Energy minima and ordering in ferromagnets with quenched randomness

TL;DR

This study uses large-scale simulations to explore magnetic ordering in 2D and 3D disordered spin systems, revealing unexpected ordering in 3D random-anisotropy systems and spin-glass behavior in random-field systems.

Contribution

It provides new insights into the magnetic ordering phenomena in disordered systems, challenging existing theoretical predictions and highlighting the role of anisotropy and random fields.

Findings

3D random-anisotropy systems order magnetically at low temperatures.

Strong random-anisotropy leads to reduced magnetization and spin-glass components.

3D random-field systems do not order but freeze into spin-glass states.

Abstract

Energy minimization at T=0 and Monte Carlo simulations at T>0 have been performed for 2D and 3D random-field and random-anisotropy systems of up to 100 million classical spins. The main finding is that 3D random-anisotropy systems magnetically order on lowering temperature, contrary to the theoretical predictions based on the Imry-Ma argument. If random-anisotropy is stronger than the exchange, which can be the case in sintered materials, the system still orders but the magnetization is strongly reduced and there is a large spin-glass component in the spin state, the heat capacity having a cusp instead of a divergence. 3D random-field systems do not magnetically order on lowering temperature but rather freeze into the correlated spin-glass state. Here, although magnetized local energy minima have lower energies than non-magnetized ones, magnetic ordering is prevented by singularities pinned by the random field.