| A true critical current density, j_c, as opposite to commonly measured relaxed persistent (Bean) current, j_B , was extracted from the Campbell penetration depth, lambda_C(T,H) measured in single crystals of LiFeAs, and optimally electron-doped Ba (Fe__0.954Ni_0.046)_2As_2 (FeNi122). In LiFeAs, the effective pinning potential is non-parabolic, which follows from the magnetic field - dependent Labusch parameter alpha. At the equilibrium (upon field - cooling), alpha(H) is non-monotonic, but it is monotonic at a finite gradient of the vortex density. This behavior leads to a faster magnetic relaxation at the lower fields and provides a natural dynamic explanation for the fishtail (second peak) effect. We also find the evidence for strong pinning at the lower fields.The inferred field dependence of the pinning potential is consistent with the evolution from strong pinning, through collective pinning, and eventually to a disordered vortex lattice. The value of j_c (2 K) = 1.22x10^6 A/cm^2 provide an upper estimate of the current carrying capability of LiFeAs. Overall, vortex behavior of almost isotropic, fully-gapped LiFeAs is very similar to highly anisotropic d-wave cuprate superconductors, the similarity that requires further studies in order to understand unconventional superconductivity in cuprates and pnictides.;In addition to LiFeAs, we also report the magnetic penetration depth in BaFe_2 As_2 based superconductors including irradiation of FeNi122. In unirradiated FeNi122, the maximum critical current value is, j_c (2K)= 3.3x10^6 A/(cm^2 ). The magnetic-dependent feature was observed near the transitiontemperature in FeTe_0.53Se_0.47 and irradiated FeNi122. Because of this feature, further studies are required in order to properly calibrate the Campbell penetration depth. Finally, we detected the crossing between the magnetic penetration depth and London penetration depth in optimally hold-doped Ba_0.6K _0.4Fe_2As_2 (BaK122) and isovalent doped BaFe_2(As_0.7P_0.3)_2 (BaP122). These phenomena probably coincide with anomalous Meissner effect reported in pnicitde superconductors [Prozorov et al. (2010b)] however more studies are needed in order to clarify this. |
