NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2:: A nesting scenario

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na0.5CoO2. Above T-N=86 K, both sites display a similar T dependence of the spin shift, suggesting that there is no charge segregation into Co3+ and Co4+ sites. Below T-N, the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition at T-MIT=51 K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive spin-density wave induced by two nestings of the Fermi surface specific to the x=0.5 Na ordering.