Ferromagnetism-induced Phase Separation in a Two-dimensional Spin Fluid

TL;DR

This paper investigates how ferromagnetic interactions induce liquid-gas phase separation in a two-dimensional spin fluid, revealing a complex interplay between magnetization and phase behavior through simulations and mean-field analysis.

Contribution

It demonstrates that ferromagnetic ordering triggers phase separation in a 2D spin fluid and characterizes the phase diagram, including a tricritical point and the nature of magnetic ordering.

Findings

Magnetization induces liquid-gas phase separation.

The coexistence region ends at a tricritical point.

Long-range order persists beyond the BKT scenario.

Abstract

We study the liquid-gas phase separation observed in a system of repulsive particles dressed with ferromagnetically aligning spins, a so-called `spin fluid'. Microcanonical ensemble numerical simulations of finite-size systems reveal that magnetization sets in and induces a liquid-gas phase separation between a disordered gas and a ferromagnetic dense phase at low enough energies and large enough densities. The dynamics after a quench into the coexistence region show that the order parameter associated to the liquid-vapour phase separation follows an algebraic law with an unusual exponent, as it is forced to synchronize with the growth of the magnetization: this suggests that for finite size systems the magnetization sets in along a Curie line, which is also the gas-side spinodal line, and that the coexistence region ends at a tricritical point. This picture is confirmed at the mean-field level with different approximation schemes, namely a Bethe lattice resolution and a virial expansion complemented by the introduction of a self-consistent Weiss-like molecular field. However, a detailed finite-size scaling analysis shows that in two dimensions the ferromagnetic phase escapes the Berezinskii-Kosterlitz-Thouless scenario, and that the long-range order is not destroyed by the unbinding of topological defects. The Curie line becomes thus a magnetic crossover in the thermodynamic limit. Finally, the effects of the magnetic interaction range and those of the interaction softness are characterized within a mean-field semi-analytic low-density approach.