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 ${\mathrm{PbMo}}_6$ ${\mathrm{S}}_8$ has been measured at temperatures 0.7≤T/${\mathit{T}}_{\mathit{c}}$≤0.95 and for fields 1≤H≤300 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 ${\mathit{H}}^5$ for the zero-field-cooled magnetization and as ${\mathit{H}}^{2.4}$ for the thermoremanent magnetization. The reduction in magnetization over 5 decades in time (0.3–3×${10}^4$ 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 δM∼[ln(t/${\mathrm{\tau }}_0$)${]}^{\mathrm{-}\mathrm{\alpha }}$. 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-${\mathit{T}}_{\mathit{c}}$ superconductors.