck effect, recently discovered in ferromagnetic metals such as permalloy, semiconductors such as GaMnAs and insulators such as YIG, consists of a thermally generated spin redistribution. This effect is measured by detecting an inverse spin Hall voltage that varies spatially across a sample due to the thermally generated local spin currents. Here, we describe measurements of the spin-Seebeck effect in metallic ferromagnetic MnAs thin films grown on GaMnAs. The difference in H$_{c}$ and T$_{c}$ of each layer allows independent measurement of spin-Seebeck signals arising from MnAs from that of GaMnAs. We discuss the effect of the exchange bias between these layers on the spin-Seebeck effect above and below the magnetic phase transition. Work support in parts by NSF, NSF-CBET-0754023, ONR, and DMR-0820414. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J15.00005: Magnetoresistance in Lateral GaMnAs devices with Nano-constrictions Bhim Paudel, Grant Riley, Leonidas Ocola, Xinyu Liu, Jacek Furdyna, Khalid Eid Mn-doped GaAs (or GaMnAs) offers opportunities to demonstrate both new device concepts with added functionality and new phenomena in condensed matter physics, since it is both a ferromagnet and a semiconductor. We will present our recent results on fabricating and characterizing GaMnAs-based nano- devices. The resistance of these deep-nanoscale devices can be manipulated either by varying the applied voltage or via an external magnetic field. The nano-devices were prepared using electron-beam lithography and wet chemical etching. The magnetoresistance of the devices was as high as 50{\%} at 4.2k and the behavior was different from previous results reported in literature. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J15.00006: A grate field-dependent change of magnetic damping in Fe/(Ga,Mn)As Satoi Kobayashi, Keita Suda, Hiro Munekata Reported here is the field dependence of the magnetic damping in the photo-induced precession of magnetization in three different samples, a simple (Ga,Mn)As, two hybrids Pt/(Ga,Mn)As and Fe/(Ga,Mn)As. The Mn content $x $is $x$ = 0.045 for all cases. In (Ga,Mn)As, the precession frequency \textit{$\omega $} increases and the precession lifetime\textit{ $\tau $} decreases with increasing a lateral, external field applied along the easy axis, whereas the \textit{$\omega \tau $} product is hardly changed. Similar trend is observed in Pt/(Ga,Mn)As, except that the \textit{$\omega \tau $} product is smaller than that of (Ga,Mn)As. An inverse value of the \textit{$\omega \tau $} product, so called the Gilbert damping constant $\alpha $, is $\alpha $ = 0.1 and 0.15, respectively, for (Ga,Mn)As and Pt/(Ga,Mn)As. The enhanced magnetic damping in Pt/(Ga,Mn)As can be understood qualitatively in terms of the spin pumping. In Fe/(Ga,Mn)As, the \textit{$\omega \tau $} product around the zero field is even smaller than that of Pt/(Ga,Mn)As, being indicative of a larger damping ($\alpha $ $\sim $ 0.26), whereas the \textit{$\omega \tau $} product increases steeply with an external field. At around 400 Oe and higher, the \textit{$\omega \tau $} product saturates at the value comparable to that of a simple (Ga,Mn)As. Taking magnetization data into account, a great field-dependent change in damping could be attributed to the spin-wave excitation at the Fe/(Ga,Mn)As interface caused by non-parallel magnetization configuration between Fe and (Ga,Mn)As in microscopic scale. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J15.00007: Interfacial Spin Filtering at Copper/GaMnAs Contacts Khalid Eid, Bhim Paudel, Grant Riley, Xinyu Liu, Jacek Furdyna We determine the spin injection efficiency using a single ferromagnetic film without the need for a spin-detection layer. This is accomplished by studying the temperature dependence of the specific contact resistance (AR$_{C})$ of a copper/GaMnAs contact using a circular transmission line method. AR$_{C}$ is as low as 5x10$^{-8} \quad \Omega $cm$^{2}$, and decreases slowly with decreasing temperature T. However, as T approaches Curie temperature T$_{C}$, AR$_{C}$ abruptly jumps to about double its initial value. We suggest that this behavior arises from the suppression of one of the two spin conduction channels, which results in substantial spin polarization. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J15.00008: Spin-dependent Transport in GaAs/MnAs Core/shell Nanowires J. Liang, J. Wang, N.S. Dellas, B.J. Cooley, S.E. Mohney, R. Engel-Herbert, M.H.W. Chan, N. Samarth The integration of a metallic ferromagnet (FM) with a semiconductor (S) in axially- and radially modulated nanowires (NWs) has the potential to open up new opportunities in nanospintronics. We describe a comprehensive study of the structure, magnetism and electrical transport in hybrid core/shell S(GaAs)/FM(MnAs) NWs synthesized by molecular beam epitaxy. This is an unusual system where the competition between magnetocrystalline and shape anisotropies in the FM shell creates a magnetic ordering regime which is distinct from conventional FM metal NWs. We report four probe measurements of the temperature dependence of conductivity and the magnetoresistance (MR) in single NWs over a temperature range 0.5 K - 300 K and in magnetic fields ranging up to 80 kOe. Assuming that electrical transport is dominated by the metallic shell, we use the measured anisotropic MR in conjunction with micromagnetic simulations to gain insight into the magnetization reversal process of the FM shell. We also discuss the possible origins of a striking negative linear MR at high field which becomes more pronounced with increasing temperature. Supported by NSF-MRSEC and ONR. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J15.00009: Ohmic spin injection from a half-metal at finite temperatures: Is the conductivity mismatch problem relevant? James Glasbrenner, Aleksander Wysocki, Kirill Belashchenko Spin injection from a normal ferromagnet into a semiconductor requires a highly-resistive tunnel or Schottky barrier at the interface to overcome the conductivity mismatch problem. This barrier limits the current that can be achieved in a device. A half-metallic ferromagnet used as a spin injector obviously overcomes this problem at zero temperature, but the situation at finite temperatures is nontrivial. We argue that the two-current model is inapplicable to half-metals, and that Ohmic (barrierless) spin injection from a half-metal is possible even at finite temperatures. This conclusion is reached using an intuitive model which sums up multiple scatterings at the interface. To complement this model, we calculate the spin injection efficiency for a half-metallic electrode using a single-band tight-binding model with explicit statistical averaging over thermal spin fluctuations. The results are contrasted with the case of a normal ferromagnet. We also consider a practically interesting case of a CrAs electrode within the tight-binding LMTO method. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J15.00010: MnAs/Al(AsSb)/InAs Heterostructure-Based Spin LEDs H. Zhang, E.D. Fraser, S. Hegde, J. Kwon, J.B. Hatch, H. Luo, G.P. Lindberg, B.A. Weinstein, B.D. McCombe MnAs is a promising spin alignment material for spin-injection into InAs-based structures due to its well studied structural and magnetic properties. A well known difficulty of spin injection from a ferromagnetic metal spin aligner into a semiconductor is the so-called conductivity mismatch, which has been overcome via tunnel barrier contacts.\footnote{B.T. Jonker \textit{et al.}, PRB \textbf{62}, 8180 (2000).}$^,$\footnote{X. Jiang \textit{et al.}, PRL \textbf{94}, 056601 (2005).} Lattice matching the tunnel barrier to the active region is important because a highly strained interface and resulting defects can reduce spin polarization of the injected carriers. We report development of a spin-LED structure with a lattice matched AlAs$_{0.16}$Sb$_{0.84}$ tunneling barrier between the MnAs spin aligner and an InAs quantum well. The composition was characterized through XRD and Raman spectroscopy. Sample growth, characterization, LED fabrication and optical polarization studies of electroluminescence in the 3 micron spectral region will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J15.00011: Efficient injection of spin-polarized electrons from MnAs contacts into GaAs quantum well LEDs Everett Fraser, Shridhar Hegde, Lars Schweidenback, Andreas Russ, Athos Petrou, Hong Luo, George Kioseoglou Recent studies of ferromagnetic MnAs have revealed a wide range of properties desirable for spintronic applications. In this work, ferromagnetic MnAs contacts have been used to inject spin polarized electrons into AlGaAs(n)/GaAs(i)/AlGaAs(p) light emitting diodes. The band-edge electroluminescence emitted from these devices has a saturation circular polarization of 26\% at 7K. The circular polarization was found to track the out of plane magnetization of MnAs, confirming spin injection. Using optical pumping measurements, the corresponding electron spin polarization was determined to be 52\%. Emission persists up to room temperature, with a saturation circular polarization of 6\%. The improved performance over similar structures is attributed to the use of MnAs/AlGaAs Schottky barrier tunneling and minimal interdiffusion of Mn ions near the materials interface. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J15.00012: Oscillatory spin polarization and magneto-optical Kerr effect in Fe$_{3}$O$_{4}$ thin films on GaAs(001) Yan Li, Wei Han, A.G. Swartz, K. Pi, J.J.I. Wong, S. Mack, D.D. Awschalom, R.K. Kawakami Magnetite is an attractive material for spin injection and detection, because the theory predicts completely negative spin polarization at the Fermi level at room temperature. We fabricated high quality Fe$_{3}$O$_{4}$ films on GaAs (001) by molecular beam epitaxy. The Fermi level spin polarization of the Fe$_{3}$O$_{4}$ film was probed using the ultrafast optical measurement of ferromagnetic proximity polarization (FPP). The systematic thickness dependence of FPP and MOKE were measured on wedged Fe$_{3}$O$_{4}$ films on GaAs(001), and similar oscillatory and sign reversing behaviors were observed even though the two measurements rely on different mechanisms (spin dependent electron reflection for FPP, and optical transitions for MOKE). Quantum confinement of the t$_{2g}$ states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J15.00013: TMR study of GaMnAs/AlGaAs:Be/GaMnAs trilayers Joseph Hagmann, Xinyu Liu, Malgorzata Dobrowolska, Jacek Furdyna, Taehee Yoo, Sungwon Khym, Sanghoon Lee GaMnAs/GaAs:Be/GaMnAs trilayers have recently demonstrated antiferromagnetic (AFM) coupling between the two ferromagnetic (FM) layers, mediated by holes in the spacer layer. In this work, GaMnAs/Al$_{x}$Ga$_{1-x}$:Be/GaMnAs trilayer samples with varying Al concentrations were fabricated into magnetic tunnel junction (MTJ) devices with range of pillar diameters to measure tunneling magnetoresistance (TMR) under various conditions. SQUID measurements were use to measure the magnetization of the samples, including switching fields for parallel and antiparallel magnetization alignments of the FM layers. TMR was observed in the sample with Al$_{0.22}$Ga$_{0.78}$As:Be spacer, but was massively suppressed in the samples with lower Al content. The presence of holes in the spacer layer is shown to suppress TMR. This illustrates the difference in conditions for TMR and for AFM interlayer coupling. [Preview Abstract] |
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