Na atomic order, Co charge disproportionation and magnetism in Na${}_x$CoO${}_2$ for large Na contents

Abstract

We have synthesized and characterized four different stable phases of Na ordered Na${}_x$CoO${}_2$, for $0.65<x<0.8$. Above 100 K they display similar Curie-Weiss susceptibilities as well as ferromagnetic $q=0$ spin fluctuations in the CoO${}_2$ planes revealed by ${}^{23}$Na NMR data. In all phases from ${}^{59}$Co NMR data we display evidences that the Co disproportionate already above 300 K into non magnetic Co${}^{3+}$ and magnetic $\approx$Co${}^{3.5+}$ sites on which holes delocalize. This allows us to understand that metallic magnetism is favored for these large Na contents. Below 100 K the phases differentiate, and a magnetic order sets in only for $x\gtrsim 0.75$ at $T_N=$22 K. We suggest that the charge order also governs the low $T$ energy scales and transverse couplings.