Na atomic order, Co charge disproportionation and magnetism in Na xCoO2 for large Na contents

Abstract

We have synthesized and characterized four different stable phases of Na ordered NaxCoO2, 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 CoO2 planes revealed by 23Na NMR data. In all phases from 59Co NMR data we display evidences that the Co disproportionate already above 300 K into non-magnetic Co3+ and magnetic ≈Co3.5+ 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≳0.75 at TN=22 K. We suggest that the charge order also governs the low-T energy scales and transverse couplings. © Europhysics Letters Association.