Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A33: Focus Session: Spin Dependent Phenomena in Semiconductors: I |
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Sponsoring Units: DMP GMAG FIAP Chair: Nitin Samarth, Pennsylvania State University Room: Morial Convention Center 224 |
Monday, March 10, 2008 8:00AM - 8:12AM |
A33.00001: Cross-sectional scanning tunneling microscopy of Ga$_{1-x}$Mn$_{x}$As/GaAs Heterostructures Pedram Roushan, Anthony Richardella, Shawn Mack, David Awschalom, Ali Yazdani We have used a cryogenic scanning tunneling microscope (STM) to perform cross-sectional imaging studies of GaMnAs heterostructures. The heterostructures, consisting of a p-type buffer followed by a 3{\%} Mn doped layer, were grown on a n-type GaAs substrate by molecular-beam epitaxy and cleaved in situ for STM measurements. The topographic measurements on the GaMnAs layer showed a variety of long range electronic structure modulations on the order of a few nm due to high level of disorder and compensation. Combining bias-dependent imaging and spectroscopy, we have used the STM to identify electronic features due to Mn dopants and other defects. In particular, we find that Mn dopants on the top most layer act as deep acceptors and exhibits similar topographic and spectroscopic features as our previous work [1] on Mn adatoms substituted into GaAs using STM manipulation techniques. [1] D. Kitchen, A. Richardella, J-M. Tang, M. Flatte, A. Yazdani, Nature 442, 436--439 (2006) [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A33.00002: Spectroscopic Mapping of Electronic States near Fermi Energy in GaMnAs Anthony Richardella, Pedram Roushan, Shawn Mack, David Awschalom, Ali Yazdani We have used atomically resolved spectroscopic mapping with the scanning tunneling microscope (STM) to probe the spatial structure of electronic states in GaMnAs/GaAs heterostructures. Mapping the electronic states over a wide range of energies across the heterostructure, we find significant differences from the p-type buffer layer to the Mn-doped layer. The features include gap narrowing and in gap resonances, some of which are due to the bulk electronic structure of GaMnAs. We focus on these bulk states, in particular a broad state that is observed near the valance band above the Fermi energy. We analyze the spatial distribution of these states in conjunction with our observations of disorder in the sample and variations in the electrostatic potential landscape due to compensation. Finally, we discuss the implications of our results to the questions of localization, band tails and whether highly doped GaMnAs has an impurity band separated from or merged with the host valance band. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A33.00003: STM studies of spin-spin interactions between Mn acceptors in p-type GaAs Donghun Lee, David R. Daughton, Jay A. Gupta We use a custom STM operating in a cryogenic, ultrahigh vacuum environment to study spin-spin interactions in semiconductors at the single-impurity level. By applying a voltage pulse with the STM tip, single magnetic impurities (e.g. Mn) can be substituted for Ga atoms in the first layer of the GaAs(110) surface. It was previously found that pairs of Mn acceptors exhibit an exchange splitting which depends on their separation and crystal orientation [2]. We are developing a capability for spin-polarized STM to better study long range magnetic ordering between pairs and larger clusters of Mn acceptors. To characterize the magnetic orientation of our STM tips, we have prepared samples such as Co/Cu(111) which exhibits out-of-plane magnetization, and Cr(001), which is an antiferromagnet with in-plane magnetic contrast between alternating terraces. http://www.physics.ohio-state.edu/$\sim $jgupta [2] D. Kitchen et al., Nature 442, 436-439 (2006) [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A33.00004: Magnetic Memory Effects in Coherent Magnetization Dynamics of GaMnAs: From Non-equilibrium to Thermal Regime Ingrid Cotoros, Jigang Wang, Xinyu Liu, Jacek Furdyna, Jaroslav Chovan, Ilias Perakis, Daniel Chemla III-Mn-V heterostructures exhibit rich spin memory effects and their magnetic properties show strong responses to external stimuli (light, electrical gate and current) via carrier density tuning. Here we report on the coherent magnetization dynamics due to laser excitation of transient carriers in GaMnAs, distinctly depending on the initial magnetic state the system is prepared in. We identify two distinct temporal regimes that reveal a complex scenario of spin reorientation, marked by the transition from a highly non-equilibrium, non-thermal, transient carrier-mediated regime ($<$ 300 femtosecond), to a thermal, lattice-heating regime (on the picosecond time scale). The ultrafast, sub-picosecond response can be used as footing for ultrafast optical detection of magnetic memory states. This observed femtosecond cooperative magnetic phenomenon may represent an as-yet-undiscovered universal principle in all carrier-mediated ferromagnetic materials, offering potential perspectives for terahertz (10$^{12}$ Hz) speed ``spintronic'' devices and functional systems. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A33.00005: Epitaxial engineering of ferromagnetism in (GaAs)/(Ga,Mn)As digital superlattices. M.J. Wilson, G. Xiang, B.L. Sheu, P. Schiffer, N. Samarth Recent theory predicts novel pathways for the epitaxial engineering of ferromagnetism in GaAs/MnAs digital superlattices grown along different crystalline directions [Franceschetti et al, PRL 97, 047202 (2006)]. This has motivated us to systematically study GaAs/(Ga,Mn)As digital superlattices grown along the [001], [110], [201] and [311] directions. We have characterized these samples using SQUID magnetometry, magnetotransport,TEM and SIMS. We observe clear trends in the Curie temperature that are correlated with the epitaxy direction and the GaAs spacer layer thickness. Our results suggest that -- for a given Mn concentration -- the Curie temperature of (Ga,Mn)As may depend on the geometrical arrangement of the Mn ions. We also explore alternative explanations, such as an orientation dependence of the Mn incorporation rate and defect formation energies. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A33.00006: Stoichiometric growth of high Curie temperature heavily-alloyed GaMnAs S. Mack, R.C. Myers, J.T. Heron, A.C. Gossard, D.D. Awschalom Previous work indicates that single-phase, high-Mn incorporation ($\ge $9{\%}) in GaAs can be obtained via MBE at very low substrate temperatures with film thicknesses on the order of a few nanometers. Here we present the properties of smooth, single-crystalline GaMnAs samples with Mn densities between 9 and 18{\%} and continuously tuned arsenic stoichiometry using a combinatorial, non-rotated growth method.[1] Systematic, reproducible thick films (100 nm) display optimal magnetic, electronic, and structural properties in a narrow band of As:Ga flux ratios at the stoichiometric condition, where the Curie temperature is maximum. Post-growth annealing increases the Curie temperature while lowering the lattice constant, indicating that Mn interstitials are the dominant compensating defect in high-Mn containing GaMnAs. Curie temperatures from many samples grown with varying conditions all reach a maximum near the previously reported maximum ($\sim$165K). \newline [1] R. C. Myers\textit{ et al.}, Physical Review B 74, 9 (2006). [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A33.00007: Ultrafast Coercivity Dynamics in GaMnAs Kimberley Hall, Jeremy Zahn, Samuel March, Xinyu Liu, Jacek Furdyna The hole-mediated ferromagnetism in III-Mn-V diluted magnetic semiconductors opens up a whole host of possibilities for future multifunctional devices. Control over the ferromagnetic properties in these materials through hole density modulation has been demonstrated using electrical gates [1] and CW optical excitation [2], and more recently using femtosecond optical excitation [3,4]. Using time-resolved magneto-optical Kerr Effect spectroscopy, we have measured the magnetization and coercivity dynamics in GaMnAs. Our experiments reveal a subpicosecond ferromagnetic to paramagnetic phase transition followed by coercivity enhancement on longer time scales. Our findings are promising for possible applications in ultrafast, nonthermal magneto-optical recording using diluted magnetic semiconductors. [1] H. Ohno et al., Nature 408, 944 (2000). [2] S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997). [3] J. Wang et al., Phys. Rev. Lett. 95, 167401 (2005). [4] J. Wang et al., Phys. Rev. Lett. 98, 217401 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A33.00008: Magnetic domain structure study of a ferromagnetic semiconductor using a home-made low temperature scanning Hall probe microscope Seongsoo Kweon, Alex De Lozanne, Nitin Samarth GaMnAs is a ferromagnetic semiconductor actively studied for basic research and for the possibility of application to spintronic devices. To study the local magnetic properties of this material the magnetic force microscope (MFM) is too invasive (by affecting the domains in the sample) or not sensitive enough (due to the weak magnetization of the GaMnAs). We have therefore built a scanning Hall probe microscope (SHPM) to complement our MFM studies. We use a lock-in amplifier to supply a bias current of 1-10$\mu $A and to measure the Hall voltage. We calibrated this home-made SHPM with a computer hard disk sample. Comparing images of this sample obtained with MFM and SHPM we show that our home-made SHPM is operating well. We observed the domain structure of 30-nm thick Ga$_{0.94}$Mn$_{0.06}$As epilayer grown on a 700nm-thick In$_{0.13}$Ga$_{0.87}$As buffer covering a GaAs substrate. We will study the magnetic domain structure as a function of temperature with varying external magnetic fields. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A33.00009: Si co-doping of GaMnAs: a solution for removing As antisites Y.J. Cho, X. Liu, J.K. Furdyna The primary defects that degrade the ferromagnetism of GaMnAs are believed to be Mn interstitials and As antisites. The former can be reduced by low temperature annealing after growth. However, so far no solution for removing As antisites in GaMnAs has been developed. In this connection we report the effect of Si co-doping on Ga$_{1-x}$Mn$_{x}$As films. For $x <$ 0.05, Si co-doping decreases the Curie temperature compared to undoped GaMnAs. However, at higher Mn concentration Si co-doping has the desirable effect of improving both ferromagnetic and structural properties of GaMnAs. To achieve a high Mn concentration in GaMnAs the growth temperature has to be sufficiently low to prevent the formation of MnAs clusters, but this introduces a high density of As antisites in GaMnAs, thus degrading its structural and ferromagnetic properties. However, when such growth of GaMnAs is accompanied by Si co-doping, x ray diffraction results show that such co-doping removes almost all As antisites in GaMnAs with high Mn concentration. Furthermore, magneto-transport and magnetization measurements show that the ferromagnetic properties of thick Si co-doped Ga$_{1-x}$Mn$_{x}$As (x $>$ 0.1) films are greatly improved compared to samples without Si. This suggests that Si co-doping provides an effective solution for removing As antisites in GaMnAs at high Mn concentrations. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A33.00010: Ferromagnetism in epitaxial InMnSb films Nidhi Parashar, Bruce Wessels The structure and ferromagnetic properties of epitaxial In$_{1-x}$Mn$_{x}$Sb semiconductor films deposited using metal-organic vapor phase epitaxy were investigated. Films were single phase as determined by x-ray diffraction for x = 0.01 to 0.05. A rocking curve width of 0.3 degrees was measured in $\theta $-2$\theta $ x-ray scans. XRD $\phi $-scans indicated that the films were epitaxial. Films are ferromagnetic at room temperature as indicated by hysteretic behavior. For an In$_{0.965}$Mn$_{0.035}$Sb epitaxial film a saturation magnetization (Ms) and coercive field (H$_{C})$ of 20 emu/cm3 and 240 G respectively were measured at 295 K. The field cooled and zero field cooled magnetization curves exhibit reversible behavior confirming the absence of any impurity phase. The temperature dependent magnetization was well-described by a Brillouin function. The Curie temperature (T$_{C})$ was above 400 K as determined from the field cooled magnetization temperature dependence. High temperature SQUID measurements will also be presented. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:36AM |
A33.00011: Self-organized magnetic GeMn nanocolumns in germanium Invited Speaker: Spintronics requires spin injectors compatible with silicon technology and operating at room temperature. Potential candidates are group-IV ferromagnetic semiconductors like Mn-doped silicon or germanium. In this presentation, we show that co-evaporating Ge and Mn on Ge(001) and GaAs(001) substrates using low-temperature MBE leads to the formation of self-assembled Mn-rich nanocolumns. These nanocolumns are observed in a wide range of growth temperatures (80\r{ }C to 180\r{ }C) and Mn concentrations (1{\%} to 11{\%}). However the deposition rate is kept very low in order to favor the 2D spinodal decomposition which promotes the growth of nanocolumns. In this talk, we first present a complete phase diagram of nanocolumns as a function of growth temperature and Mn concentration focusing on their size, density, crystalline structure and magnetic properties. In particular, we could demonstrate that at low growth temperature it is possible to tune the columns density and at higher growth temperatures their size distribution. Moreover vertical self-organization of nanocolumns in (GeMn/Ge) superlattices could be achieved. At low growth temperatures, nanocolumns exhibit the diamond structure of Ge and contain up to 30 {\%} of Mn. By combining ab-initio calculations and EXAFS measurements, we could suggest a realistic building block of the nanocolumns. In parallel we have studied the crystalline structure of nanocolumns using grazing incidence x-ray diffraction on synchrotron radiation facilities. We then correlated the magnetic properties like magnetic anisotropy of nanocolumns to their structure by combining SQUID and EPR measurements in a three-dimensional geometry. Finally magneto-transport measurements were performed to evidence the coupling between carriers and the magnetic nanocolumns. CIP measurements mostly give information on the Ge matrix electronic properties and CPP measurements on the nanocolumns. We show the first CPP measurements on a single nanocolumn using nanocontacts. [Preview Abstract] |
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