Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H10: Spin Glasses and Other Systems with Disorder |
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Sponsoring Units: DMP Chair: Daniel Silevitch, University of Chicago Room: Colorado Convention Center Korbel 1E |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H10.00001: Ferromagnetism and Glassiness in a Disordered Ising Magnet C. Ancona-Torres, T.F. Rosenbaum, G. Aeppli LiHo$_{0.20}$Y$_{0.80}$F$_{4}$ is a diluted, dipolar-coupled Ising magnet on the boundary between ferromagnet and spin glass. Application of a magnetic field transverse to the Ising axis introduces quantum tunneling modes that speed relaxation and drive an order-disorder transition. We determine the Transverse Field - Temperature phase diagram through measurements of the linear and nonlinear magnetic susceptibility, as well as the spectral response of the system. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H10.00002: The Disordered Ising Ferromagnet in Transverse Field D.M. Silevitch, D. Bitko, J. Brooke, G. Aeppli, T.F. Rosenbaum The phase diagram and critical behavior of the model quantum magnet LiHo${}_x$Y${}_{1-x}$F${}_4$ were studied for x=0.44, 0.65, and 1.0. Measurements of the phase boundary and critical exponents show mean-field behavior in the classical variable, temperature, but significant deviations from mean-field character emerge in the presence of the quantum variable, transverse magnetic field. Rounding and hysteresis of the magnetic susceptibility in the highly disordered and low temperature limit point to an amplified role for non-equilibrium physics in this regime. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H10.00003: Specific Heat of the Dilute Ising Magnet LiHo$_x$Y$_{1-x}$F$_4$ Jeffrey Quilliam, Chas Mugford, Lauren Lettress, Jan Kycia We will present specific heat results on the dilute dipolar-coupled Ising magnet LiHo$_x$Y$_{1-x}$F$_4$. This material was previously observed to change from a spin glass to an unusual ``anti-glass'' state at a Ho concentration of $x\simeq 0.045$. This state showed dynamics that are very different from those of a spin glass\footnote{S. Ghosh \emph{et al.}, Science {\bf 296}, 2195 (2002)} and also exhibited sharp features in its specific heat at around 100 and 300 mK\footnote{S. Ghosh \emph{et al.}, Nature {\bf 425}, 48 (2003).}. In contrast, our measurements of the heat capacity do not reproduce these sharp features and instead find broad curves for three concentrations (1.8\%, 4.5\% and 8.0\%). Integrating $C/T$ reveals a residual entropy $S_0$ which is 0 for 8.0\% Ho but increases with lower concentration (to $0.31R$ at 1.8\% Ho). This provides some evidence for a change to a different magnetic ground state below 8.0\% Ho and is qualitatively consistent with Monte Carlo simulations\footnote{J. Snider and C. C. Yu, Phys. Rev. B {\bf 72}, 214203 (2005).}. AC susceptibility measurements probing the dynamics of this system are currently being performed and results will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H10.00004: Study of magnetism at the atomic level in highly doped LiHo$_x$Y$_{1-x}$F$_4$ J.A. 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 $\mathrm{LiHo_xY_{1-x}F_4}$ is believed to be a physical realization of the ferromagnetic transverse field Ising model. Nevertheless, studies show deviations from the theoretical expectations. One of these differences is the appearance of an anomalous paramagnetic phase after the spin glass state is destroyed by dilution. $\chi_{AC}$ measurements motivated some authors to refer to this phase as an ``anti-glass". Most of the experimental results on the anti-glass phase have been performed at a single Ho concentration ($x=4.5$\%). In order to better understand the magnetic properties of this system, we performed zero field and longitudinal field $\mu$SR measurements in three highly diluted samples (x = 0.02, 0.045 and 0.08), each of which was expected to lie within the anti-glass phase. Our measurements probe the dynamic behavior of the system in a higher frequency window than the existing $\chi_{AC}$ studies, and have the advantage of being performed with a microscopic probe. We found that there is no qualitative difference on the magnetic behavior upon dilution. In this talk we will discuss the fluctuation of magnetic moments down to base temperature ($\sim$20mK) and in the presence of a transverse magnetic field. We will also discuss our results on the basis of an independent-ion picture. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H10.00005: ABSTRACT HAS BEEN MOVED TO H11.00013 |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H10.00006: Substitution effects in the spin-gap compound BiCu$_{2}$PO$_{6}$ A.V. Mahajan, B. Koteswara Rao, J. Bobroff BiCu$_{2}$PO$_{6}$ has a structure where the Cu spins appear to form a two-leg ladder. From our susceptibility measurements, we indeed find an exponential decrease of the spin-susceptibility ($\chi _{spin}$(T)) below a broad maximum with a spin-gap ($\Delta )$ of about 40 K. Analysis of $\chi _{spin}$(T) indicates that the leg and rung exchange couplings are nearly equal and further that the inter-ladder coupling is not negligible. No long-range order was observed down to 1.8 K. Heat capacity measurements yield $\Delta \quad \approx $ 40 K. Isovalent substitutions (Zn and Ni) at the Cu site destroy the spin-gap and induce a transition to a spin-glass/disordered magnetic state below about 5 K, as seen from our susceptibility, heat capacity, and $\mu $SR data. Heterovalent substitutions (Pb, Sr) at the Bi site, which should release one hole per substituent, do not significantly change the magnetic behavior other than a low-T Curie term in the susceptibility. On the other hand, Na doping (which should give rise to two holes per Na) gives rise to an additional peak in $\chi _{spin}$(T) at about 20 K, below the broad maximum at 60 K. The nature of this transition is currently being investigated. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H10.00007: Magnetism of weakly hole-doped, anisotropic CuO$_2$ spin chains in La$_5$Ca$_9$Cu$_{24}$O$_{41}$ R. Leidl, R. Klingeler, B. B\"uchner, M. Holtschneider, W. Selke Experiments suggest that for fields $B > 4\textrm{T}$ (applied along the easy axis) the spin chain system in La$_x$Ca$_{14-x}$Cu$_{24}$O$_{41}$, $x\ge5$, is characterized by short-range spin order and quasistatic (quenched) charge disorder. Using realistic estimates for the interaction parameters, we present extensive Monte-Carlo simulations of a (classical) two-dimensional Heisenberg model with randomly distributed, static holes. We find, in particular, that the spin-flop transition of the pure model (without holes) is destroyed and transformed into a smooth anomaly. A qualitative agreement with the experimentally observed magnetic susceptibility curves in La$_5$Ca$_9$Cu$_{24}$O$_{41}$ is obtained. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H10.00008: Magnetic Phase Separation in La$_{1-x}$Sr$_{x}$CoO$_{3}$ Single Crystals using $^{139}$La NMR. Robert X. Smith, Michael J.R. Hoch, Philip L. Kuhns, William G. Moulton, Gregory S. Boebinger, Arneil P. Reyes, Chris Leighton, John Mitchell Nano-scale phase separation appears to occur in a number of doped transition metal oxides and has been shown to be important in the cobaltites. LSCO is a mixed valence (Co$^{3+}$, Co$^{4+})$ near-cubic perovskite. We report $^{139}$La NMR experiments on nanoscale phase separation in single crystals complementing earlier work on sintered powders$^{1}$. Concentrations in the range x=0.05 to 0.30, spanning the metal-insulator transition at x=0.18, were used. Spectra for x=0.30 reveal a single peak showing long-range FM order with an internal transferred hyperfine field of 2.5T, at the La site. NMR spectra for samples with x$<$0.30 show asymmetric peaks that can be well fitted with two Gaussian's, evidence of FM and spin/cluster-glass magnetic phases. A droplet model has been proposed in the literature, where Co neighbors interact via double exchange in hole-rich regions to form the FM phase. We find magnetic phase separation persists over a narrower x range in the single crystals than in previously studied sintered samples$^{1}$. 1) M.J.R Hoch et al. PRB 70, 174443 (2004) [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H10.00009: Spin-glass ordering in the layered III-VI Diluted Magnetic Semiconductor Ga$_{1-x}$Mn$_{x}$S Tom Pekarek, E.M. Watson, J. Garner, P.M. Shand, I. Miotkowski, A.K. Ramdas A spin-glass transition has been observed in a class of materials based on a layered III-VI semiconducing host. We have performed dc magnetization and ac susceptibility measurements on the diluted magnetic semiconductor Ga1-xMnxS ($x$ = 0.09). A scaling analysis of the nonlinear magnetization just above the transition gives \textit{Tc} = 11.2 +/- 0.2 K, and the critical exponent values gamma~= 4.0 +/- 1.0 and beta = 0.8 +/- 0.2. The non-linear magnetization scaling for Ga1-xMnxS follow the same universal scaling function characterized with the same values of $\gamma $ and $\beta $ as Zn$_{0.49}$Mn$_{0.51}$Te over many orders of magnitude along each axis. The values for the critical exponents $\gamma $ and $\beta $ obtained in this work are in excellent agreement with values reported for other spin-glass materials. These results represent convincing evidence that the III{\-}VI diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$S undergoes a true spin-glass transition and is in a subset of the class of insulating spin-glass materials with short-range interactions. The observed spin-glass transition in Ga1-xMnxS is unprecedented in the published literature on III-VI DMS. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H10.00010: Spin-Glass Like Phase in the Weak-Coupling Limit of the Double-Exchange Model Randy Fishman Recent work has demonstrated that in the weak-coupling or RKKY limit, the double-exchange (or Kondo lattice) model supports a spin-glass like (SGL) phase with short-range but not long-range magnetic order. The magnetic susceptibility and Edwards-Anderson order parameter q of this SGL phase have been evaluated using dynamical mean-field theory (DMFT), which becomes exact in infinite dimensions. We find that q=M(T/T$_{SGL})^{2}$, where M is the classical Brillouin function and T$_{SGL}$ is the SGL transition temperature. The correlation length of the SGL phase is determined by a correlation parameter Q that simultaneously maximizes T$_{SGL}$ and minimizes the free energy. The magnetic susceptibility has a cusp at T$_{SGL}$ and reaches a nonzero value as the temperature goes to zero. Analytic results for the SGL phase of a model with classical spins but without quenched disorder and geometric frustration should provide new avenues of investigation into SGL behavior. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H10.00011: Chaos in spin glasses Helmut G. Katzgraber, Florent Krzakala We study the effects of small temperature as well as disorder perturbations on the equilibrium state of three-dimensional Ising spin glasses via an alternate scaling ansatz. By using Monte Carlo simulations, we show that temperature and disorder perturbations yield chaotic changes in the equilibrium state and that temperature chaos is considerably harder to observe than disorder chaos. Results in two space dimensions are also discussed. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H10.00012: Spin glass phenomena caused by dipole interactions and the Onsager reaction field Derek Walton The effects of dipole interactions have been the subject of study for decades, however with few exceptions the Onsager reaction field has been neglected. It will be shown that this field leads to waiting time efeects and rejuvenation. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H10.00013: Universality in spin glasses: A Monte Carlo study Helmut G. Katzgraber, Mathias Koerner, A. Peter Young We study universality in three-dimensional Ising spin glasses by large-scale Monte Carlo simulations of the Edwards-Anderson Ising spin glass for several choices of bond distributions, with particular emphasis on Gaussian and bimodal interactions. A finite-size scaling analysis suggests that three-dimensional Ising spin glasses obey universality. Results in two space dimensions are briefly discussed. [Preview Abstract] |
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