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

Abstract

We study random close packed systems of magnetic spheres by Monte Carlo simulations in order to estimate their phase diagram. The uniaxial anisotropy of the spheres makes each of them behave as a single Ising dipole along a fixed easy axis. We explore the phase diagram in terms of the temperature and the degree of alignment (or texturation) among the easy axes of all spheres. This degree of alignment ranges from the textured case (all easy axes pointing along a common direction) to the non-textured case (randomly distributed easy axes). In the former case we find long-range ferromagnetic order at low temperature but, as the degree of alignment is diminished below a certain threshold, the ferromagnetic phase gives way to a spin-glass phase. This spin-glass phase is similar to the one previously found in other dipolar systems with strong frozen disorder. The transition between ferromagnetism and spin-glass passes through a narrow intermediate phase with quasi-long-range ferromagnetic order.