Vortex Pinning and the Mechanism in CaKFe 4 As 4 Revealed by Dynamical Magnetization Relaxation

We have studied the vortex dynamics of CaKFe4As4 single crystals using a dynamical magnetization relaxation method. A large region of vortex elastic motion is observed indicating strong vortex pinning ability. Temperature dependence of normalized flux pinning energy of g(t) is determined based on the data of transient current density Js and dynamical relaxation rate Q by the generalized inversion scheme (GIS), which suggests that both the δ(Tc)- and δ(l)-pinning may be responsible for the vortex dynamical behaviors in CaKFe4As4 under field parallel to c-axis (H // c). The value of Js for field parallel to ab-plane (H // ab) is obviously larger than that of H // c, this indicates that the flux pinning mechanisms are distinct in different crystal orientations. Under H // ab, a dip structure in the magnetic hysteresis loops (MHLs) is obtained around zero field, which is ascribed to planar defects in ab-plane. We further analyze the flux pinning mechanisms and find that the abnormal flux pinning behaviors should originate from its unique structure in CaKFe4As4.