Doping evolution of the second magnetization peak and magnetic relaxation in (Ba1-xKx) Fe2As2 single crystals

We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of (Ba1-xKx)Fe2As2 (0.18 <= x <= 1). The critical current density J(c) reaches maximum in the underdoped sample x = 0.26 and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier U-0 sharply decreases in the overdoped sample x = 0.70. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples x = 0.38, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in T-c become small in the samples x = 0.43 and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the (Ba1-xKx)Fe2As2 crystals: (i) strong delta l pinning, which results from the fluctuations in the mean free path l and delta T-c pinning from the spatial variations in T-c in the underdoped regime, and (ii) weak delta T-c pinning in the optimally doped and overdoped regime.