Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site-diluted lattice

We study partially occupied lattice systems of classical magnetic dipoles which point along randomly oriented axes. Only dipolar interactions are taken into account. The aim of the model is to mimic collective effects in disordered assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we obtain the following equilibrium results. The zero-temperature entropy approximately vanishes. Below a temperature Tc, given by kBTc= 0.95 0.1 x d, where d is a nearest-neighbor dipole-dipole interaction energy and x is the site occupancy rate, we find a spin-glass phase. In it, 1 the mean value q , where q is the spin overlap, decreases algebraically with system size N as N increases, and 2 q 0.5 q T /x, independently of N, where q is the root-mean-square deviation of q .