Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U9: Glassy and Random Magnets |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Bulbul Chakroborty, Brandeis University Room: LACC 153A |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U9.00001: Experimental Study of the Phase Diagram of LiHo$_x$Y$_{1-x}$F$_4$ J. A. Quilliam, C.G.A. Mugford, L. Lettress, J.B. Kycia The diluted dipolar-coupled Ising spin system LiHo$_{x}$Y$_{1-x}$F$_4$ has been found to have a very rich phase diagram and some unique magnetic properties. Most notably, as the concentration of magnetic Ho ions is reduced, the material appears to leave the spin glass phase and enter the so-called ``anti-glass'' phase where spins do not freeze down to very low temperatures (D. H. Reich \textit{et al}). This doping regime has also shown some unusual features in the temperature dependence of the specific heat. We are performing measurements on a variety of stoichiometries to refine the phase diagram of LiHo$_{x}$Y$_{1-x}$F$_4$. More specifically we are attempting to better understand the nature of the ``anti-glass'' phase and the range of stoichiometries over which it exists. Recent results from heat capacity and SQUID-based AC magnetic susceptibility measurements at dilution refrigerator temperatures will be presented. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U9.00002: Aging dynamics across a dynamic Almeida-Thouless line in three dimensional Ising spin glass Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite bi-intercalation compound Masatsugu Suzuki, Itsuko Suzuki Cu$_{0.5}$Co$_{0.5}$Cl$_{2}$-FeCl$_{3}$ graphite intercalation compound is a three-dimensional short-range Ising spin glass with a spin freezing temparature $T_{g}$ ($= 3.92 \pm 0.11$ K). The stability of the spin glass phase in the presence of a magnetic field $H$ has been studied from the time dependence of zero-field cooled (ZFC) susceptibility $\chi_{ZFC}$ after a ZFC aging protocol with a waittime $t_{w}$ ($= 1.0 \times 10^{4}$ and $3.0 \times 10^{4}$ sec). The relaxation rate $S(t)$ (= d$\chi_{ZFC}$/d$\ln t$) exhibits a local maximum at a characteristic time $t_{cr}$, The $t$ dependence of $\chi_{ZFC}$ is well described by a stretched exponential relaxation ($\approx \exp [-(t/\tau)^{1-n}$]) in the vicinity of $\tau \approx t_{w}$, where $t \approx t_{cr}$. The $H$ dependence of $t_{cr}$ and $\tau$ is measured at the fixed temperature $T$ (2.9 K $\leq T < T_{g}$): $\tau$ ($t_{cr}$) decreases with increasing $H$. Contour plots of $H$ and $T$ with constant $\tau$ form lines in the $H$-$T$ plane, depending on the value of $\tau$ chosen. We find that the line with $\tau \approx 2.0 \times 10^{3}$ sec coincides with an Almeida-Thouless (AT) line where the irreversible effect of succeptibility appears. This result indicates that the spin glass phase is unstable in the presence of $H$. There is no AT line for short-range Ising spin glass. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U9.00003: Correlation length of the two-dimensional Ising spin glass with bimodal interactions Helmut Katzgraber, Lik Wee Lee We study the correlation length of the two-dimensional Edwards-Anderson Ising spin glass with bimodal interactions using a combination of parallel tempering Monte Carlo and a rejection-free cluster algorithm in order to speed up equilibration. Our results show that the correlation length grows $\sim \exp(2J/T)$ suggesting through hyperscaling that the degenerate ground state is separated from the first excited state by an energy gap $\sim 4J$, as would naively be expected. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U9.00004: The Compressible Ising Spin Glass: Simulation Results Adam Marshall, Bulbul Chakraborty, Sidney Nagel We have studied the compressible Ising spin glass using the Edwards-Anderson model with $\pm J$ interactions, primarily in two dimensions. Compressibility is introduced by the addition to the standard spin-glass Hamiltonian of a term which couples the spin-spin interactions to the distance between neighboring particles. A dimensionless parameter $\mu$ relates the strength of the coupling term to the original spin-glass energy. In the simulations, the spin dynamics are modeled via single-spinflip Monte Carlo, while the lattice is relaxed using conjugate-gradient minimization with respect to the particle positions. We find that the total energy of a given spin configuration is shifted from its incompressible value, $E^0$, by an amount which is proportional to $\mu$ and quadratic in $E^0$. Furthermore, the previously discrete energy levels broaden into bands that overlap for sufficiently large $\mu$; in the thermodynamic limit, even an infinitesimal coupling renders the spectrum continuous. Using these results, we have constructed a simple analytic model and investigated the effects of the compressibility on the original spin-glass transition. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U9.00005: Phase Transition of the Random Field Ising Model at Zero Temperature and Positive Temperature Yong Wu, Jonathan Machta The random field Ising model (RFIM) is studied numerically at both zero and positive temperature. Thermal states and thermodynamic properties are obtained for all temperatures using the the Wang-Landau algorithm. The specific heat and susceptibility display sharp peaks in the critical region for most realizations of random fields for large systems and strong enough disorder. These sharp peaks result from large domains flipping and are strongly correlated with the domains found in ground states. Although the correlation is higher for stronger disorder, it remains for relatively low disorder. The correlation between ground states and thermal states is a concrete manifestation of the zero temperature fixed point scenario. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U9.00006: Behaviour of spin glasses in a magnetic field Matteo Palassini The low-temperature phase structure of spin glasses is still not understood beyond the mean-field level. In particular, it has not been possible to establish whether the ordered spin-glass phase is stable or unstable against the application of a weak external magnetic field. An answer to this question would allow to discriminate among widely discussed competing theories of spin glass ordering. In this talk, I will present recent numerical investigations of this issue for short-range spin glasses in three, four, and five dimensions, as well as dilute spin glasses, and discuss the results in the light of the various alternative theories. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U9.00007: Spin-glass behavior probed at the local level using information theoretic measures Debra A. Kenneway, Susan R. McKay, David P. Feldman This study shows the equivalence of the entropy calculated using an information theoretic method and the thermodynamic entropy in two-dimensional quenched random systems. The information theoretic entropy is calculated via histograms of spin configuration occurrences in shapes planted in the lattice during a Monte Carlo simulation. This method yields a local entropy density, one for the region around each site. These local entropies differ markedly in value across the lattice, but their average coincides with the thermodynamic entropy. Thus, these calculations show how the entropy is spread unevenly across the lattice in glassy systems. We have also calculated the excess entropy in terms of local contributions, which provides a measure of spatial structure and memory in the spin glass. Results yield a way to examine the connections between frustration and the local entropy and excess entropy densities and also provide comparisons between the bimodal and Gaussian cases in two dimensions in terms of these local measures. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U9.00008: Renormalization study of Ising Spin Glasses in a transverse field David Carpentier, Pierre Pujol We study an ensemble of Ising spins coupled by random antiferromagnetic/ferromagnetic couplings, and submitted to a (random) transverse field in low dimensions. We use an extension of the Ma-Dasgupta decimation procedure to study the low energy behaviour of this quantum spin glass. Particular attention is paid to the relevance of frustration near infinite disorder fixed points. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U9.00009: Memory and Quantum Erasure in a Disordered Ising Magnet C. Ancona-Torres, T.F. Rosenbaum, G. Aeppli Glasses are well known for their slow relaxation and complex history dependence, including their memory of previous aging after a cycle in temperature or field. We study the breakdown of memory via quantum relaxation by measuring the time-dependent AC susceptibility in LiHo$_{0.20}$Y$_{0.80}$F$_{4}$, a diluted, dipolar coupled Ising magnet. Application of a magnetic field transverse to the Ising axis introduces quantum tunneling modes that speed relaxation and provide new pathways through the hierarchical distribution of states. It is possible to compare directly in the same system magnetic aging and memory effects driven by classical (thermal) and quantum (tunneling) channels. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U9.00010: Magnetic and chemical superstructures in Gd2PdSi3 studied using synchrotron radiation A. Kreyssig, J.-W. Kim, L. Tan, D. Wermeille, A. I. Goldman, M. Frontzek, M. Loewenhaupt $R_{2}$PdSi$_{3}$ compounds have stimulate a recent interest due to the coexistence of long-range antiferromagnetic order and spin-glass like behavior. We present x{\-}ray resonant magnetic scattering studies on a Gd$_{2}$PdSi$_{3}$ single crystal at temperatures between 8~K and 30~K performed on the MUCAT beamline, APS, Argonne. Charge satellite reflections were observed with the propagation vectors (1/2~0~1/8) and (1/2~0~1/3). The corresponding chemical superstructure can be described by an ordered distribution of the Pd and Si ions on the B site in the hexagonal AlB$_{2}$ derived structure. Below $T_{N}$~=~23~K antiferromagnetic satellite reflections appear. The propagation vector is incommensurate with temperature dependent values between (0.14~0~0) and (0.13~0~0). This emphasizes the dominance of the RKKY interaction in Gd$_{2}$PdSi$_{3}$ in comparison to the strong influence of crystal electric field effects in the other $R_{2}$PdSi$_{3}$ compounds. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U9.00011: Frustrated Ternary Insulating Mixed Magnetic Co/Mn/Fe Dichloride Dihydrate G.C. DeFotis, C.D. Wallo, R.L. Smith, D.B. Bodkin, G.L. Miribalio, T.R. Leftwich, M.G. Kim, Z.D. Reed Mixed ternary insulating Co/Mn/Fe dichloride dihydrate has been examined by dc susceptibility and magnetization measurements over a broad range of compositions. The three components are isomorphous antiferromagnets with different ordered spin arrangements. Orthogonal spin anisotropies occur between the Fe and Co, and the Fe and Mn, components. Competing (sign) exchange interactions occur between the Fe and Mn, and the Co and Mn, components. The consequences of these competitions has been determined in previous work on the three possible binary mixtures. Examined here, for the ternary mixture, are high temperature paramagnetic phase properties, and low temperature properties including magnetic ordering and nonequilibrium behavior vs composition. The latter includes field cooled vs zero-field cooled susceptibilities, and thermoremanent magnetization as a function of time and temperature. A T-H irreversibility line is determined for one spin-glass-like composition, and temperature-time scaling of the thermoremanent magnetization confirmed for several compositions. A magnetic phase diagram (T vs composition) is also determined, involving two independent composition variables. *Supported by NSF-Solid State Chemistry-Grant No. DMR-0085662. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U9.00012: Mixed magnetic behavior in Ce doped perovskites Eugenio E. Vogel, Jaime Cartes, Patricio Vargas, Gabriela Alejandro, Laura Steren La$_{(0.67-x)}$Ce$_x$Ca$_{0.33}$MnO$_3$ samples have been prepared for $x$=0.0, 0.10, and 0.20. We report here magnetization measurements leading to hysteresis curves showing several interesting features: low coercive field, magnetization jumps at magnetic fields less than coercive field, loops not quite reproducible, and virgin curve outside of the main loop. These and other characteristic properties are more pronounced for the sample with $x$=0.20. We present a simple model based on an Ising Hamiltonian with mixed but segregated ferromagnetic and antiferromagnetic interactions to qualitatively explain these phenomena. We conclude that groups of spins with in-plane ferromagnetic interactions are overturned at critical values of the field for which pinning due to off-plane antiferromagnetic interactions is compensated. The role of the Ce doping is discussed. [Preview Abstract] |
|
U9.00013: Absence of an Almeida-Thouless line in Ising spin glasses Helmut G. Katzgraber, A. Peter Young We present results of Monte Carlo simulations on Ising spin glasses in the presence of a (random) field. A finite-size scaling analysis of the correlation length shows no indication of a transition in three dimensions, in contrast to the zero-field case. This suggests that there is no Almeida-Thouless line for short-range Ising spin glasses. We also present results on the behavior in a field of the one-dimensional Ising chain with long-range power-law interactions. [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