On the phase diagram of a three-dimensional dipolar model

Abstract

The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on a FCC lattice has been investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude $\lambda_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic and spin-glass regions, was thus considered in the temperature, $\lambda_u$ plane. Here we interpret this phase diagram in terms of the more convenient variables namely the bare dipolar interaction and anisotropy energies $\epsilon_d$ and $\epsilon_u$ on the one hand and the volume fraction $\Phi$ on the other hand and compare the result with that corresponding to the random distribution of particles in the absence of anisotropy. We also display the nature of the ordered phase reached at low temperature by the ensemble of dipoles on the FCC lattice in terms of both the dipolar coupling and the texturation of the easy axes distribution when the latter is no more random. This system is aimed at modeling the magnetic phase diagram of supracrystals of magnetic nanoparticles.