Three-Dimensional Spin Fluctuations in Na0.75CoO2

TL;DR

This study investigates the magnetic excitations in Na0.75CoO2 using neutron scattering, revealing three-dimensional spin fluctuations with ferromagnetic in-plane and antiferromagnetic interlayer correlations, and quantifying exchange interactions.

Contribution

It provides the first detailed characterization of three-dimensional magnetic fluctuations and exchange interactions in Na0.75CoO2 using neutron scattering techniques.

Findings

Ferromagnetic correlations within cobalt layers

Antiferromagnetic correlations between layers

Interlayer exchange constant estimated at 12.2 meV

Abstract

We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na0.75CoO2. The data confirm ferromagnetic correlations within the cobalt-layers and reveal antiferromagnetic correlations perpendicular to the layers, consistent with an A-type antiferromagnetic ordering. The magnetic modes propagating perpendicular to the layers are sharp, and reach a maximum energy of ~12 meV. From a minimal spin wave model, containing only nearest-neighbour Heisenberg exchange interactions, we estimate the inter- and intra-layer exchange constants to be 12.2 +/- 0.5 meV and -6 +/- 2 meV, respectively. We conclude that the magnetic fluctuations in Na0.75CoO2 are highly three-dimensional.