Magnetic relaxation in an isotropic extreme type-II superconductor

The time dependence of the zero-field-cooled and thermoremanent magnetization of the isotropic extreme type-II superconductor PbMo6S8 has been measured at temperatures 0.7T/Tc0.95 and for fields 1H300 G. The results show that the time dependence of the decay can be divided into three different field regimes: (1) A low-field regime with a closely logarithmic decay and a well-established critical state. The relaxation rate [M/ ln(t)] increases as H5 for the zero-field-cooled magnetization and as H2.4 for the thermoremanent magnetization. The reduction in magnetization over 5 decades in time (0.3"3×104 s) is, at most, a few percent. (2) A crossover-field regime with a change from a logarithmic to a nonlogarithmic decay. (3) A high-field regime where the time dependence of the decay is well described by the expression M[ln(t/0)]-. The reduction in magnetization in the crossover- and high-field regimes is as large as 25"30 % in the measured time interval. The results in the high-field regime are interpreted within the theory of collective flux creep. Comparisons are made with the magnetic relaxation found in anisotropic high-Tc superconductors. © 1991 The American Physical Society.