Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R18: Focus Session: Spin-Dependent Phenomena in Semiconductors - GaMnAs |
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Sponsoring Units: GMAG DMP FIAP Chair: John Peters, Northwestern University Room: 320 |
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R18.00001: Experimental observations of optical spin-transfer and spin-orbit torques in magnetic semiconductors Invited Speaker: Petr Nemec The spin transfer torque (STT) is a non-relativistic phenomenon where angular momentum of spin polarized carriers electrically injected into a ferromagnet from an external polarizer is transferred to the magnetization [1]. In the absence of an external polarizer a distinct phenomenon can occur in which carriers in a magnet under applied electric field develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) [1]. We show, using the experimental data observed in the ferromagnetic semiconductor (Ga,Mn)As, that there exists optical counterparts of STT (OSTT) [2] and SOT (OSOT) [3]. In OSTT a circularly polarized femtosecond pump laser pulse acts as the external polarizer and it induces a coherent magnetization precession due to the angular momentum transfer, in a direct analogy to the current induced STT [2]. The absence of an external polarizer in OSOT corresponds to photo-carrier excitations which are independent of the polarization of the pump laser pulses and the phenomenon relies on spin-orbit coupling of non-equilibrium carriers, as in the case of the current induced SOT [3]. Our work demonstrates the possibility to study the spin-transfer and spin-orbit torques on the sub-picosecond time-scales using the optical pump-and-probe experimental technique.\\[4pt] [1] A. Bratas et al., Nature Materials 11, 372 (2012).\\[0pt] [2] P. Nemec et al., Nature Physics 8, 411 (2012).\\[0pt] [3] N. Tesarova et al., submitted, arXiv: 1207.0307. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R18.00002: Spin-orbit current-induced torques in (Ga,Mn)As Erin K. Vehstedt, Liviu P. Zarbo, Karel Vyborny, Hidekazu Kurebayashi, Pierre Roy, Joerg Wunderlich, Andrew J. Ferguson, Tomas Jungwirth, Jairo Sinova Electrical control of magnetic domains has the potential to overcome key challenges to the development of new non-volatile and down-scalable logic and memory devices. We study the spin-orbit torque induced by an unpolarized electric current in the dilute ferromagnetic semiconductor, (Ga,Mn)As. The current-induced torque (CIT) is modeled as the interaction between the uniform magnetization and an effective magnetic field representing the non-equilibrium carrier spin-polarization. We calculate the current-induced field (CIF) using the Kubo linear-response formalism for a broad range of material parameters. We find that the CIF is composed of a dominant term due to the inverse spin galvanic effect and a small component which is dependent on the relative orientation of the current, magnetization, and crystal axes. In conjunction with experimental studies, we investigate the magnetization dynamics using the phenomenological Landau-Lifschitz-Gilbert equation. The study of (Ga,Mn)As opens the door to a comprehensive theory of CITs in uniform magnetic semiconductors. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R18.00003: Observation of a Photon Echo and Measurement of Interband Dephasing in GaMnAs Kimberley Hall, Murat Yildirim, Sam March, Reuble Mathew, Angela Gamouras, Xinyu Liu, Margaret Dobrowolska, Jacek Furdyna The carrier-mediated ferromagnetism exhibited by III-V diluted magnetic semiconductors (DMS), together with their large magneto-optical response, makes these materials promising for applications in optoelectronics, including integrated optical isolators and ultrafast optically-addressable memory elements. The time scale for decay of coherence in the carrier system (T$_2$) is a key parameter in models of coherent magnetization rotation [1], yet very little is known about the coherent response in DMS. We present results of four-wave mixing measurements of T$_2$ in GaMnAs. We observe a dramatic reduction in the dephasing time with the incorporation of Mn, consistent with earlier experiments in CdMnTe [2]. This fast dephasing process, which leads to an upper bound on T$_2$ of 40 fs for x $\geq$0.014\%, is attributed to spin-flip scattering between the optically excited holes and Mn ions, providing new insight into exchange coupling and nonequilibrium magnetization dynamics in these materials. Direct measurement of the envelope of the four-wave mixing emission reveals a photon echo in GaMnAs, despite the complexity of exchange coupling and defect-induced band tailing.\\[4pt] [1] J. Chovan et al., Phys. Rev. Lett. 96, 057402 (2006).\\[0pt] [2] S. T. Cundiff et al. J. Opt. Soc. Am. B 13, 1263 (1996). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R18.00004: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R18.00005: Ferromagnetism and infrared electrodynamics of GaMnAs B.C. Chapler, S. Mack, R.C. Myers, K.S. Burch, N. Samarth, D.D. Awschalom, D.N. Basov In this work we experimentally address both the magnetic and the electronic properties of the prototype dilute magnetic semiconductor Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As using infrared (IR) spectroscopy. We first examine the relationship between the carrier density, determined through a sum-rule analysis of our data and additional IR data available in the literature, and the ferromagnetic transition temperature T$_{\mathrm{C}}$. Our analysis supports the conclusion that the Fermi level resides within a Mn-induced IB, and that the location of the Fermi level within the band plays a key role in controlling T$_{\mathrm{C}}$. Additionally, we perform a detailed examination of the spectral features observed in the IR data of our Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As films, and show that these features are also consistent only with a Mn-induced IB scenario. In this latter vein, we will discuss and resolve controversies in the literature related to the peak in a broad mid-IR resonance observed in Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As IR spectra. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R18.00006: Infrared magnetic linear dichroism spectroscopy of (Ga,Mn)As N. Tesarova, J. Subrt, P. Maly, P. Nemec, K. Vyborny, C.T. Ellis, Alok Mukherjee, J. Cerne The sensitivity of magnetic linear dichroism (MLD) to in-plane magnetization makes it well suited to study diluted magnetic semiconductors such as (Ga,Mn)As, where MLD can be used to probe electronic excitations in the material. The band structure supporting these excitations yields rich in-plane magnetization effects, which include anisotropic magnetoresistance and four non-perpendicular, in-plane easy axis orientations. Observation of these effects provides insights into the electronic structure of (Ga,Mn)As. In this work we introduce a new, low-temperature, infrared MLD measurement technique that reduces instrumentation artifacts and enables broadband (0.1 eV \textless E$_{\mathrm{ph\thinspace }}$\textless 2.7 eV) capabilities. Through these MLD measurements we sensitively and systematically probe electronic structure in (Ga,Mn)As samples with Mn concentrations varying from 3{\%}-14{\%}. In general, the data show an MLD enhancement in the visible and infrared regimes, which are indicative of interband transitions between the valence and conduction bands and optical transitions within the valence band, respectively. We find that the behavior of these MLD features with increasing Mn concentration is in reasonable agreement with theoretical predictions. We acknowledge financial support provided by NSF-DMR1006078 and the Faculty of Mathematics and Physics, Charles University in Prague. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R18.00007: Numerical studies of non-Drude ac-conductivity and infrared magneto-optics in (Ga,Mn)As Huawei Gao, Jairo Sinova Optical absorption experiments on (III,Mn)V diluted magnetic semiconductors (DMS's) show that the ac-conductivity has non-Drude behavior at low frequency. STM study show many states deep in the band gap. The numerical simulation of the first problem has been done previously using the effective Hamiltonian model with various treatments of the disorder effects. We re-examine the previous works with a similar numerical method to establish the nature of the transitions in the low to the high-doped regime and also the properties of states in the gap. We use the effective Hamiltonian k.p model to describe the holes introduced by Mn impurities and treat the Mn impurities exactly using the envelope function approximation. We use participation ratios to characterize the localization properties of quasi particle states. This allows us to study the ac-conductivity contributions due to delocalized states to deep in-gap localized states transitions and how the spectral weight is distributed. We will also report on numerical results of the magneto-optical response with this ~treatment of the effect of disorder. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R18.00008: AC-transport measurements of ion beam irradiated GaMnAs semiconductors Elis Sinnecker, Marcelo Sant'Anna, Tatiana Rappoport, Joaquim Mendes, Mauricio Pires, Germano Penello, Deivid Souza, Sergio Mello, Jacek Furdyna, Xinyu Liu GaMnAs is a diluted magnetic semiconductors in which lattice atoms have been partially substituted by magnetic atoms, thus inserting a local magnetic moment into the lattice. Recently it was shown that ion beam irradiation can be an effective tool to modify the magnetic and electronic properties of Ga$_{1-x}$Mn$_x$As thin films [1, 2]. We observed that an increase of the structural disorder by irradiation leads to a systematic decrease on the saturation magnetization. Here, we provide further information on the electronic properties of irradiated samples. Measurements of ac-resistivity, magnetoresistance and Hall resistance were performed from 5K to 300K applying a DC magnetic field up to 7T. The results show an interesting frequency dependence of the ac-transport of measured irradiated samples. For the sake of comparison, data on irradiated non-magnetic semiconductor, grown on the same conditions as Ga1-xMnxAs thin films, are provided.\\[4pt] [1] E. H. C. P. Sinnecker et al., Phys. Rev. B 81, 245203 (2010).\\[0pt] [2] Lin Li et al., J. Phys. D: Appl. Phys. 44, 045001 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R18.00009: Andreev Reflection Measurement of Spin Polarization in GaMnAs Khalid Eid, Diana Dahliah, Robert Tolley, Taylor Reid, Xinyu Liu, Jacek Furdyna Current measurement geometries in high-resistivity materials suffer from a large extra resistance that comes from the bulk of the ferromagnet. We use the Circular Transfer Line Method (CTLM) [1-2] to measure the Andreev reflection effect at GaMnAs/superconductor interface and to extract GaMnAs spin polarization. This technique works especially well for high-resistivity films. It has multiple advantages over the point contact and planar geometries, like eliminating the extra resistance contribution from the bulk, producing actual conductance values and not normalized conductance, and eliminating the broadening of the superconducting gap. The effect of the Schottky barrier at the GaMnAs/superconductor interface plays a crucial role and will also be discussed.\\[4pt] [1] K.F. Eid et al, Appl. Phys. Lett 100, 212403 (2012) \\[0pt] [2] K.F. Eid et al, IEEE Trans. Magn. 47, 2636 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R18.00010: Electron diffusivity above and below the Curie temperature of GaMnAs Chris Weber, Kassie Mattia, Eric Kittlaus, Xinyu Liu, Jacek Furdyna Using a transient-grating pump-probe experiment, we measure the diffusion of photoexcited electrons in samples of (Ga,Mn)As with doping levels of 5\%, 6\%, and 7\% Mn. At both 15 K and 80 K the diffusivity increases with density of photoexcited carriers, indicating the degeneracy of both majority holes and minority electrons. We measure electron diffusion in (Ga,Mn)As as rapid as $\sim100$ cm$^2$/s. Converting diffusivity to mobility using the Einstein relation yields $\mu_e\sim 8000$ cm$^2$/Vs, similar to that of GaAs. This high mobility demonstrates that neither the density of states nor the scattering rate of the (Ga,Mn)As conduction band is significantly influenced by Mn doping or by ferromagnetism. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R18.00011: The contribution of critical spin fluctuations to scattering and spin lifetimes in GaMnAs near the ferromagnetic transition Matthew Mower, G. Vignale As GaMnAs transitions between the paramagnetic and ferromagnetic phases, the resistivity exhibits a peak due to enhanced scattering from critical spin fluctuations. Existing work typically focuses on the ferromagnetic side, or to a lesser extent the paramagnetic side, far away from the transition; the effect of strong spin fluctuations near the transition has received little attention. We present a simple model of spin exchange mediated by dynamic spin fluctuations, calculated in the GW approximation. This produces a finite peak in the resistivity that is qualitatively accurate. We then use this model to calculate hole spin lifetimes from the relevant spin relaxation mechanisms. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R18.00012: Anomalous Fermi level behavior in GaMnAs at the onset of ferromagnetism Iriya Muneta, Hiroshi Terada, Shinobu Ohya, Masaaki Tanaka The origin of the ferromagnetism and the metal-insulator transition (MIT) has been a long-debated issue in the prototype ferromagnetic semiconductor GaMnAs. Previously, the valence band (VB) conduction picture has been widely accepted in this material, where the MIT of GaMnAs was understood by the Fermi level crossing over the VB similarly to p-type GaAs doped with non-magnetic acceptors. Here, we carefully analyze the VB structure and the Fermi level position in a series of Ga$_{1-x}$Mn$_x$As from the unexplored insulating region ($x \simeq 0.01$\%) to the metallic region ($x = 3.2\%$) by using resonant tunneling spectroscopy. We find that the Fermi level never crosses over the VB near the MIT: The Fermi level becomes closest to the VB top at $x = 1.0\%$ at the onset of the ferromagnetism, but it moves away from the VB with increasing or decreasing $x$ from 1.0\%. This anomalous behavior of the Fermi level is completely different from that of GaAs doped with other non-magnetic shallow acceptors~[1]. This work was partly supported by Grant-in-Aids for Scientific Research including Specially Promoted Research, Project for Developing Innovation Systems of MEXT, and FIRST Program of JSPS. \\[4pt] [1] I. Muneta, H. Terada, S. Ohya, and M. Tanaka, {\it submitted}; arXiv:1208.0575. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R18.00013: Does the physics of (Ga,Mn)N differ from (GaMn)As qualitatively or quantitatively? Is valance of Mn impurity 2+ or 3+? Ryky Nelson, Tom Berlijn, Wei Ku, Juana Moreno, Mark Jarrell (Ga,Mn)N is a promising material for spintronics due to its potential high currie temperature (Tc) [1]. However, unlike for (Ga,Mn)As, some of the experiments on (Ga,Mn)N are still controversial [2,3] on the intrinsic nature of the magnetism. Furthermore, under debate are the spin and charge state of the disordered Mn impurities in (Ga,Mn)N [4,5] and whether its local moments interact via the same exchange mechanism as in (Ga,Mn)As [6,7]. To address these issues we will present ab-initio-based analyses of disorder and correlation via the recently developed Wannier function based methods [8,9]. [1] T. Dietl et al., PRB 63, 195205 (2001) [2] H. Hori et al., Physica B 324, 142 (2002) [3] S. Dhar et al., APL 82, 2077 (2003) [4] A. Titov et al., PRB 72, 115209 (2005) [5] J. I. Hwang et al., PRB 72, 085216 (2005) [6] T. Dietl et al. Science 287, 1019 (2000) [7] K. Sato et al., RMP 82, 1633 (2010) [8] T. Berlijn et al., PRL 106, 077005 (2011) [9] W.-G. Yin et al., PRB 79, 214512 (2009) [Preview Abstract] |
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