Volume 82, Number 1, April 2008
|Number of page(s)||6|
|Section||Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties|
|Published online||04 March 2008|
Na atomic order, Co charge disproportionation and magnetism in NaxCoO2 for large Na contents
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
Accepted: 5 February 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.
PACS: 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
© EPLA, 2008
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