Low-temperature spin-glass behavior in a diluted dipolar Ising system

Abstract

Using Monte Carlo simulations, we study the character of the spin-glass (SG) state of a sitediluted dipolar Ising model. We consider systems of dipoles randomly placed on a fraction x of all L 3 sites of a simple cubic lattice that point up or down along a given crystalline axis. For x 0.65 these systems are known to exhibit an equilibrium spin-glass phase below a temperature Tsg ∝ x. At high dilution and very low temperatures, well deep in the SG phase, we find spiky distributions of the overlap parameter q that are strongly sample-dependent. We focus on spikes associated with large excitations. From cumulative distributions of q and a pair correlation function averaged over several thousands of samples we find that, for the system sizes studied, the average width of spikes, and the fraction of samples with spikes higher than a certain threshold does not vary appreciably with L. This is compared with the behavior found for the Sherrington-Kirkpatrick model.

Cooling field and temperature dependent exchange bias in spin glass/ferromagnet bilayers

“…Recently, spin glasses (SG) have attracted much attention and stimulated intensive studies because the combination of quenched spin spatial randomness and frustration create a complex free energy landscape with multiple local energy minima and finding the stable spin states formidable challenging 1 2 3 . In spite of many theoretical attempts of developing renormalization group 1 4 , mean-field approximation 5 and Monte Carlo simulation 3 6 7 8 based analyses, the low-temperature ( T ) spin phases as well as the out-of-equilibrium aging dynamics of such materials remain matters of strong debates. Not surprisingly, the complex nature of SG spin structures gives rise to the unique and diverse macroscopic properties in many fields of science.…”

Magnetic ordering of random dense packings of freely rotating dipoles

“…This SG phase is similar to the one found in systems of Ising dipoles with strong structural disorder. In particular, this type of phases have been seen in textured systems with strong dilution, 39,43 as well as in dense non-textured systems, that is with high disorder in the frozen directions of the Ising dipoles. With the data gathered so far we can find the contours of the several FM and SG phases.…”

“…The thermal equilibration times t 0 were estimated after examining the plateaux for large time t of the overlap parameter q (see next Section) starting from different ini-tial configurations as described at length in Refs. 18,39 We also verify the symmetry in the thermal distributions of magnetization and the SG overlap parameter under the global inversion {σ i } → {−σ i } as an additional check that all samples are well equilibrated. 18 We used the first t 0 MC sweeps to equilibrate the samples and all thermal averages were extracted in the interval [t 0 , 2t 0 ].…”

Phase diagram for ensembles of random close-packed Ising-like dipoles as a function of texturation

“…In order to test the value thus obtained for t 0 , we observed that a second overlap q(t 0 , t 0 + t) calculated for pairs of configurations of a single replica taken at times t 0 and t 0 + t remains stuck to q 0 as t increases. 30 It is found that the less textured the system is, the longer the equilibration time appears. This is due to the large roughness of the free-energy landscapes for non-textured systems.…”