Influence of charge order on the magnetic properties of Na$_{x}$CoO$_{2}$ for $x>0.65$

TL;DR

This study investigates how charge ordering influences the magnetic properties of Na$_{x}$CoO$_{2}$ for $x>0.65$, revealing that charge order subtly affects low-temperature magnetic behavior and interlayer couplings.

Contribution

It provides a detailed characterization of Na$_{x}$CoO$_{2}$ phases, linking charge disproportionation to magnetic properties and identifying the conditions for magnetic ordering.

Findings

Co disproportionates into Co$^{3+}$ and Co$^{3.5+}$ above 300K

Similar Curie-Weiss susceptibilities and ferromagnetic fluctuations above 100K

3D magnetic order only at $x=0.75$ below 100K

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.