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 ( B a 1 − x K x ) F e 2 A s 2 ( 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 ( B a 1 − x K x ) F e 2 A s 2 crystals: (i) strong δl pinning, which results from the fluctuations in the mean free path l and δ T c pinning from the spatial variations in T c in the underdoped regime, and (ii) weak δ T c pinning in the optimally doped and overdoped regime. Disciplines Condensed Matter Physics | Materials Science and Engineering Authors Yong Liu, Lin Zhou, Kewei Sun, Warren E. Straszheim, Makariy A. Tanatar, Ruslan Prozorov, and Thomas A. Lograsso This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/112 PHYSICAL REVIEW B 97, 054511 (2018) Doping evolution of the second magnetization peak and magnetic relaxation in (Ba1−xKx)Fe2As2 single crystals Yong Liu,1,* Lin Zhou,1 Kewei Sun,1 Warren E. Straszheim,1 Makariy A. Tanatar,1,2 Ruslan Prozorov,1,2 and Thomas A. Lograsso1,3 1Division of Materials Sciences and Engineering, Ames Laboratory, U.S. DOE, Ames, Iowa 50011, USA 2Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 3Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA (Received 11 June 2017; revised manuscript received 26 November 2017; published 16 February 2018) 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 Jc 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 U0 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 Tc 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 δl pinning, which results from the fluctuations in the mean free path l and δTc pinning from the spatial variations in Tc in the underdoped regime, and (ii) weak δTc pinning in the optimally doped and overdoped regime. DOI: 10.1103/PhysRevB.97. 054511