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We report the critical current density ( J  c ) and vortex pinning properties in single crystals of a novel iron-based superconductor (IBS) KCa 2 Fe 4 As 4 F 2 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 KCa 2 Fe 4 As 4 F 2 single crystals on transition temperature T  c and J  c , including its dose dependence, are systematically studied. J  c ~8 MA/cm 2 under a self-field at 2 K in the pristine crystal is strongly enhanced up to 19.4 and 17.5 MA/cm 2 by irradiation of 2.6 GeV U-ions and 3 MeV protons, respectively. Suppression of T  c and dose dependence of J  c in KCa 2 Fe 4 As 4 F 2 is different from that in a representative IBS of (Ba,K)Fe 2 As 2 , which can be explained by considering the presence of embedded defects in pristine KCa 2 Fe 4 As 4 F 2 . The vortex dynamics in the pristine and proton irradiated KCa 2 Fe 4 As 4 F 2 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 KCa 2 Fe 4 As 4 F 2 is similar to those in (Ba,K)Fe 2 As 2 to some extent, and different from that in anisotropic Li 0.8 Fe 0.2 OHFeSe. Large anisotropy, due to the presence of insulating blocking layers in KCa 2 Fe 4 As 4 F 2 , which leads to much lower irreversibility field ( H  irr ) compared with 122-type IBSs, strongly affect the vortex dynamics.

Critical Current Density and Vortex Dynamics in Pristine and Irradiated KCa2Fe4As4F2