Role of positional disorder in fully textured ensembles of Ising-like dipoles

TL;DR

This study uses numerical simulations to analyze how positional disorder affects magnetic ordering in ensembles of Ising-like dipoles, revealing phase transitions and the impact of structural anisotropy.

Contribution

It introduces a detailed phase diagram for disordered dipolar systems and compares isotropic and anisotropic particle arrangements, highlighting the role of positional disorder.

Findings

Long-range ferromagnetic order at high volume fractions

Spin-glass phase at lower volume fractions

Structural anisotropy enhances ferromagnetic order

Abstract

We study by numerical simulation the magnetic order in ensembles of randomly packed magnetic spherical particles which, induced by their uniaxial anisotropy in the strong coupling limit, behave as Ising dipoles. We explore the role of the frozen disorder in the positions of the particles assuming a common fixed direction for the easy axes of all spheres. We look at two types of spatially disordered configurations. In the first one we consider isotropic positional distributions which can be obtained from the liquid state of the hard sphere fluid. We derive the phase diagram in the $T$-$\Phi$ plane where $T$ is the temperature and $\Phi$ the volume fraction. This diagram exhibits long-range ferromagnetic order at low $T$ for volume fractions above the threshold $\Phi_{c} = 0.157$ predicted by mean-field calculations. For $\Phi \lesssim \Phi_{c}$ a spin-glass phase forms with the same marginal behavior found for other strongly disordered dipolar systems. The second type of spatial configurations we study are anisotropic distributions that can be obtained by freezing a dipolar hard sphere liquid in its polarized state at low temperature. This structural anisotropy enhances the ferromagnetic order present in isotropic hard sphere configurations.