Substantial reduction of the anisotropy in the critical current densities J c of Ni-doped CaKFe4As4 single crystals by chemical and irradiation-induced disorder

Abstract

We demonstrate that the anisotropy in the critical current densities, J c, of CaK(Fe1−x Ni x )4As4 (x = 0, 0.015, 0.025, and 0.030) single crystals reduces with increasing chemical and irradiation-induced disorder. The magnetic field dependences of J c are analyzed by performing magnetization measurements with H applied parallel and perpendicular to the crystallographic c-axis. The results show that undoped crystals display large anisotropies in J c due to an enhancement of the vortex pinning with H applied parallel to the crystallographic ab-planes. This anisotropy reduces substantially as Ni addition increases. Moreover, we found that random disorder introduced by proton irradiation enhances mainly the vortex pinning for H parallel to the c-axis. Consequently, using adequate fluencies, the vortex pinning at low temperatures in both undoped and doped samples becomes close to isotropic. These results make the CaKFe4As4 system promising for applications that require isotropic J c under magnetic fields.