Doping Dependence of the Second Magnetization Peak, Critical Current Density, and Pinning Mechanism in BaFe_2–xNi_xAs₂ Pnictide Superconductors

A series of high quality BaFe 2– x Ni x As 2 pnictide superconductors were investigated using magnetic relaxation and isothermal magnetic measurements to study the second magnetization peak (SMP) and critical current behavior in the Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas its magnetic field dependence hints at a vortex-lattice structural phase transition. The activation energy ( U ) estimated using the magnetic relaxation data was analyzed in detail for slightly underdoped, slightly overdoped, and an overdoped sample, using Maley’s method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of the 122-family. In addition, we also investigated the doping dependence of the critical current density ( J c ) and the vortex-pinning behavior in these compounds. The observed J c is higher than the threshold limit (10 5 A/cm 2 ) considered for the technological potential and even greater than 1 MA/cm 2 for the slightly underdoped Ni content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen et al., which suggest the dominant role of δ l -type pinning.