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

TL;DR

This study investigates a spin-glass phase in a lattice of magnetic dipoles with random axes, revealing a phase transition at a critical temperature and characterizing the properties of the spin overlap in the system.

Contribution

It introduces a model of disordered magnetic dipoles with random anisotropy axes and provides equilibrium results from Monte Carlo simulations, including the identification of a spin glass phase.

Findings

Spin glass phase appears below T_c proportional to dipolar interaction energy and site occupancy.

Mean spin overlap decreases algebraically with system size in the spin glass phase.

Root mean square deviation of |q| scales with temperature and occupancy, independent of system size.

Abstract

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 T_c, given by k_B T_c= (0.95 +- 0.1)x e_d, where e_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) D|q| = 0.5 <|q|> (T/x)^1/2, independently of N, where D|q| is the root mean square deviation of |q|.