Critical current density and magnetic-field irreversibility of $Na0.35CoO2⋅1.3H2O$ superconductor

Critical current density $Jc$ of $Na0.35CoO2⋅1.3H2O$ superconductor $(Tc=4.4 K)$ has been estimated from magnetization measurements: $Jc(1.9 K,0 T)=5×104 A/cm2,$ $Jc$ $(1.9 K,0.4 T)=4×102 A/cm2$ and $Jc$ $(3.4 K,0 T)=103 A/cm2.$ $Jc$ versus applied magnetic field H cannot be fitted, as for high-$Tc$ superconductors (HTS), by a power law for any temperature. The temperature dependence for a given H is not of the linear type as for $MgB2,$ but is close to quasiexponential (associated with the thermally activated flux creep) as for HTS. Irreversibility field data points $Hirr$ versus $(1−T/Tc)$ obey a power law with power exponent $n=3.4.$ The irreversibility line is located below, but in vicinity of, the line for Bi-2223. Details of morphology are also presented.