Magnetic properties of spin-orbital polarons in lightly doped cobaltates

TL;DR

This paper models spin-orbital polarons in lightly doped cobaltates to explain their unusual magnetic properties, including susceptibility and intra-layer interactions, and predicts related excitations affecting spin dynamics.

Contribution

It introduces a numerical model of spin-orbital polarons that accounts for complex magnetic behaviors in Na_xCoO_2 at low doping levels, providing new insights into their internal degrees of freedom.

Findings

Polarons explain large magnetic susceptibility.

Internal degrees of freedom lead to negative Curie-Weiss temperature.

Predictions on excitation locations and their effects on spin-wave damping.

Abstract

We present a numerical treatment of a spin-orbital polaron model for Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intra-layer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.