Na atomic order, Co charge disproportionation and magnetism in NaxCoO2 for large Na contentsH. Alloul1, I. R. Mukhamedshin1, 2, G. Collin3 and N. Blanchard1
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
received 31 December 2007; accepted in final form 5 February 2008; published April 2008
published online 4 March 2008
We have synthesized and characterized four different stable phases of Na ordered NaxCoO2, 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 Co3.5+ 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 TN = 22K. We suggest that the charge order also governs the low-T energy scales and transverse couplings.
71.27.+a - Strongly correlated electron systems; heavy fermions.
76.60.-k - Nuclear magnetic resonance and relaxation.
75.30.-m - Intrinsic properties of magnetically ordered materials.
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