Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H32: Focus Session: Theory and Simulation of Spin-Dependent Effects and Properties I: (First-Principles Calculations) |
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Sponsoring Units: GMAG DCOMP DMP Chair: Andre Petukhov, South Dakota School of Mines Room: 336 |
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H32.00001: Longitudinal spin fluctuations in itinerant ferromagnets Invited Speaker: Finite-temperature properties of magnetic materials strongly depend on their magnetic configuration given by relative orientation of local magnetic moments. In itinerant ferromagnets, an additional degree of freedom becomes important: the value of local magnetic moment, which is sensitive to the magnetic state and the local chemical environment of atoms as well as the thermal electronic excitations of different type, leading to the so-called longitudinal spin fluctuations (LSF). LSF play an important role in itinerant magnetic systems at high temperatures. In particular, they are responsible for the existence of finite local magnetic moments on atoms in the paramagnetic state, which affect different physical properties. At the same time, an ab initio based description of the LSF is problematic. The LSF are ignored in the local density and related approximations of density functional theory, which for instance leads into a substantial underestimation or complete disappearance of the local magnetic moments on atoms in the itinerant ferromagnets in the corresponding calculations at high temperature in the paramagnetic state. Although the LSF can be included in more elaborate schemes, such as the dynamical mean-field theory, the application of such techniques to real systems is too cumbersome in most cases. In this work a generalized form of classical magnetic Hamiltonian is suggested, which includes both transverse and longitudinal spin fluctuations on equal footing. Parameters of the Hamiltonian can be determined in the first-principles calculations, within the local spin density approximation. The method is applied to the calculations of high-temperature magnetic properties of Fe, Co and Ni, including the Curie temperature. The effect of the LSF in the high-temperature paramagnetic state on chemical interactions and other physical properties is demonstrated for several alloys, including fcc Fe-Cr-Ni alloys, which is the basis of austenitic stainless steels. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H32.00002: Spin excitations in solids from first principles Arno Schindlmayr, Ersoy Sasioglu, Manfred Niesert, Christoph Friedrich, Stefan Bl\"ugel The long-range order of the electron spins in magnetic solids gives rise to additional excitation modes that preserve the charge density but change the total spin of the electron system. While Stoner excitations, which correspond to spin-flip transitions between the majority and minority channels, can be described within a single-particle picture, spin waves are collective modes that result from the spin-dependent exchange interaction between the electrons. Here we discuss different approaches that we have explored for material-specific spin-wave calculations from first principles. All of these methods focus on the non-local and dynamic transverse spin susceptibility, whose spectral function can be directly related to experimental spectroscopies, but employ either time-dependent density-functional theory or many-body perturbation theory to treat exchange and correlation. In the latter case, maximally localized Wannier orbitals are used to efficiently obtain the electron-hole vertex of the multiple-scattering $T$ matrix, which is constructed with full frequency and wave-vector dependence. The implementation uses the full-potential linearized augmented-plane-wave (FLAPW) method. For ferromagnetic transition metals like Fe, Co or Ni our results are in good agreement with experimental data and reproduce all important spectral features. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H32.00003: Transverse spin susceptibility in Ni, Fe and Co Liqin Ke, Takao Kotani, Mark van Schilfgaarde, Vladimir Antropov We calculate the full transverse spin susceptibility $\chi^{\pm}(\bf q, \omega)$ in the time-dependent local density approximation(TDLDA) for elemental Ni, Fe and Co. We extract the Heisenberg exchange parameters from both the energy-dependent $\chi^{\pm}$ and the static one. The results are compared with those given by a method assuming the rigid rotation of the magnetic moments at each site(ref.kotani2008). We observe some differences between these two methods, especially around the Brillouin zone boundaries. We also calculate $\chi^{\pm}$ starting from the non-interacting Hamiltonian generated from the quasiparticle self-consistent GW(QSGW) approximation (ref.Kotani2007). We analyze how the QSGW potential alter the LDA results. \\ Ref.\\ T.Kotani and M van Schilfgaarde, J. PHYS. C 20, 295214 (2008) T.Kotani, M. van Schilfgaarde, S. V. Faleev Phys. Rev. B 76, 165106 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H32.00004: Ab-initio electronic structure calculations of periodic systems in the presence of arbitrary magnetic fields Alfredo A. Correa, Eunseok Lee, Wei Cai, Giulia Galli Ab initio electronic structure calculations in the presence of magnetic fields have been mainly performed for isolated systems, or, in the case of periodic systems, by adopting perturbative approaches. Building on a recent formulation of electronic structure calculations in the presence of magnetic fields [1,2], we will discuss calculations for periodic systems under arbitrary conditions, which include arbitrary (finite) magnetic field, arbitrary periodic cell shapes, and magnetic field spatial variations. Preliminary results based on a planewave numerical approach and local approximations to Density Functional Theory will be presented.[1] W.Cai, G.Galli, Phys. Rev. Lett. 92, 186402 (2004).[2] E. Lee, W. Cai, G. Galli, J. Comput. Phys. 226, 1310 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H32.00005: Real space first-principles derived semiempirical pseudopotentials applied to tunneling magnetoresistance Kirk Bevan, Tony Low, Hong Guo We present a real space density functional theory (DFT) localized basis set semi- empirical pseudopotential (SEP) approach. The method is applied to iron and magnesium oxide, where bulk SEP and local spin density approximation (LSDA) band structure calculations are shown to agree within approximately 0.1 eV. Subsequently we investigate the qualitative transferability of bulk derived SEPs to Fe/MgO/Fe tunnel junctions. We find that the SEP method is particularly well suited to address the tight binding transferability problem because the transferability error at the interface can be characterized not only in orbital space (via the interface local density of states) but also in real space (via the system potential). To achieve a quantitative parameterization, we introduce the notion of ghost semi-empirical pseudopotentials extracted from the first-principles calculated Fe/MgO bonding interface. In general the results underscore the need for separate tight binding interface and bulk parameter sets when modeling conduction through thin heterojunctions on the nanoscale. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H32.00006: Rational Design of Half-Metallic Alloys William Butler, Claudia Mewes, Chunsheng Liu, Mairbek Chshiev A half-metal is a material that is a metal for one spin-channel and an insulator or semiconductor in the other. Half-metals are potentially important for spintronic applications such as magnetic sensors for hard drives and magnetic random access memory. We show using very simple ideas that it is possible to rationally design a class of magnetic alloys by placing a gap at the center of one of the d-bands and placing the Fermi energy in this gap. We will present design rules that can be used to make an infinite number of half-metallic heterostructures. We will also show how the half-metallic feature may be maintained at surfaces and interfaces. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H32.00007: Density-functional theory study of the Co$_2$MnSi/MgO interface Bj\"orn H\"ulsen, Peter Kratzer, Matthias Scheffler Magnetic memory devices that exploit the tunneling magneto-resistance (TMR) effect depend crucially on the spin polarization of the electrode materials. Using ferromagnetic half-metals, such as the full Heusler alloy Co$_2$MnSi, perfect electrodes with 100\% spin polarization could possibly be realized, at least at zero temperature. Here, we use density functional theory (DFT) calculations to model an epitaxially grown Co$_2$MnSi/MgO(001) interface in a prospective TMR device. The stability, the electronic and magnetic properties of different terminations of Co$_2$MnSi (stoichiometric Co- and MnSi- and non-stoichiometric Mn- and Si- planes) and different registry with respect to the insulating barrier (Mg-top, O-top, bridge and hollow site) are investigated. We find that the electronic and magnetic properties (including the existence of the spin gap) depend strongly on the termination. The formation energy of the various interfaces is presented in form of a phase diagram. Both the interface Co/O (Co at O top site) with a high spin polarization of $P = 70$\%, and the interface MnSi/O with only small $P$ form part of the thermodynamically accessible region. The MnMn/O interface preserves the half-metallicity of the bulk, but is found to be only metastable. Interface band structures are presented, and magnetic moments are compared to experimental data. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H32.00008: Magnetoelectric polarizability and axion electrodynamics in crystalline insulators Andrew Essin, Joel Moore, David Vanderbilt Spin-orbit coupling can lead in two- and three-dimensional solids to time-reversal-invariant insulating phases that are ``topological'' in the same sense as the integer quantum Hall effect and similarly have protected edge or surface states. The three-dimensional topological insulator is known to have unusual magnetoelectric properties referred to as ``axion electrodynamics'': it supports an electromagnetic coupling $\Delta{\cal L}_{EM} = (\theta e^2 / 2 \pi h) {\bf E} \cdot {\bf B}$ with $\theta=\pi$, giving a half-integer surface Hall conductivity $\sigma_{xy}=(n + 1/2) e^2 / h$. An approach to $\theta$ in any three-dimensional crystal is developed based on the Berry-phase theory of polarization: $\theta e^2/ 2 \pi h$ is the bulk orbital magnetoelectric polarizability (the polarization induced by an applied magnetic field). We compute the orbital magnetoelectric polarizability for a simple model and show that it predicts the fractional part of surface $\sigma_{xy}$, computed using a slab geometry. Although $\theta$ is not quantized once time-reversal and inversion symmetries are broken, it remains a bulk quantity for the same reasons as ordinary polarization. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H32.00009: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H32.00010: Flexomagnetic effect in Mn-based antiperovskites Renat Sabirianov, Pavel Lukashev We report our theoretical results on the induced magnetization appearance in antiferromagnetic antiperovskites, such as Mn$_{3}$GaN, due to the gradient of applied external strain (flexomagnetic effect). We model the external flexure by forming a 40-atom Mn$_{24}$Ga$_{8}$N$_{8}$ supercell with 4 domains under external strain gradient. This structure shows a net magnetization which increases parabolically up to 0.03$\mu _{B}$ (per Mn atom) in the (0,-1,1) direction reflecting non-linear contribution to local piezomagnetric effect in the considered range of up to 0.005{\%} external strain gradient. The calculated flexomagnetic effect is found to be relatively small with induced magnetic moment order of magnitude smaller than that of piezomagnetic contribution. The flexomagnetic effect can be especially important in the nanostructures, where the stress gradients are usually large due to the surface tension. All calculations were performed using the projector augmented wave method. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H32.00011: Layered antiferromagnetism with high Neel temperature in the intermetallic compound Mn$_{2}$Au Sergii Khmelevskyi, Peter Mohn On the basis of earlier experimental studies the intermetallic compound Mn$_{2}$Au has been characterized as a non-magnetically ordered material. Here we report the results of first-principles calculations based on Local Spin-Density Approximation which describe Mn$_{2}$Au to have a narrow band antiferromagnetic ground state with rigid local moments on the Mn sites. Calculations of the inter-atomic exchange constants based on the magnetic force theorem and a Monte-Carlo modeling of the resulting Heisenberg-like Hamiltonian predict a very high Neel-temperature of $\sim $1580K. This temperature is considerably higher than for the other known high-temperature antiferromagnetic L1$_{0}$-type Mn based binary alloys, which are widely used in magnetic storage applications. The source of the difficulties in determining magnetic order from the earlier experiments is discussed. The observed meta-magnetic like behavior and a susceptibility anomaly at low temperatures are linked to the frustrated magnetism on Mn anti-site impurities. We believe that the high temperature antiferromagnetism of Mn$_{2}$Au may have quite an impact in technology. In particular, it can be considered as a candidate for the application as a ``pinning'' layer in GMR devices. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H32.00012: 3d impurities in wide gap oxides---magnetism and carrier doping. Hannes Raebiger, Stephan Lany, Alex Zunger 3d transition metal impurities in wide-gap oxides exist in multiple charged configurations[1], and may function as (i) donor/acceptor defects to modify carrier concentrations, (ii) magnetic elements to induce collective magnetism, and (iii) shift the host band edges. While previous investigations have addressed some of these phenomenologies separately, we link them together, and present the chemical trends for electronic properties, carrier doping, and magnetism along the series of 3d1...3d8 impurities in the paradigmatic wide-gap oxide hosts ZnO and In2O3. For these general trends we find that, in In2O3 most 3d impurities are amphoteric and exhibit deep transitions, whereas in ZnO the early 3d impurities (Sc-V) have shallow donor transitions, and only the late 3d's (Co, Ni, Cu) have acceptor transitions inside the band gap. Ferromagnetic interactions emerge upon the partial filling of 3d levels resonant inside the conduction band, an in general require electron doping from additional sources. [1] H. Raebiger, S. Lany, and A. Zunger, Nature {\bf 453}, 763 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H32.00013: Multifarious-magnetism in copper oxide nanostructures from first-principles X.-Y. Cui, A. Soon, B. Delley, S.-H. Wei, C. Stampfl Driven by the ever-increasing demand for novel spin-dependent advanced materials, investigation of nanoscale magnetic materials is currently actively pursued. With the latest developments focusing more on magnetic semiconducting oxides, materials based on cuprous oxide, Cu$_2$O, are of high interest as potential $p$-type semiconducting candidates. Thus developing an understanding of how intrinsic defects influence both its electronic and magnetic properties is important. We perform density-functional theory calculations [1] and analyze both the electronic and magnetic properties of native defects in both bulk Cu$_2$O and its surfaces, as well as their respective formation/surface energies under different growth conditions. We find that under oxygen-lean conditions, the experimentally observed ferromagnetic behaviour [2] could originate from copper vacancies on Cu$_2$O(111) while under oxygen-rich conditions, low energy bulk oxygen interstitials might explain the ferromagnetic moment found in the same material. This suggests that the origin of observed magnetism in sub-stoichiometric copper oxide nanoparticles could be multifarious, highlighting the complimentary role of bulk and surface native magnetic defects.\\[0pt] [1] A. Soon \textit{et al}. submitted\\[0pt] [2] A. Ye. Yermakov\textit{et al.} J. Magn. Magn. Mater. \textbf{310}, 2102 (2007) [Preview Abstract] |
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