Monte Carlo study of the spin-glass phase of the site-diluted dipolar Ising model

Abstract

By tempered Monte Carlo simulations, we study site-diluted Ising systems of magnetic dipoles. All dipoles are randomly placed on a fraction x of all L3 sites of a simple cubic lattice, and point along a given crystalline axis. For xc<x≤1, where xc≃0.65, we find an antiferromagnetic phase below a temperature which vanishes as x→xc from above. At lower values of x, we find an equilibrium spin-glass (SG) phase below a temperature given by kBTsg≃xεd, where εd is a nearest-neighbor dipole-dipole interaction energy. We study (a) the relative mean-square deviation Δq2 of |q|, where q is the SG overlap parameter and (b) ξL/L, where ξL is a correlation length. From their variation with temperature and system size, we determine Tsg. In the SG phase,we find (i) the mean values q and q2 decrease algebraically with L as L increases, (ii) double peaked, but wide, distributions of q/ q/ appear to be independent of L, and (iii) ξL/L rises with L at constant T but extrapolations to 1/L→0 give finite values. All of this is consistent with quasilong-range order in the SG phase.