Understanding vortex dynamics in CaK(Fe,Ni)4As4 and Ba(Fe,Co)2As2 single crystals under the influence of random point disorder

Abstract

Abstract We report on the influence of doping on vortex dynamics in 3 MeV proton-irradiated single crystals of CaK(Fe1−xNix)4As4 (1144, x = 0.015, 0.025, and 0.03) and Ba(Fe1−xCox)2As2 (x = 0.04, 0.062, 0.066 and 0.074). Non-irradiated crystals of the 1144 system display superconducting critical temperatures ranging from 31 K for x = 0.015 to 20.5 K, as doping increases to 0.03. On the other hand, pristine crystals of the 122 system show Tc values between 14.6 and 23.6 K, with the maximum Tc occurring at intermediate doping levels. The fluence was set at 3x1016 p/cm2, resulting in a decrease in the Tc by around 1.5 K for all samples and significantly affecting the vortex dynamics by reducing the flux creep relaxation compared to previously reported values for unirradiated crystals. Parameters such as vortex pinning energy U0 and the glassy exponent μ dependencies on doping and magnetic field strength are identified. For the 1144 system, U0 reaches values approaching 500 K for small fields in samples with Tc = 29.3 K (x= 0.015), systematically decreasing to around 200 K as Tc falls below 20 K. Furthermore, U0 decreases as the field increases to 3 T for the same sample, varying from approximately 250 K to 100 K as Tc decreases. These changes are typically accompanied by modifications in μ, gradually increasing from values around 1 towards 1.5, corresponding to small bundle relaxation in the collective creep theory. Despite differences in the substitutional disorder and magnetic phase diagram with respect to the 1144 system, the results for 122 single crystals follow a similar tendency in which U0 usually reduces and μ increase rise as the applied magnetic field is increased. Due to moderate U0 in these systems (few hundreds of kelvins), the resulting decay of persistent current at liquid helium temperatures is primarily determined by a balance between U0 and bundle size contribution. These findings provide valuable insights for potential applications of these systems, particularly in the context of intrinsic superconducting parameters and the resulting pinning landscape.