Vanishing vortex creep at the transition from ordered to disordered vortex phases in Ba_(0.64)K_(0.36)Fe₂As₂

By measuring the dynamical and conventional magnetization relaxation of the Ba$_{0.64}$K$_{0.36}$Fe$_2$As$_2$ single crystals, we found strong second peak effect on the magnetization hysteresis loops. It is found that there is a kink of magnetization at a field between the valley and maximum magnetization. Interestingly, the magnetization relaxation rate has a deep minimum at the field with the kink, indicating a diminished vortex creep. The relaxation rate at this field is clearly smaller than the so-called universal lower limit of the relaxation rate characterized by ($S_0\approx \ Gi^{1/2}(T/T_\mathrm{c}$). This diminished vortex creep is associated with the origin of the SMP effect and attributed to the strongly hindered flux motion when experiencing the transition from the quasi-ordered to disordered vortex phases.