Haberkorn, H., M. Xu, W. R. Meier, J. Schmidt, S. Suárez, S. L. Bud’ko, and P. C. Canfield. "Substantial reduction of the anisotropy in the critical current densities Jcof Ni-doped CaKFe4As4single crystals by chemical and irradiation-induced disorder." Superconductor Science and Technology 34, no. 3 (2021): 035013.
Abstract: We demonstrate that the anisotropy in the critical current densities, Jc, of CaK(Fe1-xNix)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 Jc 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 Jc 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 Jc under magnetic fields.
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Haberkorn, N., M. Xu, W. R. Meier, S. Suárez, S. L. Bud’ko, and P. C. Canfield. "Enhancement of critical current density in CaKFe4As4 single crystals through 3 MeV proton irradiation." Superconductor Science and Technology 33, no. 2 (2020): 025008.
Abstract: We study the influence of random point disorder on the vortex dynamics and critical current densities Jc of CaKFe4As4 single crystals by performing magnetization measurements. Different samples were irradiated with a proton (p) beam at constant energy of 3 MeV to fluencies from 2 × 1015 p cm−2 to 4 × 1016 p cm−2. The results show the addition of extrinsic random point disorder enhances the Jc values at low and intermediate temperatures over the entire range of magnetic fields applied. The optimum pinning enhancement is achieved with a proton fluence of 3 × 1016 p cm−2, increasing Jc at 5 K by factors ≈5 and 14 at self-field and μ0H = 3 T, respectively. We analyze the vortex dynamics using the collective creep theory. The enhancement in Jc matches with a systematic reduction in the flux creep relaxation rates as a consequence of a gradual increase in the collective pinning energy U0. The substantial increment in Jc produced by random point disorder, reaching values of 9 MA cm−2 at 5 K and self-field, makes CaKFe4As4 a promising material for applications based on current carrying capacity at high magnetic fields.
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