Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W35: Focus Session: Spins in Semiconductors -- DMS: III-V and Devices |
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Sponsoring Units: GMAG DMP FIAP Chair: Masaaki Tanaka, University of Tokyo Room: E145 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W35.00001: Ultrafast Coherent Magnetism in Ferromagnetic GaMnAs Jigang Wang, M.D. Kapetanakis, I.E. Perakis, K.J. Wickey, C. Piermarocchi, Xinyu Liu, J.K. Furdyna We reported a femtosecond collective spin tilt triggered by nonlinear, near-ultraviolet, coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the (111) equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W35.00002: High Frequency FMR spectroscopy of Thin MnAs Epilayers grown on (111) and (100) GaAs M. Cubukcu, H.J. von Bardeleben, J.L. Cantin, M.J. Wilson, D. Rench, P. Schiffer, N. Samarth The magnetic anisotropies of thin ferromagnetic epilayers can be conveniently studied by ferromagnetic resonance spectroscopy [1] with 9 or 35 GHz spectrometers. The case of $\alpha$-MnAs -- a metallic ferromagnet of interest for hybrid semiconductor spintronics -- presents technical challenges because the large structure-related anisotropy field limits such measurements to close to easy axis orientation. We have overcome this difficulty by applying high frequency FMR at 115 GHz with magnetic fields up to 11 T. This allows us to map out complete angular variation patterns and to thus deduce the corresponding anisotropy constants. We report on a systematic investigation of the influence of epilayer thickness and temperature on the magnetic anisotropy.\\[4pt] [1] Kh.Khazen et al, Phys.Rev.B {bf 77}, 165204 (2008). [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W35.00003: Ultra-fast Creation and Destruction of Ferromagnetic nanowires Kenneth Burch, F. Chen, A. Azad, J. O'Hara, S. Mack, A.M. Dattelbaum, G. Montano, V.S. Zapf, D.D. Awschalom, R.D. Averitt, A.J. Taylor The field of nano-spintronics aims to deliver novel multi-functional devices by manipulating magnetism on unprecedented length and time scales. Here we present a technique for ultra-fast creation and destruction of ferromagnetic nanowires at room temperature. We focus on MnAs films where ferromagnetic nanowires are observed within $\pm20$ degrees of room temperature. The existence of the nanowires is monitored via Terahertz Time Domain spectroscopy (THz-TDS) through the anisotropic transmission that they induce. Upon optical excitation, the ferromagnetic nanowires are either created or destroyed, depending on the temperature and history of the sample. Thus we provide a new method for the detection and manipulation of magnetism on the nanoscale. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:27PM |
W35.00004: Induced ferromagnetic order in (Ga;Mn)As in epitaxial Fe/(GaMn)As heterostructures Invited Speaker: One of the most interesting areas of spintronics research is the control of interface-induced phenomena in artificially grown systems, with capabilities that surpass the limitations of current technologies . In semiconductor spintronics, research on Diluted Magnetic Semiconductors (DMS) resulted in remarkable examples of the integration - in a single material - of the spin degree of freedom in a semiconducting environment. However, potential integration of DMS in applications rely on a firm understanding of the magnetic ordering mechanism as well as on the ability to induce ferromagnetism above room temperature. In the most frequently studied DMS material (Ga,Mn)As, the Curie temperature is today limited to about 200 K, while ferromagnetic behavior well beyond room temperature would be required in future spintronics devices. Here we demonstrate that the growth of Fe/(Ga,Mn)As heterointerfaces can be efficiently controlled by epitaxy, and that a robust ferromagnetism of the interfacial Mn atoms is induced by the proximity effect at room temperature. Chemically selective probes, supported by theoretical calculations, were used to monitor both the temperature and magnetic field dependence of the Mn magnetic moment in the semiconducting host. We identify distinct Mn populations, each of them with specific magnetic character. These results trace a possible path to interface-controlled ferromagnetism in DMS-based devices. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W35.00005: Observation of Antiferromagnetic Interlayer Exchange Coupling in a GaMnAs/GaAs:Be/GaMnAs Tri-layer Jonathan Leiner, Xinyu Liu, Jacek Furdyna, Margaret Dobrowolska, Hakjoon Lee, Taehee Yoo, Sanghoon Lee A series of GaMnAs/GaAs:Be/GaMnAs trilayers were fabricated by low-temperature molecular beam epitaxy in the hope of establishing conditions under which antiferromagnetic (AFM) interlayer exchange coupling (IEC) between the GaMnAs layers can occur in such structures. Magnetotransport measurements revealed that such AFM IEC has been achieved in the sample with the thinnest GaAs spacer (5 nm) and highest Be doping in the series. The AFM coupling is revealed by a distinct increase in resistance occurring before the applied field is reduced to zero. SQUID measurements on this sample show a lowered magnetization at temperatures below 35K, which is another characteristic of AFM coupling in that temperature range. Hysteresis loops also show two distinct steps when measured below 40 K. AFM was not observed in other samples in the series, thus providing a valuable limit for the spacer thickness and Be doping in which AFM IEC can be expected. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W35.00006: FMR study of magnetic anisotropy and interlayer exchange coupling in (Ga,Mn)As/GaAs/(Ga,Mn)As K. Dziatkowski, Z. Ge, X. Liu, J. K. Furdyna, B. Clerjaud, R. Jakiela, A. Barcz, A. Twardowski An interplay between magnetic anisotropy and interlayer exchange coupling (IEC) in (Ga,Mn)As-based heterostructures was studied experimentally. Ferromagnetic resonance (FMR) measurements were performed on a series of (Ga,Mn)As/GaAs/(Ga,Mn)As trilayers with different thickness $d_ {\mathrm{GaAs}} $ of nonmagnetic GaAs spacer. The manifestation of single- or double-resonance spectra was a foundation for determining $d_ {\mathrm{GaAs}}$-dependent regimes of strong ($d_{\mathrm{GaAs}} \leq 3$ nm) and weak ($d_{\mathrm{GaAs}} \geq 12$ nm) IEC. Under weak IEC conditions, two resonances of different intensity were consistently addressed by means of Landau- Lifshitz model of magnetization dynamics and recognized as the acoustic (strong) and optic (weak) modes of FMR. The unexpected change in the ordering of acoustic/optic modes in FMR spectra on the rotation of applied magnetic field was explained by detailed evaluation of Zeeman split energy levels of the magnetization. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W35.00007: $sp^{3}$ tight binding model of Ga$_{1-x}$Mn$_{x}$As within the Dynamical Mean Field Approximation Abdolmadjid Nili, Unjong Yu, Dana Browne, Juana Moreno, Mark Jarrell We present the results of a self-consistent dynamical mean field treatment of the ferromagnetic order in GaAs doped with Mn. We use the $sp^{3}$ tight binding model as the non-interacting Hamiltonian for the parent material's (GaAs) band structure. The spin-orbit interaction is introduced as $\lambda_{\alpha} s.l$ where $\alpha$=c and a for cation (Ga) and anion (As) respectively. Tight binding parameters are fitted in order to capture the correct band structure around the center of the Brillouin zone within the relevant energy scale. We model the interaction between randomly distributed magnetic ions and itinerant charge carriers with a modified double- exchange coupling. Band repulsion resulting from the inclusion of the conduction band improves our model by confining the impurity band. We investigate the effect of the double-exchange coupling on the magnetic and transport properties of the material. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W35.00008: Tunnel magnetoresistance and current-induced magnetization switching in mesoscale (Ga,Mn)As magnetic tunnel junctions M.J. Wilson, P. Mitra, M. Zhu, P. Schiffer, N. Samarth, L. Xue, K.V. Thadani, D.C. Ralph, M.E. Flatte We report measurements of the tunneling magnetoresistance (TMR) and current-induced magnetization switching in exchange-biased (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junctions (MTJs) with dimensions in the mesoscale regime ($2 - 10 \mu {\rm{m}}^2$). We observe thermally activated quenching of the TMR over two decades in temperature (0.35 K$ < T < $40 K), characterized by a functional form: $\rm{TMR}(T) = \rm{TMR}(0) [1 - \exp(-T_0/T)]$, with $T_0 \sim 10$ K. This behavior is interpreted using a model in which tunneling is dominated by the hole state bound to a Mn spin in the depletion region near the (Ga,Mn)As/GaAs interface. For $T < T_0$, such spins have a preferred orientation relative to the ``bulk'' ferromagnetic region, but at higher temperatures they become decoupled from the latter. We also discuss experiments that probe current-induced magnetization switching in these mesoscale devices. We find that in addition to complete switching between high- and low-resistance states of a MTJ, current pulses can lead to stable, intermediate resistance states, possibly due to fragmentation into multiple magnetic domains by heating. Supported by ONR-MURI. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W35.00009: Submicron measurements of domain wall dynamics in patterned (Ga,Mn)As devices A. L. Balk, M. J. Wilson, D. Rench, P. Schiffer, N. Samarth, M. Nowakowski, D. D. Awschalom Contemporary interest in proof-of-concept semiconductor spintronic devices provides a clear motivation for fundamental studies of magnetic domain walls (DWs) in ferromagnetic semiconductors such as (Ga,Mn)As. Here, we use the anomalous Hall effect to measure the magnetic DW position and velocity in micropatterned (Ga,Mn)As devices with submicron spatial resolution. Our measurements focus on the early stages of the creep regime. Statistical analysis of temperature-dependent Barkhausen jumps provides insights into thermally activated DW hopping, while field-dependent measurements of the DW position show behavior suggestive of reversible nanoscale DW flexing. Finally, we demonstrate a feedback technique to map out submicron variations in the DW pinning potential. These measurements are complemented by dynamic magneto-optical Kerr effect imaging of our devices. This work was supported by ONR, NSF and ONR-MURI. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W35.00010: Spin-dependent transport properties of three-terminal ferromagnetic-semiconductor heterostructures with a GaMnAs quantum well and double barriers: Control of quantum levels and TMR Iriya Muneta, Shinobu Ohya, Masaaki Tanaka We investigate the spin-dependent tunneling properties of three-terminal GaMnAs quantum-well (QW) double-barrier devices containing a QW electrode to control the spin-dependent quantum levels of the GaMnAs QW. We successfully modulate the quantum levels of the GaMnAs QW and control the spin-dependent current by changing the voltage of the QW electrode ($V_{QW}$). Also, tunneling magnetoresistance increase is observed at resonant levels by changing $V_{QW}$. This work was partly supported by Grant-in-Aids for Scientific Research, the Special Coordination Programs for Promoting Science and Technology, R\&D for Next-generation Information Technology by MEXT, and PRESTO of JST. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W35.00011: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W35.00012: InMnAs based bipolar junction transistor Nikhil Rangaraju, John Peters, Bruce Wessels InMnAs is a p-type dilute magnetic semiconductor. The characteristics of InMnAs/n-InAs/p-InAs bipolar junction transistors at temperatures from 5 to 300 K and magnetic fields up to 9T were investigated. Amplification as high as 30 is observed in these devices at room temperature and zero magnetic field. The amplification of the transistor is observed to be proportional to $I_{c}^{(1-1/\eta)}$ where $I_{C}$ is the collector current and $\eta$ is the ideality factor of the transistor. The observed amplification behavior of the transistor can be attributed to several factors such as base width modulation, the effect of generation-recombination current, resistance of the base, and geometry of the device. The role of spin on the magneto-transport properties of these devices was investigated and the resulting differences in the behavior of the transistor when the magnetic InMnAs layer acts either as an emitter or a collector was examined. A change in amplification with magnetic field was observed that makes these devices potential candidates for new computing architectures. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W35.00013: Strain-Engineered Magnetic Anisotropy in Insulating, Ferromagnetic Ga$_{1-x}$Mn$_{x}$P$_{1-y}$N$_{y}$ Peter R. Stone, Oscar D. Dubon, Jeffrey W. Beeman, Kin M. Yu, Lukas Dreher, Martin S. Brandt Ga$_{1-x}$Mn$_{x}$P is a ferromagnetic semiconductor in which exchange is mediated by localized holes [Scarpulla \textit{et al.}, Phys. Rev. Lett. \textbf{95,} 207204 (2005)]. We demonstrate a direct connection between the magnetic easy axis in Mn-doped GaP and epitaxial strain by a combined ferromagnetic resonance, X-ray diffraction and SQUID magnetometry study. The magnetic easy axis of Ga$_{1-x}$Mn$_{x}$P is gradually rotated from the in-plane [0$\overline{1}$1] direction towards the film normal [100] through alloying with isovalent N which changes the strain state of the film from compressive to tensile. For a nearly lattice-matched film the out-of-plane uniaxial anisotropy field is close to zero emphasizing the importance of epitaxial strain in determining this parameter. Both in-plane and out-of-plane magnetization reversal processes are explored by a simple model that considers the combination of coherent spin rotation and noncoherent spin switching. These results indicate that holes localized within a Mn-derived impurity band are capable of mediating the same anisotropic exchange interactions as the itinerant carriers in the canonical Ga$_{1-x}$Mn$_{x}$As system. [Preview Abstract] |
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