Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P15: Focus Session: Current and Magnetization Driven Effects |
Hide Abstracts |
Sponsoring Units: GMAG DCOMP DMP Chair: Allan MacDonald, University of Texas at Austin Room: Colorado Convention Center Korbel 4E |
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P15.00001: Nonlinear and Nonequilibrium Spin Injection in Magnetic Tunneling Junctions Invited Speaker: Quantitative analysis of charge and spin quantum transport in spintronic devices requires an atomistic first principles approach that can handle nonlinear and nonequilibrium transport conditions. We have developed an approach for this purpose based on real space density functional theory (DFT) carried out within the Keldysh nonequilibrium Green's function formalism (NEGF). We report theoretical analysis of nonlinear and nonequilibrium spin injection and quantum transport in Fe/MgO/Fe trilayer structures as a function of external bias voltage. Devices with well relaxed atomic structures and with FeO oxidization layers are investigated as a function of external bias voltage. We also report calculations of nonequilibrium spin injection into molecular layers and graphene. Comparisons to experimental data will be presented. Work in collaborations with: Derek Waldron, Vladimir Timochevski (McGill University); Ke Xia (Institute of Physics, Chinese Academy of Science, Beijing, China); Eric Zhu, Jian Wang (University of Hong Kong); Paul Haney, and Allan MacDonald (University of Texas at Austin). [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P15.00002: Fermi-Surface Evaluation of Anomalous Hall Conductivity using Wannier Interpolation Xinjie Wang, David Vanderbilt, Jonathan Yates, Ivo Souza Recently, Haldane showed that the nonquantized part of the intrinsic anomalous Hall conductivity (AHC) can be represented as a Fermi-surface property.\footnote{F.D.M. Haldane, Phys. Rev. Lett. {\bf 93} 206602 (2004).} The time-consuming integration of the Berry curvature over the entire Brilliouin zone is thereby converted into a more efficient integral over the Fermi surface only. Here we present an ab-initio approach for computing the AHC which combines a Haldane-like strategy with Wannier interpolation of the Bloch functions. First, a conventional electronic-structure calculation is performed and maximally-localized Wannier functions are constructed by a post-processing step, in order to transform the full ab-initio problem into an ``exact'' tight-binding form. Second, the Brilliouin zone is sampled by a large number of equally spaced parallel slices oriented normal to the total magnetization. We find the intersections of each Fermi surface sheet with every slice, organize these into a set of closed loops, and compute the Berry phase of the Bloch states as they are transported around these loops. The AHC is then just proportional to the sum of the Berry phases of all the loops on all the slices. The method is used to calculate the intrinsic AHC of Fe, Co and Ni. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P15.00003: Spin Hall effect: from the ballistic to diffusive regime Roksana Golizadeh-Mojarad, Supriyo Datta We describe a model based on the Non-Equilibrium Green's function (NEFG) method that allows us to study the spin Hall effect continuously from the ballistic to the diffusive regime. Our numerical results show good agreement with recent experiments by Sih et. al. [PRL 97, 096605 (2006)]. Analytical expressions for the spin accumulation density will also be presented that describe the numerical results very well as different parameters are varied. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P15.00004: Berry-phase blockade in single-molecule magnets Gabriel Gonzalez, Michael Leuenberger We formulate the problem of electron transport through a single- molecule magnet (SMM) in the Coulomb blockade regime taking into account topological interference effects for the tunneling of the large spin of a SMM. The interference originates from spin Berry phases associated with different tunneling paths. We show that in the case of incoherent spin states it is essential to place the SMM between oppositely spin-polarized source and drain leads in order to detect the spin tunneling in the stationary current, which exhibits topological zeros as a function of the transverse magnetic field. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P15.00005: The anomalous Hall effect and Nernst effect in CuCr$_{2}Se_{4-x}Br_{x}$: First principles studies Zhong Fang, Yugui Yao The non-vanishing Berry curvature of Bloch states in ferromagnetic crystals with spin-orbit coupling (broken time reversal symmetry) can act as gauge field in the momentum space, which in turn affects the transport behavior of electrons in real space, and produces the fascinating phenomena in solid crystals. Typical example is the intrinsic anomalous Hall effect (IAHE). Recent progresses in this field not only deepen our understanding of the physics behind, but also enable quantitative evaluations of the effects from the parameter-free electronic structure calculations. In this presentation, the recent progresses in this field will be addressed with emphasis on the quantitative evaluations of IAHE and Nernst effect in ferromagnetic spinel CuCr$_{2}Se_{4-x}Br_{x}$ from the first- principles calculations. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P15.00006: Chern Number effective Hamiltonian for Mn clusters in GaAs. Tor Olof Strandberg, Carlo M. Canali, Allan H. MacDonald Small numbers of Mn atoms can be manipulated into arbitrary spatial arrangements on the $<110>$ surface of GaAs by means of a novel STM atom-by-atom substitution technique, which enables the replacement of individual Ga atoms by Mn[1]. The tunnelling differential conductance over an isolated Mn atom reveals a large and broad resonance in the GaAs energy gap. For a Mn pair placed less than 1nm apart, the resonance splits into two peaks, whose spacing is thought to be related to the exchange-energy interaction between Mn ions. We report on theoretical results for the local density of states and the Mn acceptor-level splittings for a Mn dimer, based on a tight-binding model of Mn substitutions on the $<110>$ GaAs surface. We compare our model with previous work which does not account for the surface. We then derive an effective quantum spin Hamiltonian for the Mn cluster, based on a Chern number theory developed recently, which includes Berry phase effects[2]. The influence of quantum fluctuations of the Mn spin orientations on the tunnelling differential conductance will be discussed. [1] D. Kitchen et al., Nature {\bf 442}, 436 (2006). [2] C.M. Canali, A. Cehovin and A.H. MacDonald, Phys. Rev. Lett. {\bf 91}, 046805 (2003) [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P15.00007: Electron corrected Lorentz forces in solids and molecules in magnetic field Davide Ceresoli, Riccardo Marchetti, Erio Tosatti We describe the effective Lorentz forces on the ions of a generic insulating system in a magnetic field, in the context of Born-Oppenheimer ab-initio molecular dynamics. The force on each ion includes an important contribution of electronic origin, which depends explicitly on the velocity of all other ions, and is given in terms of a Berry curvature, directly suitable for classical dynamics simulations. The formulation is valid at strong magnetic field, where a scheme for ab-initio simulations based on plane wave methods is outlined. As a simple analytical demonstration we present the dynamics of an H$_2$ molecule in a weak field, describing the electrons approximately through Slater's variational wavefunction. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P15.00008: Tunneling anisotropic magnetoresistance driven by resonant surface states Athanasios Chantis, Kirill Belashchenko, Evgeny Tsymbal, Mark van Schilfgaarde Fully-relativistic first-principles calculations of the Fe(001) surface demonstrate that resonant surface (interface) states may produce sizeable tunneling anisotropic magnetoresistance in magnetic tunnel junctions with a single magnetic electrode. The effect is driven by the spin-orbit coupling. It shifts the resonant surface band via the Rashba effect when the magnetization direction changes. We find that spin-flip scattering at the interface is controlled not only by the strength of the spin-orbit coupling, but depends strongly on the intrinsic width of the resonant surface states. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P15.00009: Fully relativistic spin torques and spin currents Peter Weinberger, Andras Vernes, Balazs L. Gyorffy In using the one-particle Dirac equation in the presence of an external electro-magnetic field an exact equation of motion for the density of the four-component Bargmann-Wigner polarization operator $T_{\mu }=(\vec{T},T_{4}) $ is presented, the various occuring terms of which can be viewed as the relativistic counterparts of \textit{ad hoc} defined non- relativistic spin-currents and spin-transfer torques. Based on the properties of the Berry phase the particle and the magnetization density can be formulated in terms of a instantanous resolvent $G(z;t)$ of the time dependent Dirac equation by means of contour integrations. The corresponding Greens function $G(\mathbf{r,r}^ {\prime }$, $z;t)$ can in turn be evaluated within a multiple scattering scheme by solving at each given time $t$ a ``quasi-stationary'' problem. In terms of this Greens function the time evolution of any single-particle density, i.e., also of $T_{\mu }=(\vec{T},T_{4})$ can be evaluated. As a first application the case of a single Fe atom is considered, for which very easily a comparison with a time- dependent first order perturbational scheme can be given. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P15.00010: First principles theory of the current-modulated exchange bias. Paul Haney, Rembert Duine, Alvaro Nunez, Olle Heinonen, Allan MacDonald Recent experiments[1] have demonstrated the influence of current on exchange-bias fields in point-contact spin-valve structures. With this motivation, we consider current induced torques in multilayer structures containing ferromagnetic, paramagnetic, and antiferromagnetic layers. Our description is based on ab initio spin-density-functional theory combined with the non-equilibrium Greens' function formalism and direct microscopic evaluation [2] of spatially resolved torques. We find that current induced torques are generically present in both ferromagnetic and antiferromagnet layers. We theoretically demonstrate that current-induced torques in an antiferromagnetic layer that is exchange coupled to a ferromagnetic layer can alter exchange bias and discuss materials combinations in which this effect can be exploited. [1] Wei et al.. cond-mat/0606462 [2] Haney et al.. cond-mat/0611534 [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P15.00011: Conventional spin current in Dirac equation Soo Yong Lee, Hyun-Woo Lee The spin current has been one of main concerns in the field of the spintronics. Recently Rashba [PRB \textbf{68}, 241315 (2003)] pointed out that in certain nonmagnetic systems with the spin-orbit coupling, the conventional definition of the spin current leads to a rather strange prediction, namely a nonzero spin current should flow even without external biases. Though the nonvanishing equilibrium spin current does not violate the time reversal symmetry, it still led many scientists to reexamine the definition of the spin current. Recalling that the spin-orbit coupling arises due to the relativistic effects, we examine in this work properties of the conventionally-defined spin current for a Dirac electron subject to an electrostatic potential V(r). Interestingly it is found that in this fully relativistic treatment, the equilibrium spin current vanishes for a wide class of V(r) including those representing the zincblende structure and the asymmetric quantum well, which is in clear contrast with the nonvanishing equilibrium spin current obtained from some effective nonrelativistic Hamiltonians. The origin of this difference is also examined. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P15.00012: Markov Chain Analysis of Stochastic Micromagnetic Simulations S. Hill Thompson, G. Brown, P.A. Rikvold Stochastic micromagnetic simulations are employed to study the magnetization dynamics of a realistic model of an iron nanopillar in an oblique applied field at nonzero temperature. The results suggest the existence of more than one reversal path, revealed by the distribution of switching times. The dynamics are further analyzed by considering the system as an absorbing Markov chain and studying the properties of the associated transition matrix. In particular, the eigenvalue spectrum provides the time to cross the free-energy saddlepoint separating the metastable well from the equilibrium configuration. Additionally, eigenvectors from individual runs are used to determine which reversal path each simulation followed, since it is likely the switching-time distributions overlap. Along with projective dynamics, this analysis shows that the evolution of the faster mode is indicative of a relatively flat free-energy landscape, while the slower-mode dynamics are dominated by a well-defined metastable well. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P15.00013: Thermoinduced Magnetization in Antiferromagnetic Heisenberg Chains Gregory Brown, Markus Eisenbach, G. Malcolm Stocks The magnetic properties of linear chains of classical three-dimensional Heisenberg spins, with antiferromagnetic nearest-neighbor exchange and uniaxial single-site anisotropy, are determined analytically and numerically to investigate the phenomenon of thermoinduced magnetization (TiM). TiM is the ferromagnetic response observed in nanoparticles of antiferromagnetic materials at low temperatures, with the ferromagnetic response increasing as temperature increases. In the strong-anisotropy limit, TiM is shown analytically to result from the relaxation of individual spins away from the anisotropy axis. In the weak-anisotropy limit, it is shown numerically that TiM occurs only at temperatures low enough for long-range ordering of the entire finite chain. In the absence of anisotropy, long-range order does not occur and TiM is not observed. Both of these results present serious challenges to current theories, which describe TiM only in terms collective motions at q=0 and in the limit of vanishing anisotropy. This work was sponsored by the Laboratory Directed Research and Development Program of ORNL (GB, ME, GMS), and by the DOE-OS through the Offices of BasicSciences, Division of Materials Sciences and Engineering (GMS). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700