Spin excitations in the antiferromagnet NaNiO2

TL;DR

This paper investigates the spin excitations in NaNiO2, revealing how bi-axial anisotropy influences magnetic behavior in a quasi-two-dimensional triangular lattice of S=1/2 spins.

Contribution

It provides a phenomenological model incorporating bi-axial anisotropy to explain the magnetic excitations observed in NaNiO2.

Findings

Identification of easy-plane anisotropy in NaNiO2

Observation of magnetic excitations up to 20 cm-1

Correlation of anisotropy direction with Jahn-Teller distortion

Abstract

In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S = 1=2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferro- magnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The bulk of the results are interpreted in terms of a phenomenological model involving bi-axial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane. The direction of the easy plane is constrained by the collective Jahn-Teller distortion occurring in this material at 480 K.