Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D9: Focus Session: Theory of Magnetic Semiconductors |
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Sponsoring Units: DCOMP DMP GMAG Chair: Andre Petukhov, SDSMT Room: LACC 153A |
Monday, March 21, 2005 2:30PM - 2:42PM |
D9.00001: Ab-initio Calculation of Zero-Field Spin Splitting in Heterostructures of III-V and II-VI Semiconductors. Athanasios Chantis, Mark Van Schilfgaarde, Vladimir Antropov We have developed a fully relativistic ab-initio transport method within the Linear Muffin Tin Orbitals (LMTO) Principal Layer Green Function framework. The calculated spin orbit induced band splittings of several III- V and II-IV bulk semiconductors are in excellent agreement with a well tested perturbative approach, as well as with previous fully relativistic results and available experimental data. The orbital magnetic moment of several 3d transition metals are also in excellent agreement with the perturbative approach and previous fully relativistic results. The method is used for a first principles investigation of Rashba and Dresselhaus effects in III-V and II-VI compounds asymmetric quantum wells, double barrier and single barrier heterostructures. The magnitude and ratio of the two effects determine the spin scattering ratio and anisotropy in quantum wells. These quantites are also important for the construction of spin modulators. With the present method periodic boundary conditions are not required; thus electric fields can be applied without the need for Berry phases. It is possible to obtain reliable values from first principles for a wide variety of geometries, and to clearly distinguish the Dresselhaus and Rashba contributions. The progress of the work and several results will be reported. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D9.00002: Charge and spin transport of diluted magnetic semiconductors M. A. Majidi, J. Moreno, M. Jarrell, R. S. Fishman We use the Dynamical Mean Field Approximation (DMFA) to study the transport properties of diluted ferromagnetic semiconductors such as GaAs doped with Mn. Our model incorporates the strong spin-orbit effects on the j=3/2 GaAs valence band and the exchange interaction between the randomly distributed local ions and the itinerant holes. We calculate the density of states and the spectra of the system for different carrier concentrations, coupling strengths and temperatures. In the ferromagnetic phase the Zeeman split GaAs bands are strongly anisotropic. This anisotropy reflects on the charge- and spin transport conductivities which are different for carriers moving parallel- or perpendicularly to the direction of the average Mn magnetic moment. We discuss how to take advantage of this intrinsic anisotropy in the design of semiconducting heterostructures. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D9.00003: Magnetism and Doping in GeMnN$_2$ M. Weinert, S. H. Cheung, L. Li Although Mn doping has played a prominent role in the search for magnetic semiconductors, the development of new materials often has been hampered by phase segregation. To avoid this problem, compounds with high Mn concentrations are appealing, with GeMnN$_2$ being a promising candidate. We present Full-potential Linearized Augmented Plane Wave (FLAPW) calculations of the structural, electronic, and magnetic properties for both wurtzite and zincblende modifications. The ground state is found to be a semiconducting antiferromagnetically ordered ``hexagonal'' oP16 structure, with large local moments of $\sim$5 $\mu_B$/Mn. The Mn atoms on the different magnetic sublattices are almost fully spin-polarized. Doping and/or intrinsic defects on the Mn sites effectively remove moments from the AFM background, resulting in a net {\it ferri}magnetic moment of 5 $\mu_B$/Ge antisite; for Cu dopants, there is a net moment of 4 $\mu_B$/Cu and the system becomes a half-metallic $p$-doped material. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:42PM |
D9.00004: Tailoring Ferromagnetic Semiconductors Invited Speaker: If magnetic semiconductors are ever to find wide application in real spintronic devices, their magnetic and electronic properties will require tailoring in much the same way that band gaps are engineered in conventional semiconductors. Unfortunately, no systematic understanding yet exists of how, or even whether, properties such as Curie temperatures and band gaps are related in magnetic semiconductors. We have explored theoretically these and other relationships within 64 members of a single materials class, the Mn-doped II-IV-V2 chalcopyrites, three of which are already known experimentally to be ferromagnetic semiconductors. Our first-principles results reveal a variation of magnetic properties across different materials that cannot be explained by either of the two dominant models of ferromagnetism in semiconductors. Based on our results for structural, electronic, and magnetic properties, we identify a small number of new chalcopyrites with excellent prospects for stable ferromagnetism. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D9.00005: First Principles Investigation on the Spatial Distribution and Magnetism in Cr doped GaN X.Y. Cui, C. Stampfl, B. Delley, J Medvedeva, A.J. Freeman Exhaustive structural and magnetic configurations have been investigated to understand the magnetism of Cr:GaN. Our results provide direct evidence that the magnetic ions have a strong tendency to get close by forming ``constrained clusters'' with short-range magnetic interactions. Substitutional clustering configurations are found much more favourable than the substitutional-interstitial complexes. Significantly, while for isolated and ``pair'' substitutional configurations the ferromagnetic state is the ground state with a magnetic moment close to 3$\mu _{B}$/Cr, for more than three Cr atoms configurations, antiferromagnetic states are found energetically more favourable where the net spin is much lower (0-1.8 $\mu _{B})$. We propose a picture that various Cr-configurations coexist and the statistical distribution and associated magnetism depend sensitively on the growth conditions. Such a view can elucidate and explain many of the hitherto puzzling experimental observations. We argue such a picture may also hold for other diluted magnetic semiconductor systems. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D9.00006: Real-Space Screened-Exchange Method for Large Systems Titus Sandu, Mark van Schilfgaarde A self-consistent screened exchange method in real-space has been developed for electronic structure calculations. Both static polarization function P0 and self-energy sigma are calculated in real space using Green's function within TB-LMTO (tight-binding linear muffin-tin orbital) method. The method is fast and does not require orthogonal orbitals, enabling us to handle large systems. We compare the results for GaAs in zincblend stucture, MnAs with NiAs-type structure, and Ga3MnAs4 in zincblend structure with those results obtained from GW approximation. Finally, the method is applied to diluted Ga(1-x)Mn(x)As. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D9.00007: Combining Dynamical Mean Field and Local Density Approximations in the study of GaMnAs J. Moreno, M. A. Majidi, Paul Kent, M. Jarrell, R. S. Fishman We use the band structure predicted by the Local Density Approximation as the starting point for a self consistent Dynamical Mean Field treatment of the ferromagnetic order in GaAs doped with Mn. Our model is an effective eight-band model including the heavy and light valence bands, the split-off valence band and the conduction band of the host material. The exchange interaction between the randomly distributed magnetic ions and the itinerant carriers is modeled using a modified double-exchange coupling. We compare our results with the ones predicted by more simplified models. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D9.00008: GdN: an excellent candidate for high performance spintronic devices Chun-gang Duan, R.F. Sabiryanov, W.N. Mei We report a detailed theoretical study of the electronic structure and magnetic properties of GdN. Based on first-principles total energy calculations, we deduced the exchange interaction parameters of GdN from fitting the total energies of different magnetic configurations to those computed from the Heisenberg model. The Curie temperature obtained by the Monte Carlo simulation agreed well with experiment. Furthermore, we found strong lattice constant dependence of the exchange parameters of GdN. Our band-structure analysis provides a convincing explanation to this phenomenon. In addition, from the band structures studies of the spin majority and minority of GdN, we expect this compound, which is an excellent half-metallic ferromagnet with a large half-metallic gap, could be utilized for spintronics devices. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D9.00009: Half-metallicity at the (001) surface of Co$_2$MnSi and magnetic properties of thin Co$_2$MnSi films on Si(001) P. Kratzer, S.J. Hashemifar, Hua Wu, M. Scheffler Recently, films of the half-metal Co$_2$MnSi have been discussed as potential spin injectors for semiconductor spintronics. We have studied the stability, the electronic and magnetic properties of the Co$_2$MnSi(001) surface for 15 different terminations using density-functional theory (DFT) calculations. The phase diagram obtained by {\em ab initio} atomistic thermodynamics shows that in practice the MnSi, pure Mn, or pure Si terminated surfaces can be stabilized under suitable conditions. Analyzing the surface band structure, we find that the pure Mn termination, due to its strong surface-subsurface coupling, preserves the half-metallicity of the system, while surface states appear for the other terminations. For ultra-thin Co$_2$MnSi films (2 or 3 double layers) on Si(001), we find that they are thermodynamically stable against decomposition, and that the magnetic moments of the Mn and Co atoms are still similar to those of bulk Co$_2$MnSi. While we see the incipient recovery of the half-metallic gap in the interior layer, this gap gets filled by electronic states at the interface with Si(001) for the atomic structures we have considered. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D9.00010: First-principles calculations of adsorption and incorporation of Mn on GaAs-(110) surfaces Xuan Luo, Richard Martin Using first-principles total-energy methods, we investigate the addition of Mn on the GaAs(110) surface. Our results show that Mn atoms can be adsorbed on GaAs(110) with a preference for the middle site between two Ga atoms along the [001] direction, that is to be adsorbed at the center of triangle formed by two surface As atoms and one surface Ga atom. The calculations show that Mn should be incorporated into the first layer since substitution of the Mn for a Ga atoms results in a larger binding energy. This is in agreement with STM images from our collaborating experimental group[1] and with previous tight-binding calculations[2]. Studies are in progress on complexes of two and more Mn atoms on the surface.\\ References:\\ 1. See talk by A. R. Richardella, D. Kitchens, and A. Yazdani.\\ 2. H. X. Fu, L. Ye, K. M. Zhang and X. D. Xie, Surface Science, vol. 341 273, (1995). [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D9.00011: Density functional determination of the magnetic state of $\beta$-MnAs Leonard Kleinman, Manish Niranjan, B.R. Sahu Ferromagnetic hexagonal $\alpha$-MnAs has a first order phase transition to $\beta$-MnAs at about 40 $^{\circ}$ C. Because it shows no long range antiferromagnetic order, most workers assume that it is paramagnetic in spite of the fact that it does not have a Curie-Weiss magnetic susceptibility. With the aid of density functional calculations, we show that it is antiferromagnetic and explain the lack of observed long range order. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D9.00012: Rocksalt MnN: A Vacancy Stabilized Structure Maosheng Miao, Walter Lambrecht Recent density functional computations showed that the zinc blende is the most stable structure for MnN. However, so far MnN has only been found in a tetragonally distorted rocksalt (RS) structure. This conflict is resolved by our full potential linearized muffin-tin orbital calculations that showed the RS structure is stabilized by a few percent of nitrogen vacancies in MnN. Our calculations show that that the Gibbs energy of formation of the vacancy is low even under quite N-rich conditions and can even be negative under N-poor conditions. On the other hand, vacancies are hard to form in the ZB structure. The vacancies also affect the magnetic moments of their surrounding Mn atoms. When several vacancies are introduced per supercell we find that in the RS structure the vacancies prefer to stay in next nearest neighbor positions from each other at low concentration while they form ordered structure at high concentration. For the ZB structure, the vacancies tend to stay close even at high concentration. Supported by ONR and NSF. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D9.00013: Relevance of short-range correlations on the ferromagnetic order of dilute magnetic semiconductors B. Moritz, K. Mikelsons, J. Moreno, M. Jarrell, R. S. Fishman We use the Dynamical Cluster Approximation (DCA) to study the ferromagnetic transition in GaAs doped with Mn. In addition to the local dynamics our approach includes the effects of short-range correlations. We study the critical temperature, the appearance of the impurity band and the development of the magnetization for a range of coupling strengths, doping, and carrier concentrations. We conclude that the understanding of short- range correlations in the ferromagnetic semiconductors is crucial for the successful design of spintronic nanostructures. [Preview Abstract] |
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D9.00014: Ferromagnetic ordering and halfmetallic state in a shandite: Co$_3$Sn$_2$S$_2$ Helge Rosner, Richard Weihrich, Walter Schnelle The recent rapid development in spintronics challenges the search for new magnetic half metals with high Curie temperatures as well as an improved understanding of the underlying microscopic properties. Here, we present a joint experimental and theoretical study of the recently reinvestigated shandite Co$_3$Sn$_2$S$_2$ [1]. From magnetic susceptibility, specific heat and resistivity measurements on powder samples we find a phase transition to a ferromagnetic metallic state at 177 K with a saturation moment of 0.87 $\mu_B$/f.u. Full potential electronic structure calculations within the local spin density approximation result in a halfmetallic ferromagnetic groundstate with a moment of 1 $\mu_B$/f.u. and a tiny gap in the minority spin channel. The calculated structure optimization and structure variations show that the size of the gap is rather sensitive to the lattice geometry. Possibilities to stabilize the halfmetallic ferromagnetic behaviour by various substitutions have been studied theoretically and will be discussed in detail.\\[0pt] [1]R. Weihrich {\it et. al.} Z. Anorg. Allg. Chem. {\bf 630}, 1767, (2004) [Preview Abstract] |
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D9.00015: Magnetic phases of $M_xAs_y$ (M=Co, Fe, Mn) compounds Fredrik Bultmark We present theoretical calculations of $X_xAs_y$, (X = Co, Fe, Mn) in different structures in order to understand how the As surrounding influence the stability of the magnetic phases. We analyse the quenching of the magnetic moment of the ferromagnetic phases in terms of the pd+hybridisation between the magnetic ion and As. A simplified model of the quenching is suggested and the dependence on the number of As neighbours and nearest neighbour distance is investigated. [Preview Abstract] |
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