Pressure dependence of magnetic and superconducting transitions in   sodium cobalt oxides NaxCoO2
  The results of DC magnetization measurements under hydrostatic (helium-gas) pressure are reported for an ambient pressure superconductor Na0.35CoO2.1.4D2O and its precursor compound, the gamma-phase Na0.75CoO2 that is known to combine a metallic conductivity with an unusual magnetic state below ~22K. The obtained data allowed us to present for the first time the pressure dependence of the magnetic transition in a metallic sodium cobaltate system. This dependence appears to be positive, with the magnetic transition rapidly shifting towards higher temperatures when an applied pressure increases. We ascribe the observed effect to the pressure-induced enhancement of the out-of-plane antiferromagnetic coupling mediated by localized spins interactions (of either superexchange or RKKY type), the scenario consistent with the A-type antiferromagnetic state suggested by recent neutron-scattering data. As for the pressure effect on the superconductivity in Na0.35CoO2.1.4D2O, our measurements established negative and linear for the entire pressure range from 1 bar to 8.3 kbar pressure dependence of Tc, the behavior quite different from the reported by previous workers strong non-linearity of the Tc (P) dependence. (Dated September 12, 2005)   PACS numbers: 74.62.Fj, 74.70.-b, 75.20. En, 75.50 Ee, 75.30 Kz. 
Y. V. Sushko (1), O. B. Korneta (1), S. O. Leontsev (1), R. Jin (2),   B. C. Sales (2), and D. Mandrus (2) ((1) Department o...