Upper critical field of aligned Na_{x}(H_{3}O)_{z}CoO_{2}\cdot yH_{2}O superconductor by magnetization measurement

We present magnetization measurements for the upper critical field $H_{c2}$ on aligned Na${}_x$(H${}_3$O)${}_z$CoO${}_2\cdot y$H${}_2$O superconductors with magnetic field parallel to CoO${}_2$ 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 $T_c$($H$) accurately determined. No superconducting transition was observed down to 1.9 K under an applied field of 8 Tesla. The result suggests that the $H_{c2}^{\mathrm{\parallel }ab}(0)$ 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 $H_{c2}^{\mathrm{\parallel }ab}-T$ phase diagram was confirmed to be intrinsic. Possible origin of the anomaly was discussed.