Novel sample-thickness-dependent flux pinning behaviors of KFe₂As₂ intercalations in CaKFe₄As₄ single crystals

We report the strong dependence of critical current density (J _c) on the sample thickness (d) along the crystalline c-axis in CaKFe₄As₄ single crystals. The J _c can be respectively enlarged more than 3 and 50 times in H parallel to c-axis ($J_{\text{c}}^c$) and ab-plane ($J_{\text{c}}^{ab}$) at temperature of 5 K and high magnetic fields of 8 T when the d is reduced from 88 to 2.0 $\mu \text{m}$. It is surprising to find that, accompanied by the enhanced J _c, the fishtail effect fades away with the decreasing d. We believe that both the origins of enhanced J _c and the fishtail effect in CaKFe₄As₄ may have close relationship with the interlayer coupling between CaKFe₄As₄ and the intercalations of KFe₂As₂. Our results not only demonstrate an encouraging prospect for application of CaKFe₄As₄, but also illuminate new light on the origin of fishtail effect in iron-based superconductors. (paper)