Magnetic ordering and spin waves in $\bf Na_{0.82}CoO_2$

TL;DR

This study investigates the magnetic order and spin wave excitations in Na$_{0.82}$CoO$_2$, revealing antiferromagnetic order and unusual spin dynamics that challenge existing models of layered cobaltates.

Contribution

First neutron scattering study of magnetic order and spin waves in Na$_{0.82}$CoO$_2$, providing new insights into its magnetic interactions and anisotropy.

Findings

Observed A-type antiferromagnetic order below 20 K.

Detected strongly energy-dependent spin wave linewidths.

Found similar in-plane and out-of-plane exchange constants.

Abstract

Na$_x$CoO$_2$, the parent compound of the recently synthesized superconductor Na$_x$CoO$_2$:$y$H$_2$O, exhibits bulk antiferromagnetic order below $\sim20$ K for 0.75 $\leq x \leq$ 0.9. We have performed neutron scattering experiments in which we observed Bragg reflections corresponding to A-type antiferromagnetic order in a Na$_{0.82}$CoO$_2$ single crystal and characterized the corresponding spin-wave dispersions. The spin waves exhibit a strongly energy-dependent linewidth. The in-plane and out-of-plane exchange constants resulting from a fit to a nearest-neighbor Heisenberg model are similar in magnitude, which is unexpected in view of the layered crystal structure of Na$_{x}$CoO$_2$. Possible implications of these observations are discussed.