Na atomic order, Co charge disproportionation and magnetism in Na_xCoO₂ for large Na contents

¹  Laboratoire de Physique des Solides, UMR CNRS 8502, Université Paris-Sud - 91405 Orsay, France, EU
²  Physics Department, Kazan State University - 420008 Kazan, Russia
³  Laboratoire Léon Brillouin, CE Saclay, CEA-CNRS - 91191 Gif-sur-Yvette, France, EU

Corresponding authors: alloul@lps.u-psud.fr Irek.Mukhamedshin@ksu.ru

Received: 31 December 2007
Accepted: 5 February 2008

Abstract

We have synthesized and characterized four different stable phases of Na ordered Na_xCoO₂, for 0.65 < x < 0.8. Above 100K 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 300K into non-magnetic Co³⁺ and magnetic ≈ Co^3.5+ sites on which holes delocalize. This allows us to understand that metallic magnetism is favored for these large Na contents. Below 100K the phases differentiate, and a magnetic order sets in only for x ≳ 0.75 at T_N = 22K. We suggest that the charge order also governs the low-T energy scales and transverse couplings.