Quantum versus thermal fluctuations in the fcc antiferromagnet: alternative routes to order by disorder

TL;DR

This paper investigates how quantum and thermal fluctuations differently influence the selection of magnetic order in frustrated antiferromagnets, leading to a temperature-driven transition between distinct ordered states.

Contribution

It demonstrates that quantum and thermal order by disorder can favor different magnetic states, revealing a competition that causes phase transitions in frustrated systems.

Findings

Zero-point oscillations favor type 3 collinear AFM at T=0

Thermal fluctuations favor type 1 collinear AFM at finite temperature

A finite-temperature transition occurs between the two collinear states

Abstract

In frustrated magnetic systems with competing interactions fluctuations can lift the residual accidental degeneracy. We argue that the state selection may have different outcomes for quantum and thermal order by disorder. As an example, we consider the semiclassical Heisenberg fcc antiferromagnet with only the nearest-neighbor interactions. Zero-point oscillations select the type 3 collinear antiferromagnetic state at T=0. Thermal fluctuations favor instead the type 1 antiferromagnetic structure. The opposite tendencies result in a finite-temperature transition between the two collinear states. Competition between effects of quantum and thermal order by disorder is a general phenomenon and is also realized in the J1-J2 square-lattice antiferromagnet at the critical point J2 = 0.5 J1.