Critical Current Density and Vortex Dynamics in Pristine and Irradiated KCa₂Fe₄As₄F₂

We report the critical current density (J_c) and vortex pinning properties in single crystals of a novel iron-based superconductor (IBS) KCa₂Fe₄As₄F₂ with large J_c in the pristine state, before and after introduction of artificial defects by swift-particle irradiation. The effects of 2.6 GeV U and 3 MeV proton irradiations in KCa₂Fe₄As₄F₂ single crystals on transition temperature T_c and J_c, including its dose dependence, are systematically studied. J_c~8 MA/cm² under a self-field at 2 K in the pristine crystal is strongly enhanced up to 19.4 and 17.5 MA/cm² by irradiation of 2.6 GeV U-ions and 3 MeV protons, respectively. Suppression of T_c and dose dependence of J_c in KCa₂Fe₄As₄F₂ is different from that in a representative IBS of (Ba,K)Fe₂As₂, which can be explained by considering the presence of embedded defects in pristine KCa₂Fe₄As₄F₂. The vortex dynamics in the pristine and proton irradiated KCa₂Fe₄As₄F₂ single crystals are also investigated from the analyses of the field dependence of J_c and the normalized magnetic relaxation rate. In addition to the contribution of embedded defects, weak collective pinning is considered for comprehensive analyses. Vortex dynamics in KCa₂Fe₄As₄F₂ is similar to those in (Ba,K)Fe₂As₂ to some extent, and different from that in anisotropic Li_0.8Fe_0.2OHFeSe. Large anisotropy, due to the presence of insulating blocking layers in KCa₂Fe₄As₄F₂, which leads to much lower irreversibility field (H_irr) compared with 122-type IBSs, strongly affect the vortex dynamics.