Influence of charge order on the magnetic properties of Na${}_x$CoO${}_2$ for $x>0.65$

Abstract

We have synthesized and characterized the four different stable phases of Na ordered Na${}_x$CoO${}_2$, for $0.65<x\lesssim 0.75$. Above 100K they display similar Curie-Weiss spin susceptibilities as well as ferromagnetic $q=0$ spin fluctuations in the CoO${}_2$ planes revealed respectively by ${}^{23}$Na NMR shift and spin lattice $T_1$ data. The Co disproportionate already above 300K into Co${}^{3+}$ and $\approx$Co${}^{3.5+}$ in all phases, which allows us to understand that magnetism is favoured. Below 100K the paramagnetic properties become quite distinct, and a 3D magnetic order sets in only for $x=0.75$, so that charge order has a subtle incidence on the low $T$ energy scales and transverse magnetic couplings.

4 pages, 4 figures