Structural phase transitions in geometrically frustrated antiferromagnets

TL;DR

This paper investigates how geometrically frustrated antiferromagnets undergo structural phase transitions influenced by magnetoelastic coupling and quenched disorder, revealing a first-order transition and disorder-stabilized phases through Monte Carlo simulations.

Contribution

It provides a detailed Monte Carlo analysis of the phase transition in frustrated antiferromagnets, showing the nature of the transition and the effects of quenched disorder, which was not previously well understood.

Findings

Transition is first-order, not supporting spin-Peierls phase.

Quenched disorder stabilizes the cubic phase.

Phase diagram resembles experimental observations in ZnCdCrO.

Abstract

We study geometrically frustrated antiferromagnets with magnetoelastic coupling. Frustration in these systems may be relieved by a structural transition to a low temperature phase with reduced lattice symmetry. We examine the statistical mechanics of this transition and the effects on it of quenched disorder, using Monte Carlo simulations of the classical Heisenberg model on the pyrochlore lattice with coupling to uniform lattice distortions. The model has a transition between a cubic, paramagnetic high-temperature phase and a tetragonal, Neel ordered low-temperature phase. It does not support the spin-Peierls phase, which is predicted as an additional possibility within Landau theory, and the transition is first-order for reasons unconnected with the symmetry analysis of Landau theory. Quenched disorder stabilises the cubic phase, and we find a phase diagram as a function of temperature and disorder strength similar to that observed in ZnCdCrO.