Chiral magnetism and helimagnons in a pyrochlore antiferromagnet

TL;DR

This paper investigates the origin of chiral helimagnetic order in a pyrochlore antiferromagnet, combining theoretical modeling and numerical simulations to explain experimental neutron scattering results and the role of lattice chirality.

Contribution

It provides a systematic theoretical analysis of the magnetic ground state and spin-wave spectrum, highlighting the interplay of Dzyaloshinskii-Moriya interactions and lattice effects in stabilizing incommensurate order.

Findings

Identification of helimagnon dispersion relation

Agreement between numerical spectrum and experimental data

Evidence that lattice chirality influences magnetic transition

Abstract

Recent neutron scattering measurements on the spinel CdCr2O4 revealed a rare example of helical magnetic order in geometrically frustrated pyrochlore antiferromagnet. The spin spiral characterized by an incommensurate wavevector Q = 2pi (0, delta, 1) with delta ~0.09 is accompanied by a tetragonal distortion. Here we conduct a systematic study on the magnetic ground state resulting from the interplay between the Dzyaloshinskii-Moriya interaction and further neighbor exchange couplings, two of the most important mechanisms for stabilizing incommensurate spin orders. We compute the low-energy spin-wave spectrum based on a microscopic spin Hamiltonian and find a dispersion relation characteristic of the helimagnons. By numerically integrating the Landau-Lifshitz-Gilbert equation with realistic model parameters, an overall agreement between experiment and the numerical spectrum, lending further support to the view that a softened optical phonon triggers the magnetic transition and endows the lattice a chirality.