Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H10: Focus Session: Magnetic Semiconductors: Electronic Structure |
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Sponsoring Units: DMP GMAG Chair: Jairo Sinova, Texas A&M University Room: LACC 153B |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H10.00001: Electronic Structure of Mn in III-V semiconductors: lessons from First-principles theory Invited Speaker: Alex Zunger Early model-theories have suggested that Mn in the III-V semiconductors such as GaAs and GaN will introduce a host-like- hole state that will interact via RKKY-type coupling with the Mn local moment to produce the observed ferromagnetism ( FM).This widely publicized model made then specific predictions thus prompting a worldwide search for host materials satisfying these conditions.I will show how modern first-principles electronic structure therory can be used to establish if the basic properties of Mn and other 3d impurities in III-V semiconductors are indeed similar to what was widely assumed in earlier phenomenological model theories.Surprizingly,such calculations ( via LDA,GGA,and LDA+U) reveal that the hole induced by Mn is not host-like,and that the ensuing FM is not RKKY-like,but has a characteristic dependence on the lattice-orientation of the Mn-Mn interactions in the crystal which is unexpected by RKKY.I will describe the chemical trends as the host is altered ( GaN/GaP/GaAs/GaSb and Chalcopyrite semiconductors such as CuInSe2 );as the impurity is altered along the 3d series,and as the Fermi level is altered ( via doping ).The effects of clustering of impurities,as well as the site-preference ( substitutional,interstitial ) will be revealed.These calculations open the way to "Inverse Design",by searching the atomic configuration of Mn in the lattice having the highest FM Tc.See Mahadevan,Sarma,and Zunger,Phys.Rev.Letters 93,177201 (2004) . [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H10.00002: Antiferromagnetic $s-d$ exchange coupling in GaMnAs R.C. Myers, M. Poggio, N.P. Stern, A.C. Gossard, D.D. Awschalom Molecular beam epitaxy growth of GaMnAs is typically performed at low substrate temperatures ($\sim$250 $^{\circ}$C) and high As overpressures leading to the incorporation of excess As and Mn interstitials that quench optical signals, such as photoluminescence. In this work, optical quality samples of paramagnetic Ga$_{1-x}$Mn$_{x}$As-Al$_{0.4}$Ga$_{0.6}$As quantum wells with x $<$ 0.0005 are achieved by performing crystal growth at a substrate temperature of 400 $^o$C. Electronic and structural measurements demonstrate that this elevated temperature reduces As defects while allowing the substitutional incorporation of Mn into Ga sites. Using time-resolved optical spectroscopy, the electron spin coherence is measured allowing for the extraction of the sign and magnitude of the conduction band spin splitting due to the $s-d$ exchange interaction ($N_{0}\alpha$), whose sign is negative and magnitude varies as a function of well width. In the limit of wide quantum wells $N_{0}\alpha$ $<$ 0 indicating that antiferromagnetic $s-d$ exchange is a bulk property of GaMnAs. Polarization-resolved photoluminescence spectroscopy is used to measure the total excitonic spin splitting due to the $sp-d$ exchange interactions, $N_{0}(\alpha-\beta)$, and thus the sign and magnitude of the $p-d$ exchange constant ($N_{0}\beta$) is found. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H10.00003: Calculation study of Curie Temperature in GaAs based diluted magnetic semiconductor Jialei Xu, Mark van Schilfgaarde, German D. Samolyuk We present calculations study of several attempts to increase the Curie Temperature in Mn doped GaAs. In our calculation, the local spin density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonion, but no significant further approximations are made. Special quasi-random structures in large unit cells are used to accurately model the disorder. Tc is computed using cluster variation method developed for the classical Heisenberg model. By artificially changing the Fermi level of Mn:GaAs, our calculation indicated the Tc of this system can reach its maximum with half filled band. This result shows a similar picture with double exchange model. This effect was also confirmed by using a virtual dopant with atomic number between Mn and Cr. This result suggests a route to increase Tc by co-doping of Mn and Cr in GaAs. However, our calculation for the disordered Mn and Cr co-doped GaAs shows the decrease in Tc, which may be due to the charge transfer from Cr into Mn. We also investigate the delta-doped Mn:GaAs. By study the different configuration and doping concentration. We found the Tc of one delta-layer structure can reach its maximum with moderate doping concentration ($\sim $25{\%} in the layer). [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H10.00004: Band structure model of magnetic coupling in II-VI and III-V semiconductors Su-Huai Wei, Gustavo M. Dalpian Mn-doped II-VI and III-V diluted magnetic semiconductors (DMS) exhibit a wide range of magnetic ordering behavior. For example, Mn doped II-VI semiconductors are mostly having antiferromagnetic (AFM) ground state, whereas Mn doped III-V semiconductors are mostly having ferromagnetic (FM) ground state. More interestingly, Mn doped GaN can be either FM or AFM, depending on the Mn concentration, carrier density, or pressure. It has been a great challenge to understand the mechanism of magnetic coupling in these systems. Several models have been used to explain the phenomena, including the Zener/RKKY model, super exchange model and double exchange models. Although these models are successful in explaining some of the systems, they often lack the universality and transparency, and is difficult to compare directly with the band structure calculations. In this work, we will present an unified model to explain magnetic ordering in Mn doped semiconductors. This model is based on the $p-d$ and $d-d$ level repulsions between the Mn ions and host elements and can successfully explain magnetic ordering observed in all Mn doped II-VI and III-V semiconductors such as CdTe, GaAs, ZnO and GaN. This model, therefore, provides a simple guideline for future band structure engineering of magnetic semiconductors. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H10.00005: Compensation, interstitial defects and ferromagnetism in III-V semiconductors Timothy Ziman, Georges Bouzerar, Josef Kudrnovsk\`y In diluted magnetic semi-conductors, e.g. Ga(Mn)As, Ga(Mn)N and In(Mn)As, the observed magnetism depends strongly on methods of preparation and sample history, with correlation between the Curie temperature T$_c$ and the conductivity for the same concentration of magnetic ions. The transport measurements show that the simple picture of substitution of Mn$^{2+}$ ions for Ga sites is insufficient and effects of defects, e.g. Mn interstitials and As anti-sites, must be included. We present a quantitative theory, using magnetic exchange interactions from carrier- and impurity- concentration dependent {\it ab initio} estimates, and a recently developed semi-analytic theory$^{1,2}$. Spin fluctuations are treated in a locally self-consistent RPA approach, and disorder exactly, by sampling. Very good agreement, without ajustable parameters, is obtained for the T$_c$ of different samples. We predict T$_c$ as a function of hole concentration for Mn$_x$Ga$_{1-x}$As and In$_x$Ga$_{1-x}$As. For fixed x, T$_c$ is non-monotonic in carrier concentration for a restricted region of carrier density and vanishes outside. [1]G. Bouzerar, T. Ziman, J.Kudrnovsk\'y, cond-mat/0405322 [2]G. Bouzerar, T. Ziman, J.Kudrnovsk\'y, Appl. Phys. Lett. (scheduled Nov. 29 2004), cond-mat/0407101 [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H10.00006: Mn electronic structure and induced Ga moment in Mn-doped GaN Lian Li, Sau Ha Cheung, Seth King, Michael Weinert , David Keavney We use soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) to examine the Mn valence, magnetic behavior, and induced Ga magnetic moment in a series of MBE-grown Mn-doped GaN films. We find two distinct magnetic states for Mn, one that is clearly Mn$^{2+}$, and another of indeterminate valence that is potentially due to Mn interstitials. A comparison of fluorescence and electron yield data suggests a depletion of Mn interstitials near the surface. A small magnetic polarization exists on the Ga site, apparently antiparallel to the Mn moment. First principle calculations suggest that most defects, such as Mn interstitials, double Mn substitutions and Ga vacancies, lead to small Ga moments that are parallel to Mn, while only the split N interstitials lead to antiparallel Ga moment. Work at the University of Wisconsin, Milwaukee was supported by NSF grant No. DMR-0094105. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31-109-Eng-38. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H10.00007: Spin-Fermion Models for Manganites and Diluted Magnetic Semiconductors: A Dynamical Mean Field Study Florentin Popescu, Gonzalo Alvarez, Elbio Dagotto Using Dynamical Mean Field Theory (DMFT) we derive general expressions for the Curie Temperature ($T_{c}$) of a spin-fermion model for any coupling constant $J$ and any concentration of localized spins $x$. In the case of manganites, we compare these results with those obtained previously for the case of an infinite Hund's coupling $J_{H}$. In the case of diluted magnetic semiconductors (DMS) we discuss the dependence of $T_{c}$ on model parameters and the effect of the inclusion of a more realistic band structure. We show that DMFT is a powerful tool to study spin-fermion models for DMS in the weak coupling regime. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H10.00008: ScN:Mn a dilute ferromagnetic semiconductor Aditi Herwadkar, Walter R. L. Lambrecht We study the electronic properties of ScN:Mn calculated using the full potential linearized muffin-tin orbital method. To model the impurity we use 64 atom supercells and fully relax the structure. Band gap corrections are included in a closely related atomic sphere approximation (ASA). Mn on a Sc site is found to induce a localized state in the middle of the band gap of ScN with $t_{2g}$ character. Its spin splitting leads to a net magnetic moment of 2-3 $\mu_B$ per Mn. Calculations of unit cell for near neighbor Mn atoms reveal that the spins prefer ferromagnetic coupling. Using mean field approximation and assuming a random distribution of Mn atoms we estimate the Curie temperature. Above room temperature $T_c$ seems possible with only 2 \% Mn concentration. The Gibbs energy of formation of the Mn impurity is found to be 3.6 eV, which is comparable to that of other magnetic semiconductor systems. The shared rocksalt structure of MnN and ScN should facilitate alloy formation. The energy of formation of the Mn pairs indicates, no tendency towards clustering. Notably, the ferromagnetic coupling in this system occurs even without the condiseration of a carrier mediated coupling mechanism. The presence of local magnetic moments on Mn results in a small spin splitting of the Sc d-like conduction bands, so one could expect to see interesting effects on carrier transport in n-type ScN:Mn. A high n-type concentration however may tend to reduce the magnetic moments. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H10.00009: First-principles Study of the Structural and Magnetic Properties of Cobalt Indium Nitride Pavel Lukashev, Walter R. L. Lambrecht In previous work we have shown that at atmospheric pressure CoN has the zincblende (ZB) equilibrium crystal structure, in agreement with experimental results of Suzuki et al. [1] The ZB lattice structure would allow for a nice match to semiconductors such as GaN and SiC if the lattice constant of CoN can be slightly increased by doping with suitable atoms with larger atomic radii. In this work we study structural and magnetic properties of Co$_{1-x}$In$_x$N alloys. The theoretical framework of our calculations is the density functional method in the local spin density approximation (LSDA). Our calculations are carried out using the full-potential linear muffin-tin orbital band-structure method (FP-LMTO). We find that the lattice constant follows Vegard's law. Furtermore this expansion of the lattice constant leads to more localized behavior for the Co $d$ states and hence the formation of magnetic moments. The magnetic moments and spin-polarization of the density of states at the Fermi level are studied as function of concentration and lattice constant. Finally, a comparison is made with corresponding Fe$_{1-x}$In$_x$N alloys. \\ \\ 1. Suzuki K, Kaneko T, Yoshida H, Morita H, Fujimori H J. Alloys Compd. {\bf 224}, 232 (1995) [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H10.00010: Theoretical modeling of epitaxial growth and properties of Mn/Ge (001) multilayers J.E. Medvedeva, A.J. Freeman, J.B. Ketterson As part of the search for useful dilute magnetic semiconductors, structural, electronic and magnetic properties of Mn/Ge (001) digital alloys and multilayers are determined using our highly precise full-potential linearized augmented plane wave (FLAPW) method\footnote{E.Wimmer, H.Krakauer, M.Weinert, A.J.Freeman, PRB {\bf 24}, 864 (1981)}. First, the calculated formation energies of the fully relaxed structures with different Mn and Ge site locations (both substitutional and interstitial), predict the lowest-energy structure in an epitaxial growth process. We found that (i) substitutional positions are energetically more favorable for one (001) monolayer of Mn in the supercell and (ii) when the number of Mn layers increased, the magnetic atoms prefer a second-layer interstitial site and form a 45$^{\circ}$-rotated fcc structure on the Ge diamond structure. For the Mn/Ge (001) multilayers, which consist of 8 layers of Ge and 1 or 4 layers of fcc Mn, we found that the experimental ferromagnetic coupling between Mn atoms can be reproduced only when Coulomb interactions are taken into account; indeed, LDA+U estimates of T$_c$ as a function of the Mn layer thickness are in good agreement with experiment\footnote{J.J.Lee, J.E.Medvedeva, J.H.Song, Y.Cui, A.J.Freeman, J.B.Ketterson (to be published)}. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H10.00011: Magnetism of silicon doped with alkaline earth elements Bang-Gui Liu, Li-Jie Shi, Steven G. Louie It would be highly desirable to fashion spintronic materials based on silicon because of the vital importance of silicon in semiconductor technology. Since it is very difficult to experimentally dope transition metals into silicon, we explore the possibility of ferromagnetism in silicon doped with alkaline earth atoms using first-principle calculations. It is found that the energy needed for doping a calcium atom into silicon is approximately equivalent to the formation energy of a vacancy in silicon. Neither silicon nor any alkaline earth metal alone is magnetic, but our calculations indicate that ferromagnetism may be obtained by appropriately doping silicon with alkaline earth atoms such as calcium. This is intriguing for basic understanding of magnetism and for possible future applications. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H10.00012: A dilute ferromagnetic semiconductor with an isolated impurity band: (Ga,Mn)P M.A. Scarpulla, B.J. Cardozo, O.D. Dubon, W.M. Hlaing Oo, M.D. McCluskey Ga$_{1-x}$Mn$_{x}$P can provide insight into the physics of ferromagnetic III-V semiconductors because it is expected to be chemically intermediate between Ga$_{1-x}$Mn$_{x}$As, in which free holes in the valence band mediate the ferromagnetic exchange, and Ga$_{1-x}$Mn$_{x}$N, where double exchange in an impurity band may provide the ferromagnetic coupling. We have recently synthesized ferromagnetic films of Ga$_{1-x}$Mn$_{x}$P with T$_{C}$ up to 65 K using Mn ion implantation followed by pulsed-laser melting. We present the first reported electrical transport measurements in Ga$_{1-x}$Mn$_{x}$P, which show hopping conduction from 10-300 K with a change in activation energy at T$_{C}$, large negative magnetoresistance, and a large anomalous Hall effect reflecting the sample magnetization. The activation energy from resistivity measurements is 29 meV, while far infrared photoconductivity spectra reveal a gap of 26 meV between localized and conducting states. Mid-infrared absorption spectra have identified an impurity band centered near the 0.4 eV binding energy of the Mn acceptor. We interpret these measurements as demonstrating that the Mn band exists and that the ferromagnetic exchange is mediated by carriers in this band. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H10.00013: Piezo-Magneto-Electric Effects in p-Doped Semiconductors Oskar Vafek, Andrei Bernevig We investigate the appearance of a uniform magnetization in strained three dimensional p-doped semiconductors with inversion symmetry breaking subject to an external electric field. We compute the magnetization response to the electric field as a function of the direction and magnitude of the applied strain. This effect could have potential application in manipulation of the collective magnetic moment of hole mediated ferromagnetism of magnetically doped semiconductors. [Preview Abstract] |
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H10.00014: Anisotropic exchange interactions in III-V diluted magnetic semiconductors Carsten Timm, A. H. MacDonald The RKKY interaction between substitutional Mn local moments in GaAs is both spin-direction-dependent and spatially anisotropic. We determine the strengths of these two types of anisotropy using a semi-phenomenological tight-binding model that treats the hybridization between Mn d-orbitals and As p-orbitals perturbatively and accounts realistically for its non-locality. We show that spin-orbit coupling, exchange non-locality, and band-structure anisotropy all play a role in determining these strengths. The results are used to estimate the degree of ground-state magnetization suppression due to frustrating interactions between randomly located Mn ions. We discuss our results for the RKKY interaction in relation to previous calculations from both phenomenological models and ab-initio approaches. [Preview Abstract] |
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