Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S12: Focus Session: III-Mn-V Ferromagnetic Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Andre Petukhov, South Dakota School of Mines and Technology Room: Colorado Convention Center Korbel 3C |
Wednesday, March 7, 2007 2:30PM - 3:06PM |
S12.00001: Large-Scale Monte Carlo Study of a Realistic Lattice Model for $\rm Ga_{\it 1-x}Mn_{\it x}As$ Invited Speaker: The properties of Mn-doped GaAs are studied at several doping levels and hole compensations, using a real-space Hamiltonian on an fcc lattice that reproduces the valence bands of undoped GaAs. Large-scale Monte Carlo (MC) simulations on a Cray XT3 supercomputer, using up to a thousand nodes, were needed to make this effort possible. Our analysis includes both the spin-orbit interaction and the random distribution of the Mn ions. The hopping amplitudes are functions of the GaAs Luttinger parameters. At the coupling exchange $J$$\sim$1.2~eV deduced from photoemission experiments, the MC Curie temperature and the shape of the magnetization curves are in good agreement with experimental results for annealed samples. The system is found to be qualitatively closer to a hole-fluid regime than to localized carriers. The methodology described here introduces a quantitative tool to analyze a variety of diluted magnetic semiconductors, valid in broad regimes of parameter space. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S12.00002: Evidence of impurity band conduction in GaMnAs Leonid Rokhinson, Yuli Lyanda-Geller, Z. Ge, S. Shen, X. Liu, M. Dobrowolska, Jacek K. Furdyna We investigate low-temperature conduction in GaMnAs, and demonstrate that the observed properties are inconsistent with the valence band transport, but consistent with the metallic transport of holes within the impurity band. We observe a peak in magnetoresistance at very small magnetic fields $B<20$ mT, which is independent of orientation of $B$ with respect to the ferromagnetic easy axis and to the direction of the electric current. The peak appears below 3.4 K and increases at lower temperatures. We attribute this effect to the anomalous negative magnetoresistance of the Aharonov-Bohm (AB) origin. The shape and magnitude of the peak is consistent with weak localization (WL) in a three dimensional (3D) conductor with vanishing spin-orbit interaction. Holes in the valence band, on the contrary, experience strong spin-orbit interaction, which would suppress weak localization in a ferromagnet. In addition to WL we observe a field-independent increase of resistance at $T<5$ K, a signature of Altshuler-Aronov (AA) electron-electron interaction effect. Temperature dependent AA contribution to resistivity is almost an order of magnitude bigger than the magnitude of magnetoresistance peak, as it should be in conventional 3D disordered conductors. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S12.00003: Conclusive Evidence for Impurity Band Conduction in Metallic Ga$_{1-x}$Mn$_{x}$As K.S. Burch, D.B. Shrekenhamer, E.J. Singley, J. Stephens, B.L. Sheu, R.K. Kawakami, D.D. Awschalom, N. Samarth, P. Schiffer, D.N. Basov The electronic structure and carrier dynamics of the dilute magnetic semiconductor Ga$_{1-x}$Mn$_{x}$As are studied via optical spectroscopy. We focus on the changes induced by annealing the samples, which is known to result in a large increase in the critical temperature. This study provides conclusive evidence that the Fermi energy ($E_{F}$) lies in a metallic impurity band. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S12.00004: Insulating ferromagnetic (Ga,Mn)As with low Mn-doping B.L. Sheu, R.C. Myers, N. Samarth, D.D. Awschalom, P. Schiffer We probe the onset of ferromagnetism in (Ga,Mn)As near the vicinity of the metal-insulator transition by studying systematic series of low Mn-doped ($<$ 2.0 atomic {\%}) GaAs epilayers [growth described in R.C. Myers \textit{et al.} Phys. Rev. B \textbf{74}, 155203 (2006)]. We have studied the temperature dependent magnetization and electronic transport properties of these insulating samples. We fit the data using the variable-range-hopping conduction model and calculate the hopping energy from the logarithm of the resistivity versus T$^{-1/4}$ slope fit. Our results indicate that a cutoff in ferromagnetism is strongly correlated with critical values of the electric conductivity and the hopping energy. This work is supported by grants from NSF, ONR and DARPA. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S12.00005: Imaging of single magnetic dopants in III-V semiconductor hosts Paul Koenraad, Cem Celebi, Andrei Silov, Andrei Yakunin, Jian-Ming Tang, Michael Flatte, Maria Kaminska We present room-temperature cross-sectional scanning tunneling microscopy (STM) topographic measurements of the acceptor state wave function for Mn dopants in InP and compare with measurements for the nonmagnetic dopants Zn and Cd as well as with previous results for Mn in GaAs[1]. We find a strongly anisotropic ``bow-tie'' shape for the Mn acceptor state wave function in InP, similar to Mn in GaAs, which has a binding energy of 210 meV (compared to 113 meV for Mn in GaAs). The shape for Mn in InP is more symmetric with respect to the 001 plane than Mn in GaAs, which agrees with a general trend for the magnetic and nonmagnetic acceptor state symmetry as a function of acceptor binding energy. We present a new theoretical model based on the surface strain of GaAs (110) that explains why the 001-plane asymmetry of acceptor states seen in STM measurements is much larger than expected from bulk calculations. [1] A. M. Yakunin, et al., Phys. Rev. Lett. 92, 216806 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S12.00006: Atom-by-Atom substitution of Mn in n-type GaAs and Electron mediated Mn-Mn interaction in GaAs Anthony Richardella, Pedram Roushan, Dale Kitchen, Ali Yazdani Using a novel atomic scale manipulation technique with a cryogenic scanning tunneling microscope (STM), we have recently demonstrated the ability to incorporate single Mn atoms in p-GaAs substrates and have used this technique to visualize hole-mediated interaction between Mn acceptors in p-type GaAs. [1] We will report on the extension of these experiments to n-type substrates, for which we have also succeeded in incorporation of single Mn acceptors and probed Mn-induced in-gap states using spatially resolved STM spectroscopy. In contrast to previous work, experiments on n-type substrates allow us to explore spin-spin interaction between Mn mediated by electrons in the valance band. Imaging and spectroscopic mapping show Friedel oscillation in the vicinity of individual Mn dopants. We will report these results and more recent experiments on the role of such oscillation on the interaction between Mn-dopants. [1] D. Kitchen, A. Richardella, J-M. Tang, M. Flatte, A. Yazdani, Nature 442, 436--439 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S12.00007: Anisotropic spin-spin interactions of Mn-Mn pairs in III-V semiconductors Jian-Ming Tang, Michael E. Flatt\'e We calculate the energy splitting of acceptor states of Mn pairs in GaAs [1]. The calculated splittings show crystalline anisotropy that is in good agreement with recent scanning tunneling measurements [2]. The splitting is large when the pair axis is along the $\langle 110\rangle$ axis and smaller when along the $\langle 100\rangle$ axis. This anisotropy can be understood from the overlap of two Mn acceptor wavefunctions that have the approximate cubic symmetry [3]. Within a double exchange model, the splitting can be linked to the energy difference between parallel and antiparallel Mn spins. Our results show that the parallel configurations always have the lower energy. This exchange coupling energy follows the same crystalline anisotropy for the splitting because the anisotropy is predominately determined by the lowest hole state. The rotational symmetry of the total spin of Mn pairs is weakly broken by the spin-orbit interaction. [1] J.-M. Tang and M. E. Flatt\'e, Phys. Rev. Lett. 92, 047201 (2004) [2] D. Kitchen, et al., Nature 442, 436 (2006) [3] A. M. Yakunin, et al., Phys. Rev. Lett. 95, 256402 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S12.00008: X-ray standing wave imaging of Mn in GaAs Jorg Zegenhagen, Tien-Lin Lee, Isabelle Joumard, Martin Brandt, Wladimir Schoch GaMnAs is a prototype of a dilute magnetic semiconductors with a Curie temperature T$_{c}$ of up to 170 K at a Mn concentration of 5{\%}. Substituting the Ga, the Mn acts as an acceptor with a local spin moment of 5/2 and ferromagnetic ordering is mediated by the itinerant holes. However, depending on the growth conditions and post-growth treatments, small fractions of the Mn may occupy magnetically inactive interstitial sites, act as a donor, compensating the hole doping, and thus decreasing T$_{c}$. Determining the site distribution of the Mn is therefore important in order to achieve an optimal T$_{c}$. We used x-ray standing waves generated by substrate (hkl) Bragg reflections to locate the Mn in the GaAs lattice for three differently treated samples, each with 4{\%} Mn in a 4 nm thick epilayer. For 22 reflections, the amplitude as well as the phase of the (hkl) Fourier component of the Mn distribution were determined by recording the Mn K fluorescence during angular scans of the sample traversing the GaAs(hkl) Bragg peaks. Thus a real-space image of Mn within the GaAs unit cell can be reconstructed via direct Fourier expansion. The majority of the Mn is substituting for Ga, but refinement shows that up to 10{\%} of the Mn occupies As interstitial sites. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S12.00009: Near field infrared spectroscopy of the ferromagnetic semiconductor Ga$_{1-x}$Mn$_x$As at the nanoscale. G.O. Andreev, M. Brehm, F. Keilmann, M.M. Qazilbash, T. Driscoll, K.S. Burch, J. Stephens, D.D. Awschalom, D.N. Basov We report on the nanoscale infrared response of a prototypical ferromagnetic semiconductor Ga$_{1-x}$Mn$_x$As at Mn doping fractions in the range of x=1.8-7.75 {\%}. These studies have been carried out using an apertureless scattering Scanning Near field Infrared Microscope (s-SNIM) with a pseudoheterodyne detection scheme operating at the wavelength near 10 $\mu $m.. For samples with doping fractions below 7{\%} we observe a contrast-free near field infrared signal, suggesting a homogenous electronic state on the length scale down to 10-20 nm determined by the spatial resolution of s-SNIM.. At doping fractions of 7.66{\%} and 7.74{\%} we find significant contrast in the form of surface clusters ranging in diameter from 15 to 30nm in both topographical and near field images. These clusters occupy approximately 15{\%} of the total image area. We will discuss possible origins of the observed contrast within the framework of the effective polarizability dipolar model of the tip-sample interaction. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S12.00010: Interlayer Magnetic Coupling in AlBeGaAs/GaMnAs/GaAs/GaMnAs Heterotructures, as Probed with Polarized Neutron Reflectometry Brian Kirby, Mike Fitzsimmons, Julie Borchers, Xinyu Liu, Zhiguo Ge, Jacek Furdyna Understanding interlayer exchange coupling between magnetic semiconductor layers could prove important for device applications. We discuss a series of AlBeGaAs/GaMnAs/GaAs/GaMnAs heterostructures, fabricated to be identical except for varying GaAs spacer layer thickness. Via hole doping, the AlBeGaAs layer alters the coercivity (Hc) and Curie temperature (Tc) for an adjacent GaMnAs layer. Therefore, in the absence of interlayer coupling, the GaMnAs layers in our heterostructures will not have equal Hc or Tc. Using polarized neutron reflectometry (PNR), we have measured the \textit{depth-dependent} magnetizations for this series of samples, as functions of applied field and temperature. Our results show the effects of interlayer spacer thickness and temperature on coupling between GaMnAs layers. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S12.00011: The role of Mn acceptors in determining the Zeeman splitting of the band edges in GaMnAs. R. Chakarvorty, Y. -Y. Zhou, Y. -J. Cho, X. Liu, J. K. Furdyna, M. Dobrowolska It has been widely accepted that ferromagnetism in Ga$_{1-x}$Mn$_{x}$As is carrier-induced, and much work has been devoted to the mechanism of coupling between the Mn ions. By comparison considerably less attention has been given to the interaction of Mn ions with electronic bands, and to the Zeeman splitting of the band edges. To address the latter issue, we use magnetic circular dichroism (MCD) to investigate how different Mn acceptor states (neutral A$^{0}$ vs. negatively charged A$^{-})$ affect the magneto-optical properties of Ga$_{1-x}$Mn$_{x}$As. Several series of Ga$_{1-x}$Mn$_{x}$As layers were fabricated for this purpose by low-temperature molecular beam epitaxy, using different Mn concentrations (0.001 $\le $ x $\le $ 0.01), As$_{2}$ fluxes, growth temperatures and co-dopings. Our MCD data show that in highly compensated samples (i.e., those rich in A$^{-}$ centers) the Zeeman splitting of the band edges disappears, thus indicating that the exchange between band carriers and Mn spins takes place entirely via the A$^{0}$ centers. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S12.00012: Ferromagnetism in InMnAsP epitaxial films Nidhi Parashar, Philip Chiu, Bruce Wessels The magnetic properties of epitaxial In$_{1-x}$Mn$_{x}$As$_{1-y}$P $_{y}$
deposited by metal-organic vapor phase epitaxy were investigated in order to
study matrix effects. Alloy concentrations of 0.01$ |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S12.00013: Magnetic Properties of Ga$_{1-x}$Mn$_{x}$ P-based Quaternary Ferromagnetic Semiconductors P.R. Stone, M.A. Scarpulla, I.D. Sharp, E.E. Haller, O.D. Dubon, E. Arenholz, J.W. Beeman, K.M. Yu Ga$_{1-x}$Mn$_{x}$P is a ferromagnetic semiconductor in which exchange is mediated by carriers localized in a Mn-derived impurity band [Scarpulla \textit{et al.}, Phys. Rev. Lett. \textbf{95} 207204 (2005)]. Despite its non-metallic nature even for x$\sim $0.042, Ga$_{1-x}$Mn$_{x}$P displays many properties that are not significantly different from those of the canonical system Ga$_{1-x}$Mn$_{x}$As including an approximately linear increase of the Curie temperature (T$_{C})$ with x and a strong spin polarization of the density of states at the Fermi energy. Here we report the effect of partial anion replacement by either S or As on the magnetic properties of Ga$_{1-x}$Mn$_{x}$P-based thin films. In Ga$_{1-x}$Mn$_{x}$P$_{1-y}$S$_{y}$ both T$_{C}$ and X-ray magnetic circular dichroism decrease monotonically with y due to compensation of ferromagnetism-mediated holes by electrons introduced by S donors. Addition of sulfur significantly enhances the uniaxial magnetic anisotropy between in-plane $<$110$>$-type directions with increasingly harder [110] axes as y increases. Finally, we explore Ga$_{1-x}$Mn$_{x}$As$_{1-y}$P$_{y}$ for which it has been predicted [Masek \textit{et al}. cond-mat/0609158v1] that T$_{C}$ will increase as y increases, thus providing a route to higher T$_{C}$ based on the well-studied Ga$_{1-x}$Mn$_{x}$As system. [Preview Abstract] |
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