Superconductivity, charge order and anomalous magnetism in sodium cobaltates

Oriented powder samples of NaxCoO2, with quasi single phase composition have been synthesized and studied by x rays, SQUID magnetometry, 23Na and 59Co NMR. We evidence that the dry and superconducting (SC) samples with x ≈ 0.35 differ from those with 0.50 ≤ x < 1 inasmuch as the Co sites display an uniform charge state in the former while they differentiate markedly in the latter. For 0.50 ≤ x < 1, the local Co charge and magnetic properties appear correlated with definite orderings of the Na+ ions. For x ≃ 0.70 we could distinguish non magnetic Co3+ sites with no spin susceptibility from more magnetic Co sites, among which Co4+ sites can be excluded. These sites form a single strongly correlated electronic band which exhibits a quasi localized T dependent magnetism which might only become Fermi liquid like below 2K. This contrasts with the Pauli like magnetism found for x ≈ 0.35 in the dry and SC compounds. Those display quasi identical metallic states, with a weak spin susceptibility which only contributes to a small shift of the 59Co NMR. Its variation below Tc reveals a strong suppression of the spin susceptibility, compatible with singlet superconducting pairing. © EDP Sciences