Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S10: Focus Session: Magnetic Semiconductor Heterostructures |
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Sponsoring Units: DMP GMAG Chair: Jing Shi, University of Utah Room: LACC 153B |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S10.00001: Magnetomechanical Dissipation in (Ga,Mn)As Nanoelectromechanical Resonators E.B. Myers, S. Masmanidis, H.X. Tang, M. Li, M.L. Roukes, K. De Greve, Geert Vermeulen, W. Van Roy When the magnetization of a ferromagnet is changed (e.g., by applying a magnetic field), a stress develops in the material that can change its shape; this is the well-known magnetostriction effect. The magnetoelastic coupling that governs this effect operates conversely as well, in that an applied stress can alter the magnetization state of the material (the magnetomechanical effect). We have studied this coupling in MHz-range nanoelectromechanical resonators fabricated out of the dilute magnetic semiconductor (Ga,Mn)As. We find that the resonator quality factor and piezoresistive signal amplitude vary strongly with both the magnitude and direction of an applied magnetic field. The data can be interpreted in terms of local AC strain in the resonator driving the Mn moments into heavily damped motion at the resonator frequency; comparisons to magnetoelastic theory will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S10.00002: In-plane uniaxial anistropy in epitaxial InMnAs on GaSb determined using the magneto-optical Kerr effect Philip Chiu, Steven May, Bruce Wessels The in-plane magnetic anisotropy of (001) oriented InMnAs/GaSb layers grown by metal organic vapor phase epitaxy was investigated by the longitudinal magneto-optic Kerr effect. The films exhibited ferromagnetic hysteresis at room temperature. An In$_{0.96}$Mn$_{0.04}$As film with a thickness of 50 nm was tetragonal and coherently strained to the GaSb substrate. The c-axis lattice constant was 1.008 times smaller than that of GaSb, in agreement with elasticity theory. These films exhibit uniaxial in-plane anisotropy with the easy axis of magnetization along the [110] direction. Along the easy axis, a square hysteresis loop was obtained with a coercive field of 990 Gauss. In contrast, an In$_{0.93}$Mn$_{0.07}$As film with a thickness of 320 nm was incoherent. The c-axis lattice constant matched that of bulk In$_{0.93}$Mn$_{0.07}$As. The shape of the hysteresis loops for the fully relaxed film was invariant with respect to in-plane rotation. The coercive field for the unstrained InMnAs film was 280 Gauss. The large difference in coercive fields between the two samples is attributed to the disparity in the uniaxial anisotropy constant ($K_{u})$. In addition, the observation of uniaxial anisotropy for only coherently strained films indicates that the anisotropy is due to the twofold symmetry of a zinc-blende (001) substrate surface. These results are consistent with a previous study of magnetic anisotropy of InMnAs epitaxial films grown on InAs and GaAs (001) substrates.$^{ }$(Chiu APL 2004) [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S10.00003: Studies of the magnetic properties of Amorphous Mn-doped GaAs Thin Films W.A. Iwamoto, R.R. Urbano, C. Rettori, P.G. Pagliuso, J.H.D. da Silva, Andr\'e L.J. Pereira, S.B. Oseroff Recently Mn-doped GaAs films have become compounds of great interest due to their potential technological application. In this work, we report experiments of magnetic susceptibility and Electron Spin Resonance (ESR) of Mn$^{2+}$ ion in amorphous thin films of GaAs doped with different concentrations of Mn (0.5 -- 10{\%}) and hydrogenated films with the same Mn concentration. A single nearly temperature independent g $\sim $ 2 line is observed for the Mn-doped films. The presence of Hydrogen is also verified in the ESR spectra by three narrows g $\sim $ 2 lines presumably due to s=1/2 centers with and without hyperfine splitting. The origin of these s=1/2 centers is unclear. Furthermore, our measurements show the absence of the ferromagnetic ordering for all measured films, in contrast to ferromagnetic ordering observed in crystalline films for Tc$\sim $ 110 K [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S10.00004: Heterojunction Engineering of Semiconductor Ferromagnetism Invited Speaker: While bandgap engineering and wavefunction engineering are established in \textit{nonmagnetic} semiconductor heterostructures, we aim to broaden their field to \textit{magnetic} heterostructures and to extend the degree of freedom in designing spin-related properties in semiconductors. In this study, we introduced delta-doping of magnetic (Mn) impurities in the quasi two-dimensional hole gas at the interface of GaAs/$p$-AlGaAs heterostructures, and successfully maximized the ferromagnetic order among the Mn spins by overlapping the hole wavefunction with the Mn delta-doping profile. Selectively $p$-doped heterostructures (Mn-delta-doped GaAs / Be-doped AlGaAs) were prepared by molecular beam epitaxy, in which holes are supplied from the Be-doped $p$-AlGaAs layer to the Mn-delta-doped channel, and ferromagnetic ordering was clearly observed in magnetotransport measurements [1]. In the heterostructure prepared with proper conditions, its ferromagnetic transition temperature ($T_{c})$ was 172 K, higher than the $T_{c}$ of InAs- or GaAs-based random-alloy magnetic semiconductors [2]. It was also found that in more suitably designed heterostructures with low-temperature annealing, $T_{c}$ can be higher than 200 K [3]. Furthermore, we show the control of ferromagnetism in the heterostructures by using gate electric field and light irradiation at relatively high temperatures ($\sim $100 K) [4]. [1] A. M. Nazmul, S. Sugahara, and M. Tanaka, Appl. Phys. Lett. \textbf{80}, 3120 (2002). [2] A. M. Nazmul, S. Sugahara, and M. Tanaka, Phys. Rev. \textbf{B67}, 241308(R) (2003). [3] A. M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, submitted. [4] A. M. Nazmul, S. Kobayashi, S. Sugahara, and M. Tanaka, Jpn. J. Appl. Phys. \textbf{43}, L233 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S10.00005: Anomalous Hall Effect in a 2DEG Containing Magnetic Impurities John Cumings, David Goldhaber-Gordon, Keh-Chiang Ku, Gang Xiang, Nitin Samarth The has been great interest recently in the role of electron spin in transport properties, specifically in the phenomena of the spin-Hall and anomalous-Hall effects. We present low- temperature transport measurements on a modulation-doped quantum well containing magnetic (Mn) impurities. The effect of the impurities is to cause a strong (free-spin) paramagnetic Zeeman- splitting of the carriers due to s-d exchange interactions. In recent measurements, we observe a Hall voltage that scales with the magnetic moment of the material, a signature of the anomalous-Hall effect. This is surprising, given that the material is not ferromagnetic and does not have a magnetically- ordered ground state. This has important implications for spin- transport in a large class of two-dimensional electron systems. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S10.00006: Coherent Spin Waves in Thin-Film GaMnAs D. M. Wang, Y. H. Ren, R. Merlin, K. Dziatkowski, X. Liu, J. K. Furdyna, M. Grimsditch We report on the observation of coherent oscillations associated with standing spin waves in thin films of the ferromagnetic semiconductor GaMnAs. The oscillations were observed in differential magnetic Kerr measurements using a standard pump-probe setup. Subpicosecond pump pulses from a Ti: sapphire laser induce a coherent precession of the magnetization which is detected by measuring the rotation of the polarization of the delayed probe pulses. The magnetic anisotropy and spin stiffness constants ($D)$ were determined from the magnetic-field dependence of the spin-wave frequencies. We obtain $D=0.7\pm 0.1\times 10^{-13}T\cdot cm^2$ for two as-grown samples ($T_{C~}$= 65 K) and $D=3.5\pm 0.5\times 10^{-13}T\cdot cm^2$ for an annealed sample with $T_{C~}$= 80 K. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S10.00007: Ultrafast Photoinduced Demagnetization in Ferromagnetic InMnAs Chanjuan Sun, Jigang Wang, Junichiro Kono, Akira Oiwa, Hiroo Munekata Mid-infrared pump-probe magneto-Kerr rotation measurements have been performed on ferromagnetic InMnAs in external magnetic fields. Systematic pump power and temperature dependent studies showed ultrafast demagnetization induced by the pump laser pulses. Complete magnetization quenching was observed at high pump powers, implying a photoinduced phase transition from the ferromagnetic state to the paramagnetic state on a femtosecond time scale. During the demagnetization process, three different time scales were revealed, indicating different mechanisms for magnetic order quenching. Interplay among different energy transfer channels will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S10.00008: Theory of carrier dynamics and coherent phonons in ferromagnetic semiconductor heterostructures G.D. Sanders, C.J. Stanton, J. Wang, C. Sun, J. Kono, A. Oiwa, H. Munekata We model generation and propagation of coherent acoustic phonons in two color time resolved reflectivity experiments on ferromagnetic InMnAs quantum wells with thick GaSb barriers grown on a GaAs substrate. Carriers are created in the InMnAs by ultrafast pumping below the GaSb band gap while differential reflectivity is measured above the GaSb gap. The electronic states in the ferromagnetic InMnAs are obtained in an effective mass model and the generation and subsequent relaxation of photogenerated carriers in the well are treated in a Boltzmann equation formalism. Coherent acoustic phonons are generated in the quantum well via electron-phonon interaction with photogenerated carriers. These propagate into the GaSb bulk layer at the LA sound speed modifying the optical properties of the GaSb layer and giving rise to an oscillatory component in the differential reflectivity with a period of $\sim 23$ ps. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S10.00009: Spin relaxation in InGaN/Ga(Mn)N quantum wells W.M. Chen, I.A. Buyanova, K. Nishibayashi, K. Kayanuma, K. Seo, A. Murayama, Y. Oka, G. Thaler, R. Frazier, C.R. Abernathy, S.J. Pearton, Jihyun Kim, F. Ren, F.V. Kyrychenko, C.J. Stanton, C. -C. Pan, G. -T. Chen, J. -I. Chyi, J. M. Zavada Spin relaxation processes in InGaN/GaN and InGaN/GaMnN multi-quantum wells are studied by transient magneto-optical spectroscopy. Nearly no photoluminescence (PL) polarization was observed immediately after pulsed laser excitation (t=0), regardless of the polarization of the excitation light. Afterwards PL gradually becomes $\sigma ^+$ polarized in an applied magnetic field. This polarization build-up is shown to correspond to an additional decay process (50 ps) of the $\sigma ^-$ PL component. With the aid of the exciton Hamiltonian and rate equations, we show that fast spin relaxation ($<$20 ps) is partly responsible for the vanishing optical polarization at t=0. The fast spin relaxation is attributed to carrier spin relaxation at high K-vectors dominated by the D'yakonov-Perel' (DP) mechanism. When the excitons are at rest (K=0), the DP spin relaxation is suppressed leading to a slower spin relaxation (50 ps). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S10.00010: Digital Ferromagnetic Heterostructure Composed of Mn in Si C. Y. Fong, M. C. Qian We examine the electronic and magnetic properties of digital ferromagnetic heterostructures (DFH) composed of Mn atoms confined in a layer of Si by using an \textit{ab-initio} algorithm. The $\delta $-layer can have the composition of Mn$_{1-x}$Si$_{x}$ with x=0 for 1ML, x=0.5 for 0.5ML, and x=0.75 for 0.25ML coverage of Mn. Each DFH is simulated by a tetragonal supercell formed by stacking 8 cubes of the Si crystal in the [100] direction. For each supercell, the value of x determines the area of the plane perpendicular to the [100] direction. For all cases, the total energy difference between the ferro- and antiferro-magnetic phases, the magnetic moments for the ferromagnetic phase, and the density of states at the Fermi energy as functions of x will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S10.00011: Memory function formalism for the optical response of diluted magnetic semiconductors Fedir V. Kyrychenko, Carsten A. Ullrich Because of the possibility to act as effective spin injectors, diluted magnetic semiconductors (DMSs) play a significant role for spintronics applications. Detailed investigation of the transport and optical properties of these materials is thus important for theory and experiment. Present theoretical calculations of the electrical conductivity and dielectric function in DMSs are based on the relaxation time approximation. Usually, the relaxation time is treated as an adjustable parameter or is calculated within the weak scattering approach. However, real III-V DMS samples are highly disordered, which casts doubt on the validity of the relaxation time approximation in general, and the weak scattering approach in particular. We go beyond the relaxation time approximation and treat disorder in DMSs in terms of the memory function formalism. As an example, we present calculations of the dielectric function in the infrared range and compare with the relaxation time approximation and with experimental data. This work is supported by DOE Grant DE-FG02-04ER46151. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S10.00012: First-principles study of ferromagnetic coupling in Zn$_{1-x}$Cr$_x$Te thin film P. Jena, Qian Wang, Qiang Sun Using gradient corrected density functional theory and supercell technique, we have calculated total energies, electronic structure, and magnetic properties of Cr doped ZnTe in both bulk and thin film configurations. Calculations with full geometry optimization for Zn$_{1-x}$Cr$_{x}$Te supercell were carried out for different Cr concentrations (x=0.095, 0.143, and 0.19) and by varying the sites Cr atoms occupy. We show that the ferromagnetic phase of Zn$_{1-x}$Cr$_{x}$Te in both bulk and thin film is energetically the most preferable state irrespective of the concentration and/or site occupation of Cr atom. The strong hybridization between Cr-3$d$ and Te-5$p$ states is found to be responsible for the ferromagnetic coupling - in agreement with recent experiments. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S10.00013: Magnetic Field Dependence of the Specific Heat of Pb$_{1-x}$Gd$_{x}$Te M. Gorska, J.R. Anderson, Y. Dagan, A. Lusakowski, A. Jedrzejczak, Z. Golacki We have measured the magnetic specific heat, C$_{m}$, of Pb$_{1-x}$Gd$_{x}$Te (x = 0.033 and 0.054) at magnetic fields up to 2 T at temperatures from 0.4 to 9 K. This study is complementary to our magnetization measurements on the same system. The samples were prepared as bulk single crystals by the Bridgman method and were n-type with carrier concentrations of about 1 x 10$^{19}$ cm$^{-3}$. The specific heat measurements were made using a Quantum Design PPMS system. We saw evidence for a maximum in C$_{m}$, which shifted to higher temperatures and became broader with increasing magnetic field. At zero magnetic field this maximum is expected to occur below 0.4 K for both samples and will be several times higher than that predicted by a model of superexchange interactions between nearest neighbors, which was based on previous experiments on Pb$_{1-x}$Eu$_{x}$Te.$^{1}$ The present data will be compared with a model that takes into account the spin splitting of the ground state of the individual Gd ions, possibly due to local lattice distortions. $^{1}$ M. G\'{o}rska, A. Lusakowski, A. Jedrzejczak, Z. Golacki, J. R. Anderson, H. Balci, Acta Phys. Pol. A \textbf{105}, 631 (2004). [Preview Abstract] |
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