Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W16: Focus Session: Theory of Magnetism: Traditional and Novel Magnets |
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Sponsoring Units: GMAG DCOMP DMP Chair: Yaroslaw Bazaliy, University of South Carolina Room: Colorado Convention Center Korbel 4F |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W16.00001: Exact Diagonalization studies of frustrated AFM Heisenberg polytopes Ioannis Rousochatzakis, Andreas Laeuchli, Frederic Mila We explore the low energy physics of the AFM $s=1/2$ Heisenberg model on a number of frustrated magnetic molecule systems using exact diagonalization (ED). Particular emphasis is given to molecules with spins occupying the vertices of symmetric polyhedra. To this end, we have extended the standard ED technique in order to exploit the full point group (permutation) symmetry, thus including higher than one-dimensional irreducible representations. Apart from classifying the energy spectra according to both spin and permutation symmetries, our method provides the exact level degeneracies. In particular, for large frustrated polytopes, we find the existence of an accordingly large number of low-lying singlets below the first triplet, similarly to the case of frustrated 2D magnets. We also study the properties of the local spectral density functions, in view of interpreting recent neutron scattering experiments in Fe$_{30}$, one of the biggest AFM frustrated molecule available (comprising 30 spins 5/2 mounted on the vertices of a icosidodecahedron). [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W16.00002: Berry-Phase Oscillations of the Kondo Effect in Single-Molecule Magnets Michael N. Leuenberger, Eduardo R. Mucciolo We show that it is possible to topologically induce or quench the Kondo resonance in the conductance of a single-molecule magnet ($S>1/2$) strongly coupled to metallic leads. This can be achieved by applying a magnetic field perpendicular to the molecule easy axis and works for both full- and half-integer spin cases. The effect is caused by the Berry-phase interference between two quantum tunneling paths of the molecule's spin. We have calculated the renormalized Berry-phase oscillations of the Kondo peaks as a function of the transverse magnetic field as well as the conductance of the molecule by means of the poor man's scaling method. We propose to use a new variety of the single-molecule magnet Ni$_4$ for the experimental observation of this phenomenon. \\ Reference: Phys. Rev. Lett. 97, 126601 (2006). [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W16.00003: Single-ion and exchange anisotropy in high-symmetry tetramer single molecule magnets Dmitri Efremov, Richard Klemm We study the effects of single-ion and both symmetric and antisymmetric exchange anisotropy in equal-spin $s_1$ tetramer single molecule magnets exhibiting the molecular group symmetries $g=C_{4h}$, $D_{4h}$, $C_{4v}$, $S_4$, $D_{2d}$, and $T_d$. The near-neighbor and next-nearest-neighbor isotropic exchange interactions are $J$ and $J'$, respectively. From the vector basis used to diagonalize the general quadratic spin-spin interaction Hamiltonian ${\cal H}$ for each site and site pairs, we impose the symmetries characteristic of each $g$ upon ${\cal H}$. Using our exact, compact forms for the four-spin single-ion matrix elements, we calculate the eigenstate energies to first order in the anisotropy interactions. Type I tetramers with $J'-J>0$ act as two dimers with maximal pair quantum numbers $s_{13}=s_{24}=2s_1$ at low temperature $T$. Type II tetramers with $J'-J<0$ are frustrated, with minimal low-$T$ pair quantum numbers. For both Type-I and Type-II antiferromagnetic tetramers, we calculate the first-order level-crossing inductions analytically. Accurate Hartree expressions for the thermodynamics, electron paramagnetic resonance (EPR) and inelastic neutron scattering cross-section are given. An EPR procedure to extract the effective microscopic parameters is provided. [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W16.00004: Double-Exchange Model for Molecule-Based Magnets Serkan Erdin, Michel van Veenendaal We report a detailed study of a model proposed for the molecule-based magnets, which is similar to the double-exchange mechanism. The model is applied to a two-dimensional periodic complex made of a transition metal and an organic molecule in which the electronic structure is described by effective $d$ orbitals of the transition metal ion at infinite Hund's coupling limit and the lowest unoccupied molecular orbital of the organic molecule. Depending on the average electron density of the organic molecules and various superexchange couplings between metal ions' core spins, magnetic states of the complex are investigated. In Monte-Carlo calculations for a model Hamiltonian, as a function of electron density on the organic molecule, the average magnetization and critical magnetic ordering temperatures are determined. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W16.00005: Stripes and hysteresis in thin film ferromagnets David Clarke, Oleg Tretiakov, Oleg Tchernyshyov Recent experimental studies have focused on the magnetic behavior of thin materials that have strong out-of-plane anisotropy. We study the behavior of such systems near the reorientation phase transition (RPT), the point at which the dipolar interaction overcomes the internal anisotropy to force the magnetization to lie in the plane. Previous studies have classified canted, polarized, and stripe domain regions of the thermodynamic phase diagram, but have not found the boundaries of metastability necessary for an understanding of observed hysteresis curves. We complete the anisotropy-applied field phase diagram near the RPT by including metastability boundaries and find the hysteresis loops characteristic of the model using analytic and numerical techniques. The system displays a line of second order transitions from a canted phase to a spin density wave (SDW) phase, and first order transitions from the SDW phase to a striped phase. We show the existence of a liquid-gas like critical point beyond which the SDW and striped phases are indistinguishable. The phase diagram is universal for thin ferromagnetic materials up to a rescaling of the applied field and effective anisotropy by a characteristic value proportional to the square of the ratio of the thickness to the exchange length. The hysteresis loops found match behavior observed in experiments. This work was supported in part by NSF Grant DMR-0520491 [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W16.00006: Stray Fields and Metastable Magnetization Configurations in Thin Films Adebanjo Oriade, Siu-Tat Chui An important aspect of the utility of magnetic tunnel junctions and the giant magneto-resistive effect devices is reversal of the magnetization of a thin film. In these devices, found in hard disk drive read heads and magneto-resistive random access memory technology, robust control of magnetization in thin films is necessary. We study, via Monte-Carlo simulations, the nature of metastable (intermediate) magnetization states in thin films and their connection to failure in the reversal process. These metastable states usually show up as plateaus in the hysteresis loop close to the switching field. The net magnetization of the film in this state is much less than the saturation magnetization. Details of the magnetization configuration in, and during reversal of, these metastable states are presented. Two mechanisms for failure are described. (1) Strong stray fields that exist during the reversal of these metastable states will affect other elements within as much as $1\mu m$ from the longest edge of an $0.2\mu m \times 1\mu m \times 50 \AA$ film. (2) Turning field off whilst the film is in a metastable state results in relaxation into a paramagnetic state, useless for application. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W16.00007: 2d order ferromagnetic resonance in nanoparticles and the dating of archaeological ceramics Derek Walton Ferromagnetic resonance is almost exclusively explored experimentally in 1$^{st}$ order where one photon decays into a single magnon, necessarily of the uniform or magnetostatic modes. In 2d order where the photon creates two magnons of equal and opposite wave-vector, it is well-known that details of the magnon spectrum become significant. An important consideration is the cut-off in the dispersion relations for magnons whose wavelength exceeds twice the scale of the particle. I will discuss the use of this property to selectively magnetize or demagnetize assemblies of single domain grains. This permits rather sensitive dating of ancient ceramics, and accurate determination of grain size distributions [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W16.00008: First-principles Calculation of the Single Impurity Surface Kondo Resonance Chiung-Yuan Lin, Antonio Castro Neto, Barbara Jones We have performed first-principles calculation of the surface and bulk wavefunctions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co [1]. By analyzing the calculated hybridization energies, we have calculated the Kondo temperature to remarkable accuracy. We find the bulk states dominate the contribution to the Kondo temperature, in agreement with a recent experiment [2]. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope on this system and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances ($<$ 6 angstroms). Our results indicate the need for a new formulation of the problem at larger distances. [1] C.-Y. Lin, A. H. Castro Neto, and B. A. Jones, Phys. Rev. Lett. \underline {97}, 156102 (2006). [2] N. Knorr, M. A. Schneider, L. Diekhoner, P. Wahl, and K. Kern, Phys. Rev. Lett. \underline {88}, 096804 (2002). [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W16.00009: Shot Noise in the SU(4) Kondo regime Vitushinskiy Pavel, Le Hur Karyn, Clerk Aashish It has recently been shown that shot noise is a direct probe of interparticle interactions which characterize the Fermi liquid fixed point of the standard Kondo model. We now examine the transport properties of the systems which are known to exhibit an unusual SU(4) Kondo correlated liquid behaviour at low temperatures. It was shown using T-matrix approach that conductance in this regime has unexpected linear in eV corrections, as dictated by the low-energy SU(4) Fermi-liquid fixed point. We confirm this result by the microscopic calculation of backscattering current using Keldysh formalism. The SU(4) symmetry in turn affects the current shot noise and thus leads to renormalized value of the effective charge. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W16.00010: Exact solution of SU(4) non-equilibrium Kondo model at the Toulouse point. Solomon Duki, Harsh Mathur SU(4) symmetry in quantum dots has become a growing interest in both semiconductor quantum dots and carbon nanotube quantum dots[1]. We investigate theoretically the properties of an SU(4) Kondo model out of equilibrium by solving the problem exactly at a special point in the parameter space. The solution reveals that, in contrast to the SU(2) model, there are two more excitations in the system other than the charge and spin excitations. We investigate the differential conductance for arbitrary voltage bias. \newline \newline [1] P. Jarillo-Herrero, J. Kong, H.S.J. van der Zant, C. Dekker, L.P. Kouwenhoven and S. De Franceschi, http://www.nature.com/openurl?url\_ver=Z39.88-2004\&rft\_val\_fmt=info:ofi/fmt:kev:mtx:journal\&rft.genre=journal\&rft. volume=434\&rft.spage=484 \&rft.date=2005 (Nature) 434, 484, (2005). [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W16.00011: Spatially dependent Kondo effect in Quantum Corrals Enrico Rossi, Dirk K. Morr We study the Kondo screening of a single magnetic impurity placed inside a quantum corral consisting of non-magnetic impurities on the surface of a metallic host system. We show that the spatial structure of the corral's eigenmodes leads to a spatially dependent Kondo effect whose signatures are experimentally measurable spatial variations of the Kondo temperature, $T_K$, and of the critical Kondo coupling, $J_{cr}$. Moreover we find that the screening of the magnetic impurity is accompanied by the formation of multiple Kondo resonances with characteristic spatial patterns that provide further experimental signatures of the spatially dependent Kondo effect. Our results demonstrate that quantum corrals provide new possibilities to manipulate and explore the Kondo effect. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W16.00012: Propagators in Position, Momentum, and Spin Variables Bailey Hsu, Jean-Francois Van Huele Propagators describe the evolution of quantum dynamical systems. Their expression depends on the dimension of the system, on the environment, and on the boundaries. Specific techniques have been developed to calculate the propagators for different functional forms of the potential, which in the case of spinless particles depend on position and momentum. Particles with spin interacting with magnetic fields in Stern-Gerlach-like systems lead to propagators involving position and spin variables. Spin-orbit-like systems combine momentum and spin variables. We investigate the applicability of methods found in the literature to systems that exhibit different combinations of momentum, position, and spin variables. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W16.00013: Size Distribution of Superparamagnetic Particles Determined by Magnetic Sedimentation Jean-Francois Berret We report on the use of magnetic sedimentation as a means to determine the size distribution of dispersed magnetic particles. The particles investigated here are i) single anionic and cationic nanoparticles of diameter D $\sim $ 7 nm and ii) nanoparticle clusters resulting from electrostatic complexation with polyelectrolytes and polyelectrolyte-neutral copolymers. A theoretical expression of the sedimentation concentration profiles at the steady state is proposed and it is found to describe accurately the experimental data. When compared to dynamic light scattering, vibrating sample magnetometry and cryogenic transmission electron microscopy, magnetic sedimentation exhibits a unique property : it provides the core size and core size distribution of nanoparticle aggregates [Preview Abstract] |
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