Pressure Effects on Superconducting and Magnetic Transitions in Layered Sodium Cobalt Oxides, NaxCoO2

The versatile family of sodium cobalt oxides emerged recently as a new target object for theoretical and experimental studies of various aspects of magnetism, superconductivity, and their interplay. The series NaxCoO2 (x<l) includes, besides a new superconductor ~, Na0.3CoOfyH20 (y =1.3) and its parent compound Na0.75CoO2, the material remarkable on its own for an antiferromagnetic order combined with metallic conductivity 2-4, also such fascinating materials as the x=2/3 "Curie-Weiss metal" exhibiting abnormally large and spin-dependent thermopower 5, and the x=l/2 compound, shown to posses an insulating low-temperature state with charge ordering on both the CoO~ and Na planes 6. It is known that both superconductivity and magnetism of NaxCoO2 compounds originate within the CoO2 layers. Onset of a magnetically ordered state in Na0.vsCoOz is signaled by the phase transition observed at -22K in various properties of the material. This transition was recently investigated in detail by Sales et al. 4 on floating zone (FZ) grown single crystals. It was found that like in polycrystalline samples, the susceptibility above 50 K follows the Z= zo+C/(T-O)dependence with a negative Weiss temperature of about 200 K. On cooling below TN = 22 K in small and moderate fields the susceptibility for both in-plane and out-of-plane orientations of magnetic field shows an abrupt increase. Magnetization curves at 1.8K for both H//a and H//c directions show typical for ferromagnet hysteresis with remnant moment o f lxl0 4 gB per Co atom,