Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T22: Focus Session: GaMnAs |
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Sponsoring Units: GMAG DMP FIAP Chair: Nitin Samarth, Penn State University Room: 324 |
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T22.00001: Electric-field manipulation of magnetization vector direction Invited Speaker: Ferromagnetism and magnetization in Mn-doped III-V semiconductors can be manipulated by various means; by changing its carrier concentration by electric fields [1] or by spin- current flowing along with the electric current [2]. This material system is thus an excellent system to study the physics involved in manipulation of magnetism as well as exploring new ways to control magnetization. Here, we show that electrical control of magnetization direction can be done through manipulating electronically the magnetic anisotropy energies [3]. The basic idea behind the effort is to control the population of carriers on spin-split anisotropic valence bands that governs the magnetic anisotropy energies, which should result in change of the direction of magnetization. In order to measure the magnetic anisotropies under a gate that applies the electric-field to the ferromagnetic semiconductor channel, we used the planar Hall effect. Analyses showed that there are biaxial as well as uniaxial anisotropies. As the sheet carrier concentration is reduced by applying electric- field to the channel, the uniaxial anisotropy field reduced its magnitude and eventually changed its sign, whereas no significant change was apparent in the biaxial anisotropy field. From the electric-field dependent anisotropy fields, one can show that the angle of the magnetization direction in the absence of magnetic fields is modulated by electric-fields by 10 degrees. This opens up a new and unique opportunity for manipulating magnetization direction solely by electronic means, not resorting to magnetic-field, spin-current, mechanical stress, nor multiferroics. The conditions for switching the magnetization direction will also be discussed. The work was done together with D. Chiba, F. Matsukura, M. Sawicki, Y. Nishitani, and Y. Nakatani. \\[4pt] [1] H. Ohno, et al. Nature 408, 944 (2000). D. Chiba, et al. Science, 301, 943 (2003). D. Chiba, et al. Appl. Phys. Lett. 89, 162505 (2006). \\[0pt] [2] M. Yamanouchi, et al. Nature 428, 539 (2004). M. Yamanouchi, et al. Phys. Rev. Lett. 96, 096601 (2006). M. Yamanouchi, et al. Science 317, 1726, (2007). \\[0pt] [3] D. Chiba, et al. Nature 455, 515 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T22.00002: Bias-controlled ferromagnetism in quantum wells with Mn-delta doping Erika Dias Cabral, Rafal Oszwaldowski, Marco Boselli, Igor Zutic, Ivan da Cunha Lima Carrier-mediated magnetism in semiconductors shows important and potentially useful differences from magnetism in metals [1] such as light- or bias-controlled ferromagnetism [2-3]. Motivated by experiments reporting in GaAs quantum wells (QWs) with Mn-delta doping higher Curie temperatures (T$_{C})$ than in bulk (Ga,Mn)As [4], we explore theoretically the feasibility of bias-controlled ferromagnetism in QWs. We calculate self-consistently indirect Mn-Mn exchange interaction [5] and apply a Monte Carlo approach to calculate T$_{C}$. Our approach allows us to systematically study the effects of quantum confinement and the position of the Mn layer on magnetic ordering and T$_{C}$, beyond the mean field approximation, which we obtain as the limiting case. We compare our findings with the experimental results and suggest paths towards improved control of ferromagnetism. Supported by CNPq, FAPEMIG, FAPERJ, CAPES, US ONR, and NSF-ECCS Career. [1] I. Zutic et al., Rev. Mod. Phys. 76, 323 (2004). [2] S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997). [3] H. Ohno et al., Nature 409, 944 (2000). [4] A. M. Nazmul et al., Phys. Rev. Lett. 95, 017201 (2005). [5] M. A. Boselli et al., Phys. Rev. B 68, 085319 (2003). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T22.00003: STM studies of an atomic-scale gate electrode formed by a single charged vacancy in GaAs Donghun Lee, David Daughton, Jay Gupta Electric-field control of spin-spin interactions at the atomic level is desirable for the realization of spintronics and spin-based quantum computation. Here we demonstrate the realization of an atomic-scale gate electrode formed by a single charged vacancy on the GaAs(110) surface[1]. We can position these vacancies with atomic precision using the tip of a home-built, low temperature STM. Tunneling spectroscopy of single Mn acceptors is used to quantify the electrostatic field as a function of distance from the vacancy. Single Mn acceptors are formed by substituting Mn adatoms for Ga atoms in the first layer of the $p$-GaAs(110) surface[2]. Depending on the distance, the in-gap resonance of single Mn acceptors can shift as much as 200meV. Our data indicate that the electrostatic field decays according to a screened Coulomb potential. The charge state of the vacancy can be switched to neutral, as evidenced by the Mn resonance returning to its unperturbed position. Reversible control of the local electric field as well as charged states of defects in semiconductors can open new insights such as realizing an atomic-scale gate control and studying spin-spin interactions in semiconductors. http://www.physics.ohio-state.edu/{\$}$\backslash $sim {\$}jgupta [1] D. Lee and J.A. Gupta (in preparation) [2] D. Kitchen et al., Nature \textbf{442}, 436-439 (2006) [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T22.00004: Probing the nature of electronic state near the Fermi level in Ga$_{1-x}$Mn$_{x}$As with STM Anthony Richardella, Pedram Roushan, Shawn Mack, David Awschalom, Ali Yazdani We have studied the electronic states near the Fermi energy in GaMnAs/GaAs heterostructures as a function of doping across the metal-insulator transition. These measurements allow us to determine the position of E$_{F}$ with respect to the valance band edge and in gap states related to the Mn induced acceptor states. As the doping level increases we observe an increase in the density of states at the Fermi energy and map their spatial dependence. Statistical analysis of these measurements can be used to find a characteristic length scale associated with growth of bulk metallic behavior for these samples. In addition, our measurements indicated a suppression of the density of states near E$_{F}$ at all doping levels, consistent with that expected for correlation effects in doped semiconductors near the metal-insulator transition. We will discuss these findings and their relation with various theoretical models for electronic states in GaMnAs that are expected to mediate the magnetic interaction in this compound. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T22.00005: Scanning Tunneling Microscopy Studies of Mn acceptor levels in Ga$_{1-x}$Mn$_{x}$As Pedram Roushan, Anthony Richardella, Shawn Mack, David Awschalom, Ali Yazdani We have used a low temperature scanning tunneling microscope (STM) to perform studies of GaMnAs/GaAs heterostructures with various Mn dopant concentrations. The STM topography of the GaMnAs showed a variety of electronic structure modulations on the order of a few nm indicating the presence of a high level of disorder and compensation. These measurements show no indication of Mn clustering as the Mn concentration is increased. On both sides of the Metal-Insulator Transition (MIT), the differential conductance (dI/dV) measurements on Mn dopants showed a broad acceptor level above 100meV from the valence band edge. Furthermore, we have mapped in energy the spatial variations of these deep acceptor levels, and their distribution will be presented for all Mn concentrations studied. The effect of disorder and coulomb correlations in modifying the local density of states close to Fermi level will be discussed for insulating as well as metallic samples. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T22.00006: Theory of STM spectroscopy in Mn clusters on GaAs surfaces. Tor Olof Strandberg, Alan MacDonald, Carlo Canali Small numbers of Mn atoms can be manipulated into arbitrary spatial arrangements on the $<110>$ surface of GaAs by means of a novel STM atom-by-atom substitution technique, which enables the replacement of individual Ga atoms by Mn [1]. The tunneling differential conductance over an isolated Mn atom reveals a large and broad resonance in the GaAs energy gap. For a Mn pair placed less than 1 nm apart, the resonance splits into two peaks, whose spacing is thought to be related to the exchange-energy interaction between Mn ions. We report on theoretical results for the local density of states and the Mn acceptor-level splittings for a Mn dimer, based on a tight-binding model of Mn substitutions on the $<110>$ GaAs surface. We compare our model with previous work which does not account for the surface. We then derive an effective quantum spin Hamiltonian for the Mn cluster, based on a Chern number theory developed recently, which includes Berry phase effects [2]. We study the transition from surface to bulk for the substitutional Mn impurity in GaAs as well as Mn-Mn interactions at the surface and in bulk at various distances and along different crystalline directions. [1] D. Kitchen et al., Nature 442, 436 (2006). [2] C.M. Canali, A. Cehovin and A.H. MacDonald, Phys. Rev. Lett. 91, 046805 (2003) [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T22.00007: Strong Magnetic Circular Dichroism in Mn Delta-doped GaAs Nazmul Ahsan, Sanjukta Ghosh, Masaaki Tanaka Delta-doping of magnetic impurities ($i.e.$ Mn) in III-V semiconductors allows locally high concentration of magnetic moments. This can lead to systematic observation of fundamental properties of the system including the enhancement of the Curie temperature and magnetic anisotropy as a function of a wider range of Mn concentration[1]. The delta-doped Mn atoms in the MBE-grown GaAs-based heterostructures are abruptly confined as confirmed by high resolution transmission electron microscopy studies[1]. Here we study the magnetic circular dichroism (MCD) of 1 monolayer (ML) Mn delta-doped GaAs layer. The structure from the growth sequence is: GaAs substrate/GaAs-buffer/Al$_{0.9}$Ga$_{0.1}$As/Be-doped Al$_{0.3}$Ga$_{0.7}$As/1nm GaAs/1ML Mn/10nm GaAs cap. The sample was chemically etched to single out the 1nm GaAs/1ML Mn layer/10nm GaAs cap to measure MCD spectra in the transmission geometry. We observed strong MCD features even at 300K, indicating ferromagnetism with zinc-blende band structure. Ref.: [1] Nazmul \textit{et al}. Phys. Rev. B \textbf{67}, 241308 (2003); J. Crystal Growth, \textbf{251}, 303 (2003); Phys. Rev. Lett. \textbf{95}, 017201 (2005); Phys. Rev. B \textbf{77}, 155203 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T22.00008: Magneto-Optical Kerr Spectroscopy of Moderately Doped GaMnAs: Evidence for Interband Transitions C. Sun, J. Kono, Y.H. Cho, A.A. Belyanin, H. Munekata The role of impurity bands as well as the nature of free holes in carrier-mediated ferromagnetism in (III,Mn)V systems are still not well understood. Previous magneto-optical studies of GaMnAs have produced an array of conflicting results, especially in terms of the nature of optical transitions involved. Here, we have performed systematic magneto-optical Kerr spectroscopy studies of GaMnAs samples with different doping densities. The Kerr angle strongly depended on the photon energy, showing positive peaks at 1.7 eV and 3 eV and a negative peak at 2.5 eV. The 1.7 eV peak clearly shifts to higher energies with Mn doping from 1{\%} to 2.4{\%} and shifts to an even higher energy after annealing. We attribute these changes to the increased hole density and effective Mn content. A 30-band {\boldmath $k \cdot p$} model with exchange interaction is adopted to simulate the spectra. The excellent agreement between the experiment and calculation leads us to conclude that Kerr rotation in GaMnAs above the band gap is dominantly determined by interband transitions. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T22.00009: Below band gap Faraday and Kerr measurements in ferromagnetic GaMnAs Gheorghe Acbas, M.-H. Kim, J. Cerne, M. Cukr, V. Novak, T. Jungwirth, M.A. Scarpulla, O.D. Dubon, J. Sinova We have studied the Faraday and Kerr effects in a series of ferromagnetic GaMnAs films in the 115-1500 meV energy range. This provides a direct magneto-optical probe of the valence band, which is critical to understanding the ferromagnetic origin in this material. Previous magneto-optical studies probed the region around the band gap (1.5 eV) where numerous contributions to the optical response were inferred: ferromagnetic, paramagnetic, valence band, impurity bands, intra-d level or intra-gap defect levels. We compare our experimental results with predictions from mean field Zener model calculations. We find that the results are consistent with a picture in which the Fermi level resides inside the spin split valence band. Many-body band renormalization effects have to be included in order to model the results quantitatively. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T22.00010: Local Magnetic Characterization of Continuous (Ga,Mn)As Film using Mechanical Force Detection I. H. Lee, Yu. Obukhov, J. Kim, X. Li, N. Samarth, D. V. Pelekhov, P. C. Hammel We report on low temperature (T = 4.2 K) studies of the local spin dynamics in ferromagnetic samples using Ferromagnetic Resonance Force Microscopy (FMRFM) and probe-induced Magnetic Force Microscopy (MFM). Both techniques are based on sensitive mechanical detection of the dipolar magnetic interaction between a micromagnetic probe mounted on a flexible micro-cantilever and magnetic moments in the sample. The probe magnet not only detects the magnetic force, but also perturbs sample spin magnetization by adding the strongly inhomogeneous magnetic field. We demonstrate that the combination of FMRFM and probe-induced MFM can be used to extract and map local magnetic properties of a continuous (Ga,Mn)As film such as saturation magnetization and anisotropy field. These new approaches to scanned magnetic force imaging open the door to powerful new tools for spatially resolved studies of nanoscale magnetism and spin-based devices. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T22.00011: Simultaneous Optical Imaging and Electrical Control of Magnetization in (Ga,Mn)As M.E. Nowakowski, G.D. Fuchs, D.D. Awschalom, A. Balk, M.J. Wilson, N. Samarth Spin dependant phenomena in metals and semiconductors promises the development of low-power logic and memory devices based on electrical control of the magnetization. To realize this potential, precise visual information of magnetic domains is required to design and control electrical structures manipulated by the spin transfer torque. We present studies of magnetization behavior in micron-scale (Ga,Mn)As channels using a recently developed video-rate magneto-optical Kerr effect microscope. Measurements record real-time, diffraction-limited, surface magnetization information including magnetic switching and domain wall motion. The optical measurements are correlated with simultaneous electrical measurements to provide insight into pinning and magnetization transport in these structures. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T22.00012: Optical conductivity of diluted magnetic semiconductors: effects of dynamical screening Fedir Kyrychenko, Carsten A. Ullrich Most theoretical studies of transport and optical conductivity in diluted magnetic semiconductors like GaMnAs treat disorder and many-body effects within the simple relaxation time and static screening models. Here we present a more complete theory of transport in charge and spin disordered media that combines a multiband ${\bf k \cdot p}$ approach with a first-principles descriptions of disorder and electron-electron interaction through the memory function formalism and time-dependent density functional theory. We discuss the effects of dynamic screening and collective electron excitations on the charge and spin scattering off Coulomb impurities and fluctuations of localized spins and compare calculated values of optical conductivity in GaMnAs with experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T22.00013: Infrared probe of Ga$_{1-x}$Mn$_{x}$As films with controlled disorder and compensation Brian Chapler, R.C. Myers, S. Mack, D.D. Awschalom, M.C. Martin, A. Dattelbaum, K.S. Burch, D.N. Basov Arsenic antisite defects (As$_{Ga})$ formed due to low temperature growth conditions are a leading cause of disorder and compensation in Ga$_{1-x}$Mn$_{x}$As. Samples grown with gradient As:Ga growth condition for 0.005$<$x$<$ 0.16 have allowed for optimized As flux minimizing As$_{Ga}$. By studying samples at this optimized location via infrared spectroscopy, a new level of precision can be attained in exploring the electronic structure and other intrinsic properties of Ga$_{1-x}$Mn$_{x}$As samples. Using optical sum rule analysis of our experimentally determined optical conductivity ($\sigma _{1}(\omega ))$, we extract the free carrier band mass (m*) and find it to be several m$_{e}$. We also comment on the levels of interstitial Mn (Mn$_{i})$, finding for x $>$ 0.03 roughly 25{\%} of Mn resides at an interstitial location. Additionally, by probing positions along the As:Ga gradient we directly measure the effects of disorder and compensation on these samples. Systematic changes in $\sigma _{1}(\omega )$ as As$_{Ga}$ content is increased are reported, and the consequences of this on our understanding of the electronic structure of Ga$_{1-x}$Mn$_{x}$As are discussed. [Preview Abstract] |
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