Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B10: Focus Session: III-V Magnetic Semiconductors |
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Sponsoring Units: DMP GMAG Chair: Nitin Samarth, Pennsylvania State University Room: LACC 153B |
Monday, March 21, 2005 11:15AM - 11:27AM |
B10.00001: Effect of carrier concentration on Magnetic Circular Dichroism (MCD) in GaMnAs/ZnSe hybrid structures with Be and Si co-doping R. Chakarvorty, K. J. Yee, X. Liu, P. Redlinski, M. Kutrowski, L.V. Titova, T. Wojtowicz, J.K. Furdyna, B. Janko, M. Dobrowolska We have investigated MCD in a series of magnetic epitaxial Ga$_ {0.99}$Mn$_{0.01}$As layers grown on hybrid ZnSe/GaAs substrates, with various levels of Be and Si co-doping. The carrier concentration, which plays a key role in ferromagnetism of GaMnAs, was controlled by the temperature of Be and Si cell during the growth. Optical absorption and MCD measurements were carried out as a function of temperature and wavelength in magnetic fields up to 6.0T using circularly polarized light combined with photo-elastic modulation. The magnitude and the spectral form of the MCD showed a clear progressive change as the hole concentration increased. The observed magnetic field and temperature dependence of the MCD data cannot be explained by the k.p model proposed by Szczytko et al. (PRB 64, 75306 (2001)). A likely reason for the failure of the latter model is that -- due to the very large number of impurities in the samples -- one must additionally include band renormalization in the calculations. Such renormalization would result in different band shapes of the two spin sub-bands, and consequently in different spectral shapes of the absorption coefficients for the $\sigma^{+}$ and $\sigma^{-}$ polarizations. *M. Kutrowski and T. Wojtowicz are also in Institute of Physics, Polish Academy of Sciences, Warsaw, Poland. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B10.00002: Magneto-optical Kerr Effect Studies of Magnetic Anisotropy in Tensile-Strained (Ga,Mn)As W. Fadgen, O. Maksimov, B. L. Sheu, G. Xiang, P. Schiffer, N. Samarth The rich variation of magnetic anisotropy with strain, carrier density, and temperature provides a valuable arena for testing different models of carrier-induced ferromagnetism in III-V ferromagnetic semiconductors. Here, we use the component resolved magneto-optical Kerr effect (MOKE) to investigate the effect of low temperature annealing on the magnetic anisotropy in a set of tensile-strained Ga$_{.94}$Mn$_{.06}$As films (thickness ranging from 15 to 120 nm) grown on strain-relaxed (In,Ga)As buffer layers. Deconvolution of the MOKE signal into the longitudinal and polar components shows that, despite the tensile strain, the as-grown samples can have a significant in-plane component of magnetization. Our measurements show that the easy axis of magnetization in as-grown samples reorients towards out-of-plane with increasing temperature, as well as when a second laser is used as a pump. Upon annealing, the in-plane component of magnetization is completely suppressed. We discuss these measurements in the context of the mean-field theory of hole-mediated ferromagnetism in these materials. Supported by DARPA, ONR and NSF. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B10.00003: Mid-infrared magneto-optical studies of Ga$_{1-x}$Mn$_x$As random alloy films G. Acbas, S. Wang, M. Cheon, G.B. Kim, A. Markelz, H. Luo, B.D. Mccombe, J. Cerne, M. Cukr, V. Novak, J. Sinova, X. Liu, J.K. Furdyna, T. Wojtowicz, M.A. Scarpulla, O.D. Dubon The mid-infrared (MIR: 115-238 meV) complex Faraday and Kerr effect are systematically studied in Ga$_{1-x}$Mn$_x$As random alloy films as a function of temperature, frequency, and Mn concentration. 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 of the same order of magnitude as predicted by effective k$\cdot$p Hamiltonian models within a mean field treatment [J. Sinova et al., PRB 2003]. Samples from from four different groups are studied, and although parameters such as the Mn-doping, annealing conditions, and resistivity vary from sample to sample, universal trends in the frequency dependence of the MIR magneto-optical response are observed. This work was supported by DARPA/ONR N0001400-1-0951. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:27PM |
B10.00004: Frontiers for Ferromagnetism in GaMnAs Invited Speaker: By careful control of MBE growth conditions and post growth annealing procedures we have produced GaMnAs epilayers which high conductivities and Curie temperature up to 173K. We demonstrate that the improvement in material properties resulting from annealing is due to the out diffusion of interstitial Mn. The high conductivies of our material makes it possible to obtain accurate hole densities for a range of Mn compositions and to make meaningful quantitative comparisons of magnetic and transport properties with theoretical predictions. We find that compensation is very low in best samples. We show that our measured Curie Temperatures, Hall conductivities and AMR are in good agreement with the mean field theory. We also find that there is no evidence of a fundamental magnetisation deficit in our material. We will also present the observation of a large tunneling anisotropic magnetoresistance (TAMR) in thin (Ga,Mn)As epilayers with lateral nanoconstrictions. The observation establishes the generic nature of the recently discovered TAMR effect, which originates from spin-orbit coupling in a ferromagnet and is not specific to a particular tunneling geometry. The lateral geometry allows us to directly link normal anisotropic magnetoresistance (AMR) and TAMR. This indicates that TAMR may be observable in other materials showing room temperature AMR, and suggest a re-examination of previous tunneling and nanocontact results. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B10.00005: Competition between cubic and uniaxial magnetic anisotropy in GaMnAs at low Mn concentrations L.V. Titova, M. Kutrowski, X. Liu, R. Chakarvorty, W.L. Lim, T. Wojtowicz, J.K. Furdyna, M. Dobrowolska We study the dependence of the cubic and uniaxial magnetic anisotropy terms in GaMnAs on hole concentration $p$ and temperature $T$. The Ga$_{0.99}$Mn$_{0.01}$As layers were grown on ZnSe buffers deposited on GaAs substrates, and co-doped by Be in the range 3.0x10$^{19} \quad < \quad p \quad <$ 8.5x10$^{19 }$cm$^{-3}$. Due to small lattice mismatch the uniaxial and cubic anisotropy terms in these samples were comparable. The magnetic anisotropy was studied by polar magneto-optical Kerr effect, which allowed us to monitor the easy axis of magnetization. The results showed that the cubic anisotropy is highly sensitive to both $p$ and $T$. Specifically, in samples with high $p$ the cubic anisotropy term is dominant at low $T$, but decreases rapidly as $T$ increases. In sharp contrast, uniaxial anisotropy shows a much weaker dependence on $p$ and $T$, thus dominating at temperatures close to T$_{C}$ even in samples with high $p$. These results open the possibility of engineering magnetic anisotropy and the magnetization reversal process in GaMnAs by controlling $T $and/or $p$. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B10.00006: Anisotropic properties of ferromagnetic GaMnAs thin films grown on misoriented GaAs substrates Konrad Dziatkowski, Weng-Lee Lim, Xinyu Liu, Zhiguo Ge, Shaoping Shen, Jacek K. Furdyna, Malgorzata Dobrowolska We discuss the magnetic anisotropy of ferromagnetic Ga$_{0.94}$Mn$_{0.06}$As films grown on vicinal GaAs substrates, with the growth plane tilted by a few degrees away from the (001) toward the (111)B plane. In addition to the well-known double-jump of in-plane magnetization, the planar Hall effect (PHE) measurements reveal an asymmetric shift of the Hall resistance. We suggest that this asymmetry arises from an out-of-plane component of the magnetization. In order to further understand the PHE results, we measured ferromagnetic resonance (FMR) on the same set of samples. We observe an asymmetrical and a symmetrical angular dependence of FMR for H in (110) and (1-10) planes, respectively. Note that the growth direction is tilted in the (110) plane. This effect is likely to originate from the fact that in films grown on misoriented substrates the [001] direction does not coincide with any of the principal directions of the demagnetization tensor. Both PHE and FMR results suggest that the direction of the easy axis is determined by magneto-crystalline rather than by shape anisotropy. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B10.00007: Effect of Sb/Ga flux ratio on magnetic properties in ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb random alloys G.B. Kim, M. Cheon, S. Wang, H. Luo, B.D. McCombe For spin-functional devices involving Ga$_{1-x}$Mn$_{x}$Sb, it is desirable to improve and control the magnetic properties, particularly hysteresis. We have carried out a systematic study of the effects of the Ga/Sb flux ratios, especially Sb overpressure, on the magnetic properties of these alloys. For a Mn composition of 0.023, the growth surface deteriorates outside a narrow range of Sb/Ga flux ratio. The magnetic properties of the samples, studied by SQUID magnetometry, showed a strong dependence on Sb/Ga flux ratio. Nearly square-like hysteresis loops were observed for Sb/Ga flux ratios between 4.6 and 5.3; coercive fields varied by nearly a factor of two, from 216 Oe to 380 Oe. The Curie temperature was independent of flux ratio over this range. No hysteresis was observed when the Sb/Ga ratio was increased to 5.6, when samples become superparamagnetic. The magnetotransport properties of these samples also show interesting behavior. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B10.00008: Magnetotransport properties of ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb random alloys with different Sb/Ga flux ratio H. Luo, G.B. Kim, M. Cheon, S. Wang, B.D. McCombe Magnetotransport measurements provide important information about electronic properties of ferromagnetic diluted magnetic semiconductors. We have carried out a study of magnetotransport properties of ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb random alloys with different Sb/Ga flux ratios and the same Mn concentration (x = 0.023). Between flux ratios of 4.6 and 5.3 the magnetization of these samples showed large, nearly square hysteresis loops with large coercive fields. The negative magnetoresistance also showed strong dependence on Sb/Ga flux ratio, decreasing with increasing values of Sb/Ga. The sheet resistance vs. temperature showed weakly localized behavior for Sb/Ga = 4.6 and metallic behavior for Sb/Ga = 5.3. The Hall resistance for the sample with Sb/Ga = 4.6 shows unusual behavior; the sign of the anomalous Hall coefficient is negative below 2.2K and changes to positive at the temperature range of 2.7K to 7K, and then back to negative above 10K. The hole densities from fitting the Hall resistance at high magnetic fields show large differences, while T$_{C}$ remains the same at 24K. These results will be discussed in terms of impurity band conduction and localization. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B10.00009: Low Temperature Magnetic Features in MnAs Films Grown on GaAs(001) Miyeon Cheon, Gibum Kim, Shumin Wang, Hong Luo Magnetic properties of MnAs grown on GaAs(001) substrates were studied as a function of temperature from 20 K to 360 K with both SQUID magnetometry and magnetic force microscopy (MFM). Magnetic domain structures in MnAs on GaAs(001) surfaces have been studied extensively with MFM near the Curie temperature. We have carried out MFM studies on MnAs films to understand their magnetic properties at temperatures both near and well below the Curie temperature. When the thickness of MnAs is below 40 nm, the samples will change from the well-studied domain patterns in the structural $\alpha $ --MnAs stripes at room temperature to a single $\alpha $ --MnAs phase with lowering temperature. However, the sample does not immediately become a single magnetic domain after it is in the single $\alpha $ --MnAs phase. Instead, there are stripe-like magnetic features mostly along the easy axis. Those features eventually disappear at even lower temperatures, leaving a single magnetic domain. Both dimensions and stability of such features show strong dependence on film thickness. In thicker MnAs films, e.g., 400 nm, such features persist down to 20 K. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B10.00010: Induced magnetic ordering control in a digital (Ga,Mn)As structure Jisang Hong, Dingsheng Wang, Ruqian Wu In our computational studies for Mn/GaAs(110) superlattices, both the thickness of the semiconductor spacers, d, and external carrier density, $\rho $, are manipulated to observe the trend of $\Delta $E$_ {FM-AF}$, a quantity that describes the effective exchange interaction between two Mn layers. The change in $\rho $ is achieved through variations in a small fraction of electrons in the valence band, accompanied by matching adjustments in positive charge in the semiconductor region. The results indicate that the $\delta $-doped (Ga,Mn)As has a strong FM ground state when d=5.56 {\AA} and d=11.31 {\AA}. The carrier induced AF-FM phase transition occurs in cases with thick spacers. When, d=16.96 {\AA}, for example, the AF configuration is the ground state without external carrier, and is further stabilized by electron doping. In contrast, the FM state gradually prevails in the hole doping side, especially when $\rho \quad >$ 1.0$\times $10$^{20}$/cm$^{3}$. This clearly explains experimental observations made by Ohno et al, who found that both magnetization and coercive force can be manipulated by applying electric field in the gated structure and changing the carrier (hole) density. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B10.00011: Magnetoelastic Measurements in a (Ga,Mn)As Nanoelectromechanical Resonator Sotirios Masmanidis, Edward Myers, Hongxing Tang, Mo Li, Michael Roukes, Kristiaan De Greve, Geert Vermeulen, Wim Van Roy We have employed nanoelectromechanical systems (NEMS) to obtain the first experimental measurements of the magnetostriction constants of a dilute magnetic semiconductor. A NEMS resonator is patterned from (Ga,Mn)As, grown epitaxially with 5.2 percent Mn and Curie temperature of 57 K. The device has a frequency and quality factor of 16.5 MHz and 6,300, respectively. Transduction is carried out by the piezoresistive effect, and using a phase-locked loop to monitor frequency. Resonance frequency shifts due to magnetostrictive strain are observed upon magnetizing the device with an applied field. By fitting a phenomenological model of magnetoelastic stress, combined with the Stoner-Wohlfarth model of magnetization reversal to the angular dependence of the frequency tuning effect, the (Ga,Mn)As magnetostriction and anisotropy field constants are derived. The connection between magnetostriction and magnetic anisotropy is discussed, together with comparisons to theoretical predictions. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B10.00012: Dynamical Mean Field Study of Ferromagnetism in Mn Doped GaAs Karan Aryanpour Ferromagnetic semiconductors such as $Ga_{1-x}Mn_xAs$ have been of great interest lately as spins in these compounds can be used to carry information (spintronics). It has been shown that the strong spin-orbit interaction in these materials together with spatial disorder leads to highly frustrated magnetic correlations. This intrinsic frustration pushes these systems into strongly spin disorder ferromagnetic regimes making them behave similarly to spin glass compounds. We solve the spin-orbit interaction analytically using the DMFA. Our results exhibit the substantial decrease of the magnetic order parameter due to the frustration originating form the strong spin-orbit coupling. There is also evidence for the formation of an impurity band inside the pure $GaAs$ gap for large values of the carrier-impurity interaction coupling. The formation of this impurity band gives rise to an increase in $T_c$ and the discontinuity of the chemical potential. The frustration is also responsible for the suppression of the carrier spin polarization which is a key quantity for the spintronics applications. [Preview Abstract] |
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