Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L5: Phase Transitions in Disordered Magnets |
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Sponsoring Units: DCMP Chair: Raymond Bishop, University of Manchester Room: Morial Convention Center RO1 |
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L5.00001: Dilute anisotropic dipolar systems as random field Ising ferromagnets Invited Speaker: We have shown the equivalence, at low energies, of dilute anisotropic dipolar magnets to the Ising model in the presence of an effective random longitudinal field and an effective transverse field, both of which are independently tunable. In the ferromagnetic (FM) regime [1], these systems constitute the first realization of the classical, as well as quantum, random field Ising model in a FM system, allowing, in particular, the application of a longitudinal field conjugate to the FM order parameter. In the spin-glass regime [2,3] we elucidate the role of both the hyperfine interactions, which couple the system to a spin bath and change the low-energy degrees of freedom, and the off-diagonal terms of the dipolar interactions, which lead to the effective random field. This resolves long standing questions regarding quantum spin glasses in general, and the quantum phase transition between the spin glass and paramagnetic phases in particular. \newline [1] $LiHo_xY_{1-x}F_4$ as a random field Ising ferromagnet, M. Schechter, Cond-mat/0611063. \newline [2] Significance of the hyperfine interactions in the phase diagram of $LiHo_xY_{1-x}F_4$, M. Schechter and P. C. E. Stamp, Phys. Rev. Lett. {\bf 95}, 267208 (2005). \newline [3] Quantum spin glass and the dipolar interactions, M. Schechter and N. Laflorencie, Phys. Rev. Lett. {\bf 97}, 137204 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L5.00002: A ferromagnet in a continuously tuneable random field Invited Speaker: The Random-Field Ising Model (RFIM) has been extensively studied as a model system for understanding the effects of disorder in magnets. Since the late 1970s, there has been a particular focus on realizations of the RFIM in site-diluted antiferromagnets. We observe random-field effects in the dilute dipole-coupled ferromagnet $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$. In the presence of a magnetic field transverse to the Ising axis ($H_t$), the behavior of $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$ becomes increasingly dominated by the influence of random-field terms in the effective Hamiltonian. This is seen experimentally in the shape of the ferromagentic-paramagnetic phase boundary and in changes to the critical exponents near the classical critical point. We find that above the classical critical point the magnetic susceptibility diverges as $H_t\rightarrow0$, and that the susceptibility both above and below the classical critical point can be collapsed onto a single universal curve using a modified Curie law which explicitly incorporates random-field contributions. The discovery of a ferromagnetic realization of the RFIM opens the door to investigation of the random-field problem with the wide variety of techniques available for probing ferromagnets, including the ability to examine both the statics and dynamics of the random-field problem. It also allows studying the effects of controlled amounts of randomness on the dynamics of domain pinning and the energetics of domain reversal. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L5.00003: Activated quantum criticality of complex systems Invited Speaker: Magnetization measurements performed on single crystals of Ho$_{x}$Y$_{1-x}$LiF$_{4}$ with x = 16.5{\%} and 4.5{\%} show the same behavior for both compositions: (i) absence of divergence of the non-linear susceptibility in a transverse field (ii) same absence of divergence in zero field. These results are in sharp contrast with earlier studies of Ho$_{x}$Y$_{1-x}$LiF$_{4}$. In (i) the observed lack divergence results from the presence of random fields induced by the applied transverse field (no spin-glass phase transition, as predicted by Schechter, Laflorencie and Stamp). In (ii) it results from important slowing down of the dynamics due to huge energy barriers. Excellent fits are obtained for the linear and non-linear susceptibilities with ln($M)$ = -$T$f($H,T)$ (f is a functional form), suggesting the possibility of a dynamical phase transition involving thermally activated tunneling states. This model may also be useful for more general quantum dynamics of complex systems at finite temperatures. \newline \newline Scaling of non-linear susceptibility in MnCu and GdAl spin-glasses, B. Barbara, A. P. Malozemoff, and Y. Imry, PRL, 7, 1852 (1981). \newline Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo$_{x}$Y$_{1-x}$F$_{4}$, P. E. J\"{o}nsson, R. Mathieu, W. Wernsdorfer, A. M. Tkachuk, and B. Barbara, PRL, 98, 256403 (2007). \newline Nuclear spin driven quantum relaxation in LiHo$_{0.002}$Y$_{0.998}$F$_{4}$, R. Giraud, W. Wernsdorfer, A. M. Tkachuk, D. Mailly, and B. Barbara, PRL, 87, 057203 (2001). \newline Significance of the hyperfine interactions in the phase diagram of LiHo$_{x}$Y$_{1-x}$F$_{4}$, M. Schechter and P. C. E. Stamp, PRL, 95, 267208 (2005). \newline Quantum spin-glass and the dipolar interactions, M. Schechter and N. Laflorencie, PRL, 97,137204 (2006). \newline Induced Random Fields in the LiHo$_{x}$Y$_{1-x}$F$_{4}$ Quantum Ising Magnet in a Transverse Magnetic Field, S. M. A. Tabei, M. J. P. Gingras, Y.-J. Kao, P. Stasiak, and J.-Y. Fortin, PRL, 97, 237203 (2006). \newline Quantum spin-glass in anisotropic dipolar systems, M. Schechter, P.C.E. Stamp, and N. Laflorencie, J. Phys: Cond. Matt., 19, 145218 (2007). \newline Activated Scaling of Classical and Quantum Spin Glasses, B. Barbara. PRL, 99, 177201 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L5.00004: Glass Phenomenology from the Connection to Spin Glasses Invited Speaker: Using an effective potential replica formalism the properties of supercooled liquids near their glass transition are related to those of an Ising spin glass in a magnetic field. Results from the droplet picture of spin glasses are used to provide an explanation of the main features of fragile glasses such as Vogel-Fulcher-like behavior of the dynamics and the growing size as the temperature is reduced of the dynamically re-arranging regions. Exact solutions of one-dimensional fluids with glass-like features have been obtained and will be used to provide illustrations of the connection between glasses and spin glasses. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:30PM |
L5.00005: Is there an Almeida Thouless line in spin glasses? Invited Speaker: One of the most striking predictions of the mean field of spin glasses is a line of transitions in the magnetic field temperature plane, called the Almeida-Thouless (AT) line, which separates a high temperature, paramagnetic phase, with finite relaxation times, from a low temperature spin glass phase with infinite relaxation times. It is therefore represents an ergodic to non-ergodic transition with no change in symmetry. Whether or not an AT line occurs in real spin glasses has been controversial. Experiments have looked to see if there is a divergent relaxation time at finite field, and Ref. [1], for example, has argued their data indicates no AT line. However, other experimental papers have come to the opposite conclusion. Theoretically, it seems best to investigate a divergent {\em static} quantity, the ``replicon'' susceptibility (which is not accessible experimentally), and the corresponding correlation length. A finite size scaling analysis of the three-dimensional Ising spin glass in a field [2] found no AT line. It is, however, possible that an AT line could occur in higher dimensions, even if it does not occur in d=3. To investigate this question we used an analogous model, a one-dimensional system with long-range interactions which fall off with a power law, in which varying the power is analogous to varying the dimension in the short-range case. We do find an AT line [3] in models corresponding to short-range systems in dimension greater than 6.\\ \\ {[1]} J. Mattsson, T. Jonsson, P. Nordblad, H. A. Katori, and A. Ito, Phys. Rev. Lett. 74, 4305 (1995).\\ {[2]} A.P. Young and Helmut G. Katzgraber, Phys. Rev. Lett. 93, 207203 (2004).\\ {[3]} Helmut G. Katzgraber and A.P. Young, Phys. Rev. E 72, 184416 (2005). [Preview Abstract] |
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