Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A32: Disordered Magnetic Materials 
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Sponsoring Units: GMAG Chair: Leonard Spinu, University of New Orleans Room: Morial Convention Center 225 
Monday, March 10, 2008 8:00AM  8:12AM 
A32.00001: Transport in strongly disordered classical spin chains Vadim Oganesyan, Arijeet Pal, David Huse We present a numerical study of diffusion of energy at high temperature through strongly disordered arrays of interacting classicals spins with Hamiltonian dynamics. We find that quenched randomness strongly supresses transport, with diffusion constant apparently becoming smaller than any power of spinspin interaction rescaled by randomness. We have looked for but not found signs of a classical manybody localization transition at any finite strength of disorder. [Preview Abstract] 
Monday, March 10, 2008 8:12AM  8:24AM 
A32.00002: Ferromagnetism in Meltspun Gd$_{0.946}$Fe$_{0.054}$ Paul Shand, Nicholas Jensen, Justin Bohnet, Jared Goertzen, Jeffrey Shield, David Schmitter, Geoffrey Shelburne, Diandra LesliePelecky The ac susceptibility and dc magnetization at various temperatures have been measured for a meltspun Gd$_{0.946}$ Fe$_{0.054}$ alloy. The grain size was $\approx $100 nm. A sharp paramagnetictoferromagnetic transition was observed at a temperature close to that of pure Gd. Effective critical exponents and the critical temperature $T_{C}$ were extracted by using modified Arrott plots and KouvelFisher analysis. The values obtained were \textit{$\beta $}$_{eff}$ = 0.398 $\pm $ 0.004, \textit{$\gamma $}$_{ eff}$ = 1.24 $\pm $ 0.02, \textit{$\delta $}$_{ eff}$ = 3.83 $\pm $ 0.05, and $T_{C}$ = 290.25 $\pm $ 0.17 K. These exponent values do not satisfy the Widom scaling relation \textit{$\beta \delta $ }= (\textit{$\beta $}+\textit{$\gamma $}). The \textit{$\beta $}$_{eff}$ and \textit{$\gamma $}$_{eff}$ values for msGd$_{0.946}$ Fe$_{0.054}$ are similar to those obtained for pure Gd in the same temperature interval around $T_{C}$. This is in consonance with xray microanalysis measurements indicating that the grains are nearly pure Gd. The lowerthanexpected value of \textit{$\delta $}$_{eff}$\textit{${\rm g}$ }may be due to the effect of increased anisotropy due to the presence of Fe in the grainboundary regions. [Preview Abstract] 
Monday, March 10, 2008 8:24AM  8:36AM 
A32.00003: Observation of spinwave mediated AltshulerAronov and weak localization corrections to the conductivity in thin films of gadolinium Rajiv Misra, Arthur F. Hebard, Khandker Muttalib, Peter Woelfle We present a study of quantum corrections to the conductivity tensor of thin ferromagnetic gadolinium films. Using the sheet resistance as a measure of disorder, \textit{in situ} magnetotransport studies were performed on a series of gadolinium films deposited onto sapphire substrates having sheet resistance $R_{0 }\equiv $~$R_{xx}$ (5K) varying over the range 428~$\Omega $ ($\sim $135{\AA}) to 4011 $\Omega $ ($\sim $35 {\AA}). For temperatures $T$~$<$~30 K and $R_{0} \quad <$ 4011 $\Omega $, we observe the simultaneous presence of two types of quantum correction to the Drude conductivity, $\sigma =\sigma _{Drude} +\Delta \sigma _{SpinWaveMediated} +\Delta \sigma _{WL} $. The characteristic feature of the first correction is an approximately linear increase with temperature of conductivity, and we attribute this as a spinwave mediated AltshulerAronov correction to conductivity. The second correction to the Drude conductivity comes from weak localization, with a characteristic logarithmic temperature dependence of conductivity with a prefactor ${e^2} \mathord{\left/ {\vphantom {{e^2} {2\pi ^2\hbar }}} \right. \kern\nulldelimiterspace} {2\pi ^2\hbar }$ in 2D. We observe a breakdown of this behavior at a sheet resistance $R_{0}$ =4011 $\Omega $, which is very close to the quantum of resistance, $\hbar \mathord{\left/ {\vphantom {\hbar {e^2\approx 4100\Omega }}} \right. \kern\nulldelimiterspace} {e^2\approx 4100\Omega }$. [Preview Abstract] 
Monday, March 10, 2008 8:36AM  8:48AM 
A32.00004: Monte Carlo study of the threedimensional Coulomb glass Brigitte Surer, Helmut G. Katzgraber, Gergely T. Zimanyi, Brandon A. Allgood The memory and hysteresis effects found in stronglydisordered electron systems can be explained by the existence of a glassy phase, the Coulomb glass. Efros and Shklovskii have predicted the emergence of a soft Coulomb gap, resulting from the longrange interactions between the localized electrons. However, the relationship between the soft Coulomb gap in the density of states and the electron's glassy behavior has been a longstanding unresolved question. Only recently has it been surmised within the framework of a mean field theory that the disordered electron system undergoes a replica symmetry breaking transition at a finite temperature, similar to the SherringtonKirkpatrick model of spin glasses. Because it is not clear, however, whether the transition persists beyond the meanfield approximation, we study in detail the critical behavior and the shape of the Coulomb gap in three space dimensions using Monte Carlo methods. Furthermore, we compare our results for the (randomenergy) Coulomb glass model to previous results on a random lattice version of the model. Since these models possess different symmetries, the equivalence of the phase diagrams is far from obvious, contrary to previous claims. [Preview Abstract] 
Monday, March 10, 2008 8:48AM  9:00AM 
A32.00005: Reentrant spinglass behavior and enhanced Curie temperature in epitaxial MnSi Eric Karhu, Samer Kahwaji, Ted Monchesky, Krista Raffel, Michael Robertson, Christian Maunders We grew single crystal MnSi(111) thin films on Si(111) substrates by molecular beam epitaxy. The 3{\%} lattice mismatch created an inplane tensile strain of $\varepsilon _{\vert \vert }$ = 0.005 $\pm $ 0.001, as measured by transmission electron microscopy, and induced an outofplane compressive strain $\varepsilon _{\bot }$= 0.0033 $\pm $ 0.0001, as determined by xray diffraction. The MnSi(111) films displayed two magnetic phases. The first transition from a paramagnetic phase to a phase with long range magnetic order occurs with an enhanced Curie temperature T$_{C}$ = 40 K as compared to bulk MnSi, which develops helical magnetic structure below T$_{C}$ = 29.5~K. This increase in T$_{C}$ can be explained by an inplane strain. A second phase transition to spin glass, below T$_{f}$ = 35 K, maybe due to geometric frustration created by the compressive outofplain strain. We propose a relationship between the reentrant spinglass behavior and the partial magnetic order reported for bulk MnSi under pressure. [Preview Abstract] 
Monday, March 10, 2008 9:00AM  9:12AM 
A32.00006: Dynamical behavior of spin clusters in La$_{1x}$Sr$_{x}$CoO$_{3}$ Robert Smith, Michael Hoch, William Moulton, Phillip Kuhns, Gregory Boebinger, Arneil Reyes, John Mitchell Previous work has provided evidence for magnetic glassy behavior in the holedoped cobaltite system La$_{1x}$Sr$_{x}$CoO$_{3 }$(LSCO). Models proposed to describe the interesting and unusual magnetotransport properties of LSCO involve holerich clusters in a holepoor matrix. The glassy properties, which are not well understood, have variously been interpreted in terms of spin glass and cluster glass components. The present $^{139}$La NMR spectral lineshape measurements on single crystal LSCO, that map the hyperfine field distribution in the x$T$ plane, confirm the presence of magnetic clusters, identified as spin polarons, and provide a phase inhomogeneity diagram. NMR relaxation rates have been used to probe the dynamical behavior of the system at the nanoscale level in macroscopically insulating and metallic samples as a function of temperature in the range 4280 K. For x less than the metalinsulator critical concentration x$_{C }$= 0.17 evidence has been obtained for two classes of glassy component with different characteristic correlation time distributions and freezing temperatures. The magnetic glass properties persist above x$_{C}$. A spin polaron model is used to explain the results. [Preview Abstract] 
Monday, March 10, 2008 9:12AM  9:24AM 
A32.00007: Local field distributions in spin glasses David Sherrington, Helmut G. Katzgraber, Stefan Boettcher Numerical results for the local field distributions of a family of Ising spinglass models are presented. In particular, the EdwardsAnderson model in dimensions two, three, and four is considered, as well as spin glasses with longrange powerlawmodulated interactions that interpolate between a nearestneighbor EdwardsAnderson system in one dimension and the infiniterange SherringtonKirkpatrick model. Remarkably, the local field distributions only depend weakly on the range of the interactions and the dimensionality and show strong similarities except for near zero local field. [Preview Abstract] 

A32.00008: ABSTRACT WITHDRAWN 
Monday, March 10, 2008 9:36AM  9:48AM 
A32.00009: The Antiferromagnetic SO(3) Heisenberg Quantum SpinGlass with Short Range Interaction Eduardo Marino, Carlos Conceicao We study the quenched disordered magnetic system which is obtained from the 2D SO(3) quantum Heisenberg model, with nearest neighbors interaction, by taking the random values of the exchange couplings as given by a Gaussian probability distribution centered in a value of the coupling that corresponds to antiferromagnetic order. Using coherent states, we map this system onto a generalization of the SO(3) nonlinear sigma model, containing different flavors, which correspond to the replicas and a quartic interaction. We then integrate over the transverse components and perform a meanfield calculation of the free energy density in the limit of zero replicas. The phase diagram of the system is then obtained and shows a critical curve, starting at a quantum critical point at T=0 separating a paramagnetic from a spinglass phase. The stability of the phases is demonstrated by an analysis of the Hessian matrix of the free energy. [Preview Abstract] 
Monday, March 10, 2008 9:48AM  10:00AM 
A32.00010: On the ordering of Ising spin glasses in a field Helmut G. Katzgraber, Thomas Jorg, Florent Krzakala We study the existence of a spinglass phase in a magnetic field in three space dimensions using a novel approach where the Monte Carlo simulations are performed along a nontrivial path in the magnetic fieldtemperature plane which must cross any putative de AlmeidaThouless line. The method is first tested on the meanfield version of the EdwardsAnderson Ising spin glass on a Bethe lattice where we compute analytically the instability line that separates the spin glass from the paramagnetic state. While the de AlmeidaThouless line is clearly reproduced by our simulations on the meanfield Bethe lattice, no such instability line can be found numerically for the shortrange threedimensional model. We thus conclude that there is no such instability line for threedimensional shortrange Ising spin glasses. [Preview Abstract] 
Monday, March 10, 2008 10:00AM  10:12AM 
A32.00011: Spin glass of a diluted Ising dipolar system KaMing Tam, Michel Gingras The diluted dipolar Ising system has been regarded as a standard example which exhibits spin glass properties. Recent studies have challenged the existence of spin glass phase transition in one of the materials in this category, $LiHo_{x}Y_{1x}F_{4}$. Using Monte Carlo simulations, we calculate various quantities to address the current controversy of a possible spin glass phase transition in this material. Beside the conventional method to locate the spin glass transition by observing the crossing of Binder ratios of magnetization moments, another crucial probe for the nature of spin glass, order parameter fluctuations, is studied via the socalled fluctuation sensitive parameters. Crossing is observed in the Binder ratio of overlap order parameter, and nontrivial structures of overlap order parameter are obtained at low temperature. [Preview Abstract] 
Monday, March 10, 2008 10:12AM  10:24AM 
A32.00012: Nonlinear and ac Susceptibility of the Dilute Ising Magnet LiHo$_x$Y$_{1x}$F$_4$ Jeffrey Quilliam, Shuchao Meng, Chas Mugford, Jan Kycia Recent work has called into question the existence of a spin glass transition in the dilute dipolar Ising magnet LiHo$_x$Y$_{1x}$F$_4$ [1]. Other work has suggested that there is an exotic spin liquid phase found at a Ho concentration of $x = 0.045$ [2]. In order to carefully study the dynamics of this system, we have put together a SQUID magnetometer which allows for measurements of ac susceptibility and nonlinear susceptibility over a large frequency range. We present results from measurements on single crystals of LiHo$_x$Y$_{1x}$F$_4$, particularly on an $x = 0.045$ sample, in an attempt to either reproduce the exotic ``antiglass'' physics that was previously observed or to detect a spin glass transition. [1] P. E. Jonnson et al. PRL 98, 256403 (2007) [2] S. Ghosh et al. Science 296, 2195 (2002) [Preview Abstract] 
Monday, March 10, 2008 10:24AM  10:36AM 
A32.00013: Lowtemperature properties of the dilute dipolar magnet LiHo$_x$Y$_{1x}$F$_4$ Anders Biltmo, Patrik Henelius The phase diagram of the rareearth compound LiHo$_x$Y$_{1x}$F$_4$ is considered as a function of dilution. At low temperatures the material is a good realization of a dipolar Ising magnet. The net magnetization vanishes at high dilution and the glassy behavior that ensues has several interesting features, including a proposed antiglass phase and anomalous peaks in the specific heat. In this talk we will show results obtained with Monte Carlo techniques and compare them with recent experimental data. [Preview Abstract] 
Monday, March 10, 2008 10:36AM  10:48AM 
A32.00014: Quantum and Classical Glass Transitions in $\mathrm{LiHo}_x\mathrm{Y}_{1x}\mathrm{F}_4$ D. M. Silevitch, C. AnconaTorres, G. Aeppli, T. F. Rosenbaum When performed in the proper low field, low frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in transverse field, $\mathrm{LiHo}_x\mathrm{Y}_{1x}\mathrm{F}_4$, prove the existence of a spin glass transition in both the classical and quantum limits for x = 0.167 and 0.198. The classical behavior tracks for the two concentrations, but the quantum glasses differ because of the competing effects of entangled spins and local random fields. [Preview Abstract] 
Monday, March 10, 2008 10:48AM  11:00AM 
A32.00015: Study of the dipolar coupled Ising system $\mathrm{LiHo_xY_{1x}F_4}$ using muon spin relaxation. Jose Rodriguez, A.A. Aczel, S.R. Dunsiger, G.J. MacDougall, G.M. Luke, P.L. Russo, A.T. Savici, Y.J. Uemura, C.R. Wiebe In $\mathrm{LiHo_xY_{1x}F_4}$ magnetic Ho ions have an Ising character and interact mainly through the magnetic dipolar interaction. For $x=1$ the ground state of the system is ferromagnetic and as $x$ decreases a disordered ground state with glassy properties arises. If $x$ is decreased further, the system enters a phase sometimes referred to as ``antiglass". Both the noncanonical glassy state and the not yet understood ``antiglass", have never been systematically studied using a microscopic probe. We performed muon spin relaxation measurements in five samples (x=0.018, 0.045, 0.08, 0.12 and 0.25) which span these disordered phases. In this talk we will show from the microscopic point of view, how does the glassy state manifests itself as well as how does the evolution from the glass to the ``antiglass" occurs. [Preview Abstract] 
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