Microwave absorption study of pinning regimes in Ba(Fe1−xCox)2As2 single crystals

Magnetic field dependent modulated microwave absorption (MMWA) measurements have been carried out to investigate vortex pinning effects in single crystals of the iron-based high-Tc superconductor Ba(Fe1−xCox)2As2 with three different cobalt doping levels of x = 0.07, 0.09 and 0.11. The dependence of the MMWA hysteresis loops on temperature, magnetic field and Co concentration have been measured and analyzed using a theoretical model of microwave absorption in superconductors. The analysis reveals that in the underdoped crystal (x = 0.07) the so called δTc-pinning due to magnetically ordered regions defines the temperature dependence of the critical current density, while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the pinning is governed by structural imperfections due to the inhomogeneous distribution of the cobalt dopant and has the so called δl character.