Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H19: Focus Session: III-V Magnetic Semiconductors II |
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Sponsoring Units: GMAG DMP Chair: Nitin Samarth, Pennsylvania State University Room: Baltimore Convention Center 316 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H19.00001: Optical Properties of Ferromagnetic Semiconductors Invited Speaker: Ferromagnetic semiconductors hold great promise for numerous magneto-optics applications. In this talk I detail recent optical spectroscopic studies of as grown and annealed thin films and digitally doped superlattices of Ga$_{1-x}$Mn$_{x}$As, prepared in the group of D.D. Awschalom (UCSB) and annealed in the group of N.Samarth (PSU). Annealing induces a large strengthening of the optical conductivity ($\sigma_{1}(\omega)$), while the frequency dependence of $\sigma_{1}(\omega)$ remains unchanged. This indicates that the scattering rate and Fermi level have not been effected by annealing, despite the large increase in hole density. Our Infrared work on Digital Ferromagnetic Heterostructures reveals a unique ability to tune their optical properties as well as their intrinsic electronic structure without changing the doping/defect level. This work is in collaboration with D.B. Shrekenhamer, E.J. Singley, D.N. Basov (University of California, San Diego) J. Stephens, R.K. Kawakami, D.D. Awschalom(University of California, Santa Barbara), B.L. Sheu, and N. Samarth (Pennsylvania State University). [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H19.00002: Faraday and Kerr effect studies in optimally annealed Ga$_{1-x}$Mn$_{x}$As random alloys. Gheoghe Acbas, A. Markelz, J. Cerne, M. Cukr, V. Novak, J. Sinova The mid-infrared (MIR: 115-366 meV) complex Faraday and Kerr effect are studied in Ga$_{1-x}$Mn$_{x}$As random alloy films (x = 0.05, and x = 0.07) as a function of frequency and temperature. The samples are optimally annealed with T$_{C}$ around 100K. The strong MIR magneto-optical response shows clear ferromagnetic behavior that is consistent with dc magnetization measurements. The real and imaginary parts of the measured Faraday and Kerr angles are in qualitative and quantitative agreement with the values predicted by an effective Hamiltonian model within a mean field treatment [E. M. Hankiewicz , J. Sinova et al., PRB 2004]. The Kerr and Faraday rotation peak at approximately 220 meV, which is consistent with theoretical predictions. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H19.00003: Ferromagnetic resonance (FMR) study of ultra-thin Ga$_{1-x}$Mn$_{x}$As films as a function of layer thickness Yingyuan Zhou, Y.J. Cho, Z. Ge, X. Liu, M. Dobrowolska, J.K. Furdyna We report a detailed study of ferromagnetic resonance (FMR) in a series of Ga$_{1-x}$Mn$_{x}$As (x = 0.05) thin films grown on (100)GaAs substrates by low-temperature MBE. The thicknesses of the specimens ranged from 10nm to 200nm. We measured angular and temperature dependences of FMR and fitted the data to obtain the cubic and the uniaxial anisotropy parameters and g- factors for each specimen. The quality of the theoretical fitting results clearly indicates that FMR data can be reliably used to determine magnetic anisotropy parameters of Ga$_{1-x} $Mn$_{x}$As. Our experiment indicates a strong thickness dependence of the in-plane uniaxial anisotropy field; on the other hand, the cubic anisotropy field, perpendicular uniaxial anisotropy field, and the g-factors depend on the hole concentration, but show little if any dependence on the film thickness, at least for as-grown specimens. The measurements and analysis of annealed specimens are presently underway. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H19.00004: Ultrafast light-induced demagnetization in (III,Mn)V ferromagnetic semiconductors L. Cywinski, L. J. Sham, J. Wang, C. Sun, J. Kono, A. Oiwa, H. Munekata An ultrafast (subpicosecond) demagnetization process induced by intense laser irradiation has been recently observed in InMnAs [1]. We propose a theoretical model of magnetization dynamics on such a timescale which is based on the sp-d exchange interaction between the delocalized carriers' spins and localized Mn spins. Light excitation heats up a population of holes, taking it out of equilibrium with the Mn spins. This triggers the process of energy and angular momentum exchange between the two spin systems. Fast spin relaxation of holes sustains the flow of spin polarization leading to the possibility of significant demagnetization of the Mn system. Ultrafast magnetization dynamics in GaMnAs will also be discussed. This work was supported by DARPA, NSF, ONR and MEXT. [1] J. Wang et al., Phys. Rev. Lett. 95, 167401 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H19.00005: Pressure Dependent Magnetotransport Properties of Dilute Magnetic Semiconductors M. Csontos, G. Mihaly, T. Wojtowicz, B. Janko, X. Liu, J. K. Furdyna The Mn$^{2+}$ ions in (III,Mn)V ferromagnetic semiconductors provide magnetic moment and at the same time they act as a source of valence-band holes that mediate the Mn$^{2+}$-Mn$^{2+}$ interactions. This coupling results in the ferromagnetic phase. By using hydrostatic pressure to continuously tune the wavefunction overlap, one can control the strength of the ferromagnetic coupling resulting in a spectacular enhancement of the magnetization and the Curie-temperature [M. Csontos et al. Nature Materials \textbf{4}, 447 (2005)]. Magnetoresistance measurements on the magnetic semiconductor (In,Mn)Sb suggest that magnetic scattering in this material is dominated by isolated Mn$^{2+}$ ions located outside the ferromagnetically ordered regions [M. Csontos et al. Phys. Rev. Lett. \textbf{95}, 227203 (2005)]. The transport properties in presence of high magnetic field and hydrostatic pressure have also been investigated by Hall-effect and thermoelectric power measurements. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H19.00006: Magnetoresistance anomalies during hard axis magnetization reversal in (Ga,Mn)As epilayers. Meng Zhu, Gang Xiang, Ben Li Sheu, Xia Li, Peter Schiffer, Nitin Samarth The ferromagnetic semiconductor (Ga,Mn)As is known to display striking magnetoresistance (MR) during magnetization reversal. Here, we use temperature- and field-dependent MR measurements to probe hard axis magnetization switching processes in (Ga,Mn)As epilayers grown on (001) GaAs. MR is measured using Hall bars oriented along the three principal crystalline directions ($[100]$, $[\bar{1} 1 0]$ and $[110]$) over a temperature range 0.35 mK $\leq T \leq$ 120 K, with the magnetic field normal to the sample plane. We find that the longitudinal MR exhibits distinct signatures corresponding to different types of magnetization reversal processes. While domain wall nucleation and propagation produce large MR anomalies ($\sim 50 \Omega -1 00 \Omega$), coherent domain rotation is accompanied by surprisingly weak MR. We discuss our experimental measurements using analytical simulations that incorporate SQUID magnetization data as additional input. Supported by DARPA/ONR and NSF. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H19.00007: Magnetotransport studies of Ga$_{1-x}$Mn$_{x}$Sb Random Alloys. M. Eginligil, G.B. Kim, S. Wang, H. Luo, B.D. McCombe We report magnetotransport and magnetization measurements from two sets of well-characterized Ga$_{1-x}$Mn$_{x}$Sb samples grown by molecular beam epitaxy: set 1 - varying Sb/Ga flux ratios and constant nominal Mn concentration of 2.3{\%}; set 2 - constant flux ratio (Sb/Ga=5.7) and varying nominal Mn content. Plots of R$_{sheet }$vs. temperature show both metallic and thermally activated behavior. Two samples (one from each set) having the highest effective Mn content (as determined from measurements of saturation magnetization at low temperatures) but different flux ratios, exhibited thermally activated transport. Hole densities in the range 2.6x10$^{19}$cm$^{-3}$ to 1.3x10$^{20}$cm$^{-3}$ were extracted from plots of R$_{Hall}$/ R$_{Sheet}^{\alpha }$ vs. B/ R$_{Sheet}^{\alpha}$ at high fields and low temperatures; for the metallic samples 1 $<$ $\alpha \quad <$2 and for the samples showing thermally activated behavior $\alpha \quad <$ 1. The two samples that showed thermally activated transport, although they had the highest measured Mn content, have hole densities at the low end of the measured range. Both of these samples also exhibit extrema in the remnant hall resistance vs. temperature, with the sample from set 1 also showing very complex behavior of the anomalous Hall coefficient (a change of sign) vs temperature. This complex behavior is attributed to movement of the chemical potential through a region of non-monotonic behavior of the density of states vs. energy. Supported in part by DARPA ONR SpinS and NSF ECS0224206. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H19.00008: Transport Properties of Ga$_{x}$Cr$_{1-x}$N thin films synthesized by MBE Stephen Wu, Hongxue Liu, Rakesh Singh, Nathan Newman The electrical transport properties of ferromagnetic single phase Cr-doped gallium nitride films grown by reactive molecular beam epitaxy were investigated by performing Hall and resistivity measurements as a function of temperature over a wide range of dopant concentrations. For Cr concentrations at 3{\%}, the films are highly resistive and the thermally activated electron transport follows the exponential law, R=R$_{o}$exp[(T$_{o}$/T)]$^{-1/4}$, characteristic of variable range hopping between localized states in an impurity band [1]. This relationship does not explicitly hold over a significant temperature range for other Cr concentrations. The physical mechanism responsible for the differences in the electrical properties of these films will be elucidated by comparing the transport processes involved with the fraction of Cr atoms situated on Ga substitutional sites as measured by RBS ion channeling and the ferromagnetic properties determined using a variable temperature vibrating sample magnetometer. [1] N.F. Mott, \textit{Philos. Mag.}, \textbf{19}, 835 (1969) [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H19.00009: Controlling ferromagnetism in GaMnAs by arsenic defect engineering R. C. Myers, A. W. Jackson, A. C. Gossard, D. D. Awschalom, B. L. Sheu, P. Schiffer, N. Samarth We manipulate the Curie temperature and hole density of Ga$_{1- x}$Mn$_{x}$As for $x = 0.75$\% to 3\% by systematically varying the arsenic compensation by As-flux control. The As:Ga flux ratio is varied using a new technique of non-rotated GaMnAs growth, where the geometry of the molecular beam epitaxy system provides a continuous variation in As-flux across a wafer. In this manner, As-stoichiometry can be calibrated in a single growth run for any Mn doping density. The hole density and Curie temperature are maximized at the same position on each wafer suggesting that minimizing As-defect compensation improves magnetic properties. Surprisingly, changes in the arsenic flux of as little as 10\% can quench ferromagnetism in the low Mn-doped material causing an apparent shift in the paramagnetic to ferromagnetic cross over transition toward higher Mn concentrations. The results suggest that proper control of arsenic compensation is critical for optimizing Curie temperatures in the low Mn-doping regime. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H19.00010: Evidence of MnAs precipitates in a diluted magnetic semiconductor (Ga,Mn)As Taesoon Hwang, Soonchil Lee, S. H. Chun, H.K. Choi, Y.S. Kim, Y.D. Park Local magnetic field in (Ga,Mn)As epilayers with 500 nm thickness having 3.6$\%$ and 5.8$\%$ Mn was studied by zero field nuclear magnetic resonance (NMR) at liquid helium temperatures. The macroscopic magnetic and transport properties of the epilayers are quite similar to other previously reported low temperature molecular beam epitaxy prepared samples. We carefully searched for NMR signal from 20 MHz to 250 MHz, but only the spectrum of MnAs precipitations of hexagonal NiAs structure was observed. No evidence for the magnetism due to the substituted Mn ions was observed by NMR within the frequency range investigated. About 1/7 of Mn atoms incorporated into the sample participates in the formation of precipitates whose size is as small as to favor formation of single magnetic domains. Temperature dependence of magnetic moment of MnAs precipitations estimated by NMR frequency below 12 K indicates that its Tc should be much lower than that of bulk. The magnetization of MnAs precipitations decreases faster with increasing temperature for samples with a lower transition temperature. The origin of this correlation between the critical temperatures of MnAs precipitates and the single phase (Ga,Mn)As is not fully understood at this moment. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H19.00011: Magnetic Properties of Ferromagnetic (Ga,Mn)As Grown on (511) GaAs Substrates Chong Xie, Y. J. Cho, R. Chakarvorty, Y. Y. Zhou, Z. Ge, W. L. Lim, X. Liu, J. K. Furdyna, M. Dobrowolska Ferromagnetic (Ga,Mn)As films were successfully grown on SI GaAs (511)A and (511)B substrates by LT-MBE. We have performed SQUID, magnetotransport, ferromagnetic resonance(FMR), and Magneto-Optic Kerr Effect (MOKE) measurements to characterize the magnetic properties of these samples. A conspicuous feature observed on these samples is an asymmetric shift of the Hall resistance caused by the superposition of the Planar Hall Effect (PHE) and the Anomalous Hall Effect (AHE), in agreement with our earlier studies of (Ga,Mn)As grown on vicinal GaAs (001) substrates. This asymmetric shift is a direct manifestation of the strength of the magnetocrystalline anisotropy, which confines the magnetization to the preferred crystalline plane, thus resulting in a non-zero component of the magnetization normal to the (Ga,Mn)As layer. The results are also compared with the magnetic properties measured in (Ga,Mn)As films grown on GaAs substrates with other orientations, e.g., (001) and (311). Supported by NSF Grant DMR02-10519. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H19.00012: Large spin polarization in Ga$_{1-x}$Mn$_{x}$P synthesized by ion implantation and pulsed-laser melting M.A. Scarpulla, P.R. Stone, R. Farshchi, I.D. Sharp, E.E. Haller, O.D. Dubon, J.W. Beeman, E. Arenholz We have recently demonstrated that the mediating holes in Ga$_{1-x}$Mn$_{x}$P reside in an impurity band separated from the valence band by a gap [Scarpulla \textit{et al.}, Phys. Rev. Lett. \textbf{95} 207204 (2005)]. For samples with x$\approx $0.06 and Curie temperature 60 K, this gap is approximately 25 meV. In this work, we demonstrate a very large spin polarization of states near the Fermi level across a range of Mn compositions using Mn L$_{3,2}$ X-ray magnetic circular dichroism (XMCD). We find that the X-ray absorption spectrum at the Mn L$_{3,2}$ edge is very similar to that observed in Ga$_{1-x}$Mn$_{x}$As [Edmonds \textit{et al.}, Appl. Phys. Lett. \textbf{84} 4065 (2004)], indicating that the Mn bonding environment is similar between Ga$_{1-x}$Mn$_{x}$As and Ga$_{1-x}$Mn$_{x}$P. The magnitude of the X-ray fluorescence XMCD signal reaches approximately 70{\%} at the Mn L$_{3}$ edge across the range of compositions, indicating a large spin asymmetry in the density of states. This is consistent with (but not exclusive to) the existence of a spin-polarized impurity band near the Fermi level. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H19.00013: Evidence of room temperature \textit{sp-d} exchange in InMnAs epitaxial films Philip Chiu, Bruce Wessels InMnAs grown by metal organic vapor phase epitaxy (MOVPE) is a promising ferromagnetic semiconductor due to its high Curie temperature ($T_{c})$ of 330 K. However, the nature of \textit{sp-d} exchange in this material is not well understood. Since enhancement of magnetic circular dichroism (MCD) at band structure critical points is indicative of \textit{sp-d} exchange, the reflection MCD of MOVPE InMnAs was measured. At room temperature and an applied field 0.5 T, the MCD spectrum is composed of two types of transitions: a negative peak centered between 2.60-2.68 eV and a broad featureless negative response from 1.6-3.0 eV. At both transitions, ferromagnetic hysteresis and saturation are observed in the MCD field dependence. The negative peak occurs between the $L$-point critical transitions of InMnAs, at 2.53 and 2.79 eV respectively. The single negative peak is also similar to that observed in MBE InMnAs in both the peak energy and width. The MCD enhancement at the $L$-point is consistent with room temperature \textit{sp-d} exchange splitting in MOVPE grown InMnAs. The \textit{sp-d} exchange is tentatively attributed to the presence of Mn dimers and trimers in the InMnAs that result in a $T_{c}$ of 326-327 K. The assignment of the broad spectral response is currently under way. [Preview Abstract] |
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