Partial order from disorder in a classical pyrochlore antiferromagnet

TL;DR

This paper explores how a weak second-neighbor interaction in a classical pyrochlore antiferromagnet lifts degeneracy, leading to a novel partially ordered phase with layered and nematic order at finite temperatures.

Contribution

It introduces a new partially ordered phase induced by weak interactions and characterizes its properties and phase boundaries analytically and via simulations.

Findings

Discovery of a partially ordered phase with layered structure and nematic order.

Phase boundaries scale with J_2 as 1.87 J_2 S^2 and 0.26 J_2 S^2.

Phase transitions are discontinuous and the stability boundary scales as J_2^2/J_1.

Abstract

We investigate theoretically the phase diagram of a classical Heisenberg antiferromagnet on the pyrochlore lattice perturbed by a weak second-neighbor interaction J_2. The huge ground state degeneracy of the nearest-neighbor Heisenberg spins is lifted by J_2 and a magnetically ordered ground state sets in upon approaching zero temperature. We have found a new, partially ordered phase with collinear spins at finite temperatures for a ferromagnetic J_2. In addition to a large nematic order parameter, this intermediate phase also exhibits a layered structure and a bond order that breaks the sublattice symmetry. Thermodynamic phase boundaries separating it from the fully disordered and magnetically ordered states scale as 1.87 J_2 S^2 and 0.26 J_2 S^2 in the limit of small J_2. The phase transitions are discontinuous. We analytically examine the local stability of the collinear state and obtain a boundary T ~ J_2^2/J_1 in agreement with Monte Carlo simulations.