Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V12: Focus Session: Magneto-optics and Spin Dynamics |
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Sponsoring Units: GMAG DMP FIAP Chair: Scott Crooker, Los Alamos National Laboratory Room: Colorado Convention Center Korbel 3C |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V12.00001: Determination of effective sp-d exchange integrals in wide-gap DMS Invited Speaker: This work presents a magneto-optical study of diluted magnetic semiconductors (DMS) based on ZnO and GaN and doped with manganese, iron and cobalt. Both host materials, ZnO and GaN, are wide bandgap semiconductors with a wurtzite structure, a weak spin-orbit coupling and a strong electron-hole exchange interaction within the excitons. In the presence of magnetic field, the magnetic ions induce in such materials giant Zeeman effect with no straightforward interpretation, e.g.: excitons anti-cross, and not only the transition energies, but also the oscillator strengths are strongly affected by the giant Zeeman effect. On thin epitaxial layers grown on (0001) sapphire, we observed the giant Zeeman splitting of $A$ and $B$ excitons which are optically active in the Faraday configuration when the propagating light is parallel to the $c$--axis. The Zeeman splitting decreases with the temperature and increases non-linearly with the magnetic field, demonstrating a dependence on the magnetization of the localized spins. A~quantitative analysis allows us to discuss the detailed behavior of the magnetization and to estimate the p-d exchange integral $\beta $ for the studied wide bandgap DMS. For the d$^{5}$ electronic configuration (Mn$^{2+ }$and Fe$^{3+})$ the magnetization follows a Brillouin function B$_{5/2}$, whereas, for d$^{7}$ or d$^{4}$ of Co$^{2+}$ and Mn$^{3+}$ respectively, the spin orbit coupling and the trigonal crystal field lead to an anisotropic magnetization, consistent with that deduced independently from the analysis of intra-ionic optical transitions. We find a positive sign of $\beta $ for GaN:Mn$^{3+}$ and GaN:Fe$^{3+}$. Assuming that the valence band ordering in ZnO is $\Gamma _{9}$, $\Gamma _{7}$, $\Gamma _{7}$ (this corresponds to usuall, positive sign of the spin-orbit coupling), we find $\beta $ to be negative for ZnO:Co$^{2+}$ and ZnO:Mn$^{2+}$. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V12.00002: Magneto-optical Kerr Spectroscopy of GaMnAs C. Sun, D. J. Hilton, J. Kono, H. Munekata, L. Cywinski, L. J. Sham We have performed static and dynamic magneto-optical Kerr measurements on ferromagnetic GaMnAs as a function of temperature, magnetic field, and photon energy. The static Kerr angle at zero magnetic field, which exists only below the Curie temperature, is a strong function of photon energy in the probed range (1.5-2.5 eV), exhibiting a sign change around 2.35 eV. We will present detailed comparison of the experimental spectra with theoretical spectra calculated based on an 8-band k.p model. In two-color ultrafast magneto-optical Kerr measurements, we observe ultrafast demagnetization, similar to what we recently reported for InMnAs [1]. The demagnetization signal (i.e., photoinduced decrease in Kerr angle), which decreases with increasing temperature and vanishes at Curie temperature, has two dynamic components: an ultrafast ($\sim $ 1 ps) drop in magnetization is followed by a slower ($\sim $ 100 ps) demagnetization process. The fast component strongly depends on the pump laser fluence. We will discuss how this dynamics changes with the photon energy and polarization of the pump beam, including both above and below band-gap excitation. 1. J. Wang et al., Phys. Rev. Lett. \textbf{95}, 167401 (2005). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V12.00003: Theory of sub-picosecond light-induced demagnetization in GaMnAs and InMnAs Lukasz Cywinski, L.J. Sham When a (III,Mn)V ferromagnetic semiconductor is excited by a strong laser pulse, its magnetization decreases on a sub- picosecond time-scale [1,2]. We explain such rapid magnetization dynamics by spin-flip scattering due to the sp-d exchange interaction between the hot carriers and the localized spins. We derive the equations for the dynamics of Mn spins and phenomenologically describe the energy and spin relaxation of carriers. For efficient demagnetization a large density of states and short spin relaxation time of carriers is necessary, so that the excited holes cause magnetization quenching. The calculation of demagnetization using spin-flip transition rates derived from the 6 band Luttinger model gives results in qualitative agreement with experiments. \newline 1. J. Wang et al., Phys. Rev. Lett. {\bf 95}, 167401 (2005) \newline 2. J. Wang et al, J. Phys.: Condens. Matter {\bf 18}, R501 (2006) [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V12.00004: Mn ion spin dynamics in GaMnAs quantum wells R.C. Myers, M.H. Mikkelsen, N.P. Stern, A.C. Gossard, D.D. Awschalom We investigate the spin precession of Mn ions within highly diluted GaMnAs-AlGaAs quantum wells grown by molecular beam epitaxy at intermediate temperatures \footnote{R. C. Myers, M. Poggio, N. P. Stern, A. C. Gossard, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{95}, 017204 (2005).}. An exciton bound to the neutral Mn acceptor (A$_{Mn}^{0}$) emits photons at an energy red-shifted from the free exciton emission, thus providing selective optical access to the Mn acceptors within the quantum wells. We observe that the Mn emission can be efficiently oriented using circularly polarized excitation, becoming increasingly efficient for narrow line width excitations close to the exciton absorption edge. In addition, Hanle effect measurements in the quantum wells reveal that the optically-induced polarization of the Mn emission tracks a sharp Lorentzian as a function of magnetic field in the Voigt geometry. We calculate the spin lifetime of the neutral Mn acceptor complex from the width of the Hanle curves and observe an exponential increase in the lifetime with decreasing Mn concentration. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V12.00005: Photo-induced precession of magnetization in ferromagnetic GaMnAs Yusuke Hashimoto, Hiro Munekata Precession of magnetization induced by the pure optical excitation with femto-second light pulses has been reported recently through the study of time-resolved magneto-optical (TRMO) signal in ferromagnetic GaMnAs layers [1]. The present work reports newer TRMO data which were obtained very recently with much precise experimental setups and wider time windows. Three different dynamic behaviors have been found in different time windows; (i) a relatively large TRMO oscillation within 300 ps which shows a strong excitation wavelength dependence, (ii) subsequent oscillatory behavior which lasts up to 1 ns with much weaker excitation wavelength dependence, and (iii) rather long TRMO decay signal in 3 ns without spin precession. Temperature and magnetic-field dependences of the signals indicate that these phenomena are associated with ferromagnetism of the sample. The modeling based on Landau-Lifshitz-Gilbert equation with three different magnetization components suggests that a change in magnetic anisotropy occurs immediately after the optical excitation and decays within 100ps, to which magnetization follows with the precessional motion in the sample plane. [1] H. Takechi et al., presented in PASPS-IV (2006); pss-c in print. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V12.00006: Time Resolved Magneto-Optical Studies of Ferromagnetic InMnSb Films Matthew Frazier, Rajeev Kini, Kanokwan Nontapot, Aliya Gifford, Giti Khodaparast, Tomasz Wojtowicz, Xinyu Liu, Jacek Furdyna Current research activities in the area of ferromagnetic semiconductor have been mainly focused on III-Mn-V alloys with small lattice constants and large effective masses of valence-band such as GaMnAs. Various theoretical models have been proposed to explain the actual mechanism of ferromagnetism in III-Mn-Vs but the microscopic mechanism is still a matter of controversy. It is therefore important to explore the opposite extreme of the III-Mn-V ternaries $i.e.$, InMnSb, which has the largest lattice constant in this family of materials. We report magneto-optical measurements in time domain of photo-induced spin and carrier in InMnSb and compare them to analogous measurements in InBeSb and InSb films. In this work, magneto-optical Kerr effect and standard pump-probe technique provided a direct measure of the photo-excited spin and carrier lifetimes, respectively. Our measurements provide new information on the dynamics and interactions in these materials systems. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V12.00007: Ultrafast Photo-induced Ferromagnetism in III-Mn-V Semiconductors Ingrid Cotoros, Jigang Wang, K.M. Dani, X. Liu, J.K. Furdyna, Daniel Chemla We report on ultrafast photoenhancement of hole-mediated ferromagnetism, and paramagnetic to ferromagnetic phase transition in III-Mn-V semiconductor GaMnAs via laser excited transient carriers. Our femtosecond UV pump/NIR probe vectorial MOKE spectroscopy reveals sub-picosecond demagnetization, precessional trajectory of the magnetization vector, and establishment of the ferromagnetic order on a 100-ps time scale. The dynamic enhancement of the magnetic ordering, manifesting as the photo-induced magnetization below and above the Curie temperature T$_{c}$, is well explained by a transient increase of T$_{c}$ via a population of photo-generated holes. This constitutes the first evidence for an ultrafast, nonthermal manipulation of the magnetic order in ferromagnetic semiconductors, thereby opening up fascinating opportunities for future high-speed spin-photon-carrier integrated devices, and above GHz magneto-optical recording. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V12.00008: Gate-tunability of electron spin precession in an InGaAs quantum well below an interdigitated ferromagnetic grating Gian Salis, Lorenz Meier, Christoph Ellenberger, Emilio Gini, Klaus Ensslin Time-resolved Faraday rotation is used to measure the coherent electron spin precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe grating. We show that the electron spin precession frequency can be modified by applying a gate voltage of opposite polarity to neighboring bars. A tunability of the precession frequency of 0.5~GHz/V is observed. Modulating the gate potential at a gigahertz frequency allows the electron spin precession to be controlled on a nanosecond timescale. Besides the contribution from a spatial displacement of electrons in the inhomogeneous stray field, we also observe spin precession induced by spin-orbit coupling of the moving electrons. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V12.00009: Dynamically Decoupled Precession of Interfacial Electron Spins in Fe/AlGaAs (001) Gunter Luepke, Haibin Zhao, Diyar Talbayev, Aubrey Hanbicki, Connie Li, Berry Jonker We report on the coherent spin precession dynamics of the interface magnetization in Fe/AlGaAs (001) heterostructures using the time-resolved magnetization-induced second-harmonic generation technique, and compare these results with the bulk spin precession dynamics obtained by time-resolved magneto-optical Kerr effect. We have measured the frequency, phase and hysteretic behavior of the precession dynamics of the interface and bulk. Our results clearly show: (a) the coherent precession of the interface magnetization is decoupled from the bulk magnetization precession even at the picosecond time scale; (b) higher frequency spin precession occurs at the interface than in the bulk; (c) the phase of the interface spin precession is opposite to that of the bulk precession at low fields; and (d) the interface and bulk precession exhibit different hysteretic behavior. This is attributed to different magnetization switching processes and vanishingly small exchange coupling between the interface magnetization and the bulk Fe. The higher precessional frequencies observed at the interface for a given field indicate that higher speed performance can be realized in nanoscale magnetic devices where interface properties dominate. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V12.00010: Development of an 8-12 GHz variable frequency microwave resonant cavity for optically-detected magnetic resonance (ODMR) of GaAs-related semiconductors J.S. Colton, L.R. Wienkes, L.R. Oestreich, P.M. Schroeder In order to do spin echo measurements of the T2 spin coherence time in GaAs and related materials, one must have a resonant microwave cavity; the resonance serves mainly to increase the strength of the magnetic field at the sample. In order to probe materials with g-factors as low as $\vert $g$\vert $ = 0.1 (such as quantum-confined GaAs samples, and bulk/quantum-confined AlGaAs alloys) in moderate magnetic fields ($<$ 7 T), we have selected a resonant frequency of around 10 GHz for the resonator design. In order to fit a 10 GHz cavity into moderately-sized ($<$ 1 inch) magnet bore, however, a high dielectric material must be used, a so-called ``dielectric resonator''. We will present the design and testing of such a resonant cavity (resonant mode: TE011), that additionally allows for optical access of the sample, as well as a highly-variable resonant frequency. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V12.00011: Simple Model for the Photon Energy Dependence of Optical Nuclear Polarization in GaAs Patrick Coles, Jeffrey Reimer It was discovered forty years ago that nuclear Zeeman order can be induced by prolonged irradiation of a bulk semiconductor with near-band-gap light. The microscopic mechanism of this optical nuclear polarization process is still under debate. Yet, we are able to predict a major feature of the data, the dramatic photon-energy dependence, from a simple model, irrespective of the spin microscopics. We discuss this model in the context of semi-insulating GaAs. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V12.00012: Localization of excitons in Cd$_{1-x}$Mn$_{X}$Te semimagnetic semiconductors near temperature of phase transition : paramagnetic-spin glass Yurij Gnatenko, Petro Bukivskij Excitons in semimagnetic semiconductors become trapped by fluctuations of the crystal field as in ordinary solid solution based on II-YI semiconductors. In addition, magnetic-polaron effects are seen in these crystals. These effects stem from an exchange interaction of electrons and holes bound in excitones with d electrons of the magnetic ions. As a result, there is an additional trapping of excitons. At magnetic ion concentration X$>$0.10 the trapping of excitons not only by fluctuation of the crystal-field potential but also by fluctuations of the crystal magnetization become important. In this paper we are reporting the study of the emission of excitons trapped by magnetization fluctuations at temperatures from 1.8 K to 100 K. Analysis the form and the temperature dependence of exciton band position allowed us to determine the temperature of the phase transitions to a spin-glass state in Cd$_{1-X}$Mn$_{X}$Te (X=0.40) semimagnetic semiconductors which corresponds to T$_{SG}$=14.5 K. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V12.00013: Magnetoplasmon excitations in a quasi-two-dimensional Rashba spintronic systems: Oscillations, resonances, and energy gaps Manvir S. Kushwaha We report on the theoretical investigation of plasmon excitations in a quasi-two-dimensional electron gas (Q2DEG) in the presence of a perpendicular magnetic field and spin-orbit (SO) interaction induced by the Rashba effect. We derive and discuss the dispersion relations for charge-density excitations within the framework of Bohm-Pines' random-phase approximation (RPA). The magnetoplasmons in a 2DEG are known to be characterized by two important properties: (i) the oscillatory behavior of the dispersion curves in the short wavelength limit (SWL), and (ii) the resonance splitting at the frequency $\omega = n \omega_c$ in the long wavelength limit (LWL); $n$ ($\ge 2$) being an integer and $\omega_c$ the cyclotron frequency. Here we study the effect of the Rashba spin-orbit interactions (SOI) on these characteristics in depth. We observe that the SOI modifies drastically both the oscillatory behavior in the SWL and yields multiple resonance splittings [at $\omega = (n \pm x_0) \omega_c$] in the LWL. Such resonance splittings make the spintronic systems potential candidates for quantum-well-based new devices as spin filters. We discuss the dependence of the magnetoplasmon energy on the propagation vector, the magnetic field, the 2D charge-density, and the Rashba parameter characterizing the SOI. [Preview Abstract] |
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