Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D23: Focus Session: MAG.THY I / Spin Structures and Dynamics |
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Sponsoring Units: GMAG DMP DCOMP Chair: Dale Koelling, Department of Energy Room: Baltimore Convention Center 320 |
Monday, March 13, 2006 2:30PM - 3:06PM |
D23.00001: Decoherence in Quantum Spin Systems Invited Speaker: Understanding decoherence of quantum spin systems strongly coupled to a bath of environmental spins (spin bath) is important for many areas of physics, such as spintronics, quantum computing, nuclear and electronic magnetic resonance. Decoherence is a complex non-equilibrium many-body phenomenon, and for many interesting situations, exact numerical simulations are needed. We have developed highly efficient and accurate numerical techniques of solving the time-dependent Schr\"odinger equation for quantum many-spin systems. We used these techniques to directly model the evolution of the central spin system and its environment. I will demonstrate usefulness of this approach for several experimentally and theoretically important cases, where the many-body correlations between the system and the bath are particularly important. I will present, among others, our studies of the NMR spin echo of Si:dopant systems, and our investigation of decoherence by a spin bath in the regime of quantum chaos. Moreover, I will present our results on decoherence of an electron spin by a bath of nuclear spins in a quantum dot, which has been recently studied in experiments, and discuss novel numerical approaches based on techniques borrowed from quantum optics. This work was supported by US DOE, and NSA, ARDA and ARO. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D23.00002: Analysis of Density Matrix Off Diagonal Correlations for NMR Spin Echoes in Dipolar Solids John Challis, Vadim Oganeysan The experiments done by Barrett et al.** have established that repeated pi pulses can slow longitudinal relaxation in a dipolar solid. This result clearly lies outside any independent spin approximation, and so off diagonal terms in the density matrix cannot be neglected. We have calculated the size and correlations of these terms as a first step in diagnosing the mechanism behind the slow decay. ** Dementyev et al. Phys. Rev. B 68, 153302 (2003) [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D23.00003: Forces on spins in a field gradient: apparent spin relaxation Yulin Chang, Mark Conradi A magnetic field gradient exerts opposite forces on spin magnetic moments oriented up and down. The result is to partially segregate spins by orientation, with more spins-up at one end of the sample and more spins-down at the other. This apparent relaxation, involving only sorting and no flipping of spins, may be dominant in systems of high diffusivity and long T$_{1}$. The sorting mechanism competes with a well-known distinct, true T$_{1}$ mechanism driven by diffusion through field gradients. The results of our calculations and numerical solutions of the Bloch-Torrey equations, modified to include the force term, will be presented in detail. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D23.00004: Finite temperature modeling of magnetization processes in exchange-coupled films R. H. Kodama, A. E. Berkowitz Recent experimental studies by Berkowitz, et al. [PRB \textbf {72}, 134428 (2005)] have explored the effect of setting the exchange anisotropy of ferromagnetic / antiferromagnetic bilayers (FM/AFM) such as Co / (Ni,Co)O by deposition in a magnetic field at temperatures well below the T$_ {N}$ or blocking temperature of the AFM material. Two remarkable effects inspired this theoretical study: (1) the magnitude and direction of unidirectional anisotropy is modified by changing the applied magnetic field during deposition of the FM overlayer, and (2) there is a ``latent period'' after deposition is complete, during which the unidirectional anisotropy is modified by changing the field. We set out to determine if these effects could be understood using orthodox models of exchange anisotropy (invoking uncompensated interfacial moments on an ensemble of AFM grains) carried to an unprecedented level of detail. Finite interactions between FM- FM, AFM-AFM, and FM-AFM grains allow modeling of domain processes in both FM and AFM films. Finite temperature is included in a novel way, using a detailed analysis of the spin system's energy surface to determine sets of activation barriers and final states for low-energy collective modes. Results indicate that some of the observed effects can indeed be simulated in this way, although some of the experimental results are more consistent with a slow, interfacial redox reaction that changes the magnetic state of the interface. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D23.00005: First Principles Approach to the Magnetic Structure of Realistic Models of Nanoparticles G. Malcolm Stocks, Aurelian Rusanu, Don Nicholson, Markus Eisenbach, Yang Wang, Sam Faulkner Magnetic nanostructures are of great interest because of their potential applications in a wide range of technologies - data storage, magneto-electronics, permanent magnets, smart drug delivery, etc. Unfortunately magnetic nanostructures present substantial theoretical challenges due to the need to treat the electronic interactions quantum mechanically whilst dealing with a, still, large number of atoms -- e.g., a 5nm cube of Fe contains $\sim $ 12,000 atoms, 4,000 of which are in the surface and subsurface layers. Here, we discuss a new implementation of the \textit{ab initio} order-N Locally Self-consistent Multiple Scattering (LSMS) method that is capable of treating tens of thousands of atoms. We will present scaling data that show the performance of the code both in the real space mode of the original method and in the k-space mode based on screened structure constants and sparse matrix solvers. Illustrative results will be shown for a BCC Fe-nanoparticle embedded in a stoichiometric B2-FeAl binary alloy, a magnetic nanostructure model containing 16,000 atoms. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D23.00006: Electronic and Magnetic Structure of Fe Nanoparticle Embedded in FeAl Yang Wang, G. Malcolm Stocks, Aurelian Rusanu, Don M.C. Nicholson, Markus Eisenbach, J.S. Faulkner Magnetic nanostructures are of great scientific interest because of their potential applications in a wide range of technologies - data storage, magneto-electronics, permanent magnets, smart drug delivery, etc. Unfortunately magnetic nanostructures present substantial theoretical challenges due to the need to treat the electronic interactions quantum mechanically whilst dealing with a, still, large number of atoms. In this presentation, we discuss our recent studies of magnetic nanoparticles using the Locally Self-consistent Multiple Scattering (LSMS) method, an order-N \textit{ab initio} method capable of treating tens of thousands of atoms. In particular, we show results for a Fe nanoparticle embedded in a stoichiometric B2-FeAl binary alloy. The Fe nanoparticle, has the shape of a BCC Wigner-Seitz cell, contains 4,409 Fe atoms, and measures about 5 nm across the diagonal corners. Including the surrounding matrix the calculation involves 16,000 atoms. We show results for the moment and charge distribution within nanoparticle, on the facets, and in the FeAl matrix. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D23.00007: Density functional calculations of the magnetic structure of FePt nano-particles P.R.C. Kent, D.M.C. Nicholson, M. Eisenbach, T.C. Schulthess The spin moment distribution in magnetic nano-particles is both scientifically interesting and technologically relevant. Here, we discuss the magnetic structure of FePt nano-particles, as determined by projector-augmented wave (PAW) and locally self-consistent multiple scattering (LSMS) local density calculations on nano-particles up to 3nm in size. The magnetic structure changes as a function of nano-particle size, composition, and chemical order, encompassing both ferromagnetic and anti-ferromagnetic tendencies. This behavior will be described and related to charge redistribution, structural relaxation, and local coordination. This work was enabled by computational resources of the Center for Computational Sciences at Oak Ridge National Laboratory and is supported by the Division of Scientific User Facilities and the Division of Materials Science and Engineering, U. S. Department of Energy. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D23.00008: Spin dynamics of a magnetic anti-vortex Hao Wang, C.E. Campbell We report the study of the spin dynamics of a magnetic anti-vortex in a submicron, asteroid shaped permalloy particle using micromagnetic simulations. A gyrotropic mode was found when a shifted anti-vortex core spirals back to the center of the asteroid. Azimuthal spin wave modes with in-plane wave vectors around the center have been excited using an in-plane pulsed field. The simultaneously excited gyrotropic mode causes a splitting of degenerate azimuthal modes. Radial spin wave modes with in-plane wave vectors along radial directions of the asteroid were excited by an out-of-plane pulsed field, which is created by a current surrounding the asteroid. The size dependence of dynamic modes has been systematically studied. The frequency of the gyrotropic mode increases with the thickness of the asteroid, but decreases with its size. The frequencies of both azimuthal and radial spin wave modes decrease with the thickness of the asteroid, and also decrease with the size of the asteroid, as may be expected by simple physical arguments. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D23.00009: Conductance and spin-flip of molecular magnets in a time-dependent magnetic field. Anh Ngo, Sergio E. Ulloa The properties of single molecular magnets and clusters with high spin, such as the Mn- or Fe-acetates, have been under intense study in recent years. The signature of magnetization tunneling on the conductance of films obtained by scanning tunneling microscopy (STM) has been recently studied theoretically [1]. We present here the effects of finite bias voltage and temperature on the STM conductance using a model for tunneling among different magnetic states. The system consists of a single molecule magnet between two conducting leads (the substrate and the STM tip). Because of the spin-phonon interaction and finite bias voltage, a time-dependent magnetic field produces tunneling to different magnetic states even away from the fully resonant condition. We use an approximation to reduce the original ($2s+1$)-level dynamical problem to a four-level sequence for the $s$-spin system. The differential conductance exhibits stepwise behavior with increasing longitudinal field that deviates substantially for increasing bias voltage and finite temperature. Analysis of the conductance vs.\ field and bias yields information on the spin-flip mechanisms. \\ ~~~ \\ $[1]$ G.-H. Kim and T.-S. Kim, Phys. Rev. Lett. 92, 137203 (2004). [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D23.00010: Spin Anisotropy Effects in Dimer Single Molecule Magnets Dmitri Efremov, Richard Klemm We present a model of equal spin $s_1$ dimer single molecule magnets. The spins within each dimer interact via the Heisenberg and the most general set of four quadratic anisotropic spin interactions with respective strengths $J$ and $\{J_j\}$, and with the magnetic induction ${\bf B}$. For antiferromagnetic Heisenberg couplings ($J<0$) and weak anisotropy interactions ($|J_j/J|\ll 1$), the low temperature $T$ magnetization ${\boldmath M}({\boldmath B})$ exhibits $2s_1$ steps, the height and midpoint slope of the $s$th step differing from their isotropic limits by corrections of ${\cal O}(J_j/J)^2$, but the position occurring at the energy level-crossing magnetic induction $B_ {s,s_1}^{\rm lc}(\theta,\phi)$, where $\theta,\phi$ define the direction of ${\boldmath B}$. We solve the model exactly for $s_1=1/2$, 1, and 5/2. For weakly anisotropic dimers, the Hartree approximation yields analytic formulas for ${\boldmath M}({\boldmath B})$ and $C_V({\boldmath B})$ at arbitrary $s_1$ that accurately fit the exact solutions at sufficiently low $T$ or large $B$. Low-$T$ formulas for the inelastic neutron scattering $S({\boldmath q},\omega)$ and the EPR $\chi(\omega)$ in an extended Hartree approximation are given. Our results are discussed with regard to existing experiments on $s_1=5/2$ Fe$_2 $ dimers, suggesting further experiments on single crystals of these and some $s_1=9/2$ [Mn$_4$]$_2$ dimers are warranted. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D23.00011: A Mean Field Theory for Superspin Glasses Derek Walton Concentrated assemblies of magnetc nanopartcles have been shown to exhibit spin glass phenomena, particularly rejuvenation. It is clear that these effects are dependent on interactions among the nanoparticles. I will describe a classic mean field theory for the local magnetic field resulting from the interactions, and show how it can account for superspin glass phenomena. [Preview Abstract] |
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