Upper critical field of an aligned Nax(H3O)zCoO2•yH2O superconductor from magnetization measurements

We present magnetization measurements for the upper critical field H-c2 on aligned Na-x(H3O)(z)CoO2 center dot yH(2)O superconductors with magnetic field parallel to CoO2 layers. The temperature-dependent part of the normal-state magnetization M(H,T) can be well scaled with Brillouin function, suggesting the existence of impurity moments. By subtracting the contribution of the magnetic impurity moments, a well defined onset of superconducting diamagnetism allows the in-field superconducting transition temperature T-c(H) to be accurately determined. No superconducting transition was observed down to 1.9 K under an applied field of 8 T. The result suggests that the H-c2(parallel to ab)(0) value is just within the Pauli paramagnetic limit, supporting spin-singlet superconductivity in the cobalt oxyhydrate superconductor. Additionally, the upward curvature near T-c in the H-c2(parallel to ab)-T phase diagram was confirmed to be intrinsic. Possible origin of the anomaly was discussed.