Magnetic relaxation and collective vortex creep in FeTe _0.6Se_0.4 single crystal

We study the vortex dynamics in a high-quality FeTe_0.6Se _0.4 single crystal by performing magnetization measurements of the screening current density J_s and flux creep rate S. The temperature dependence of S shows a plateau in the intermediate-temperature region with a high creep rate ∼ 0.03, which is interpreted in the framework of the collective creep theory. A crossover from elastic to plastic creep is observed. The glassy exponent and barrier height for the flux creep are directly determined by the extended Maley's method. J_s with flux creep, obtained from magnetic hysteresis loops, is successfully reproduced based on the collective creep analysis. We also approach the critical current density without flux creep by means of the generalized inversion scheme, which proves that the and pinning coexist in the FeTe_0.6Se_0.4 single crystal.