Order by Distortion and String Modes in Pyrochlore Antiferromagnets

TLDR

The study investigates how magnetoelastic couplings affect pyrochlore antiferromagnets and discusses experimental implications. Landau theory and classical analyses are used to show that magnetoelastic couplings induce bond order through a spin‑Peierls transition. The transition leads to various low‑temperature Néel phases, and in the collinear phase a dispersionless string‑like magnon mode dominates the spectrum, signaling the underlying frustration.

Abstract

We study the effects of magnetoelastic couplings on pyrochlore antiferromagnets. We employ Landau theory, extending an investigation begun by Yamashita and Ueda for the case of S = 1, and classical analyses to argue that such couplings generate bond order via a spin-Peierls transition. This is followed by, or concurrent with, a transition into one of several possible low-temperature Néel phases, with most simply collinear, but also coplanar or mixed spin patterns. In a collinear Néel phase, a dispersionless stringlike magnon mode dominates the resulting excitation spectrum, providing a distinctive signature of the parent geometrically frustrated state. We comment on the experimental situation.