Magnetic-field-dependent pinning potential in LiFeAs superconductor from its Campbell penetration depth

A theoretical critical current density j(c)(T, H), 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. The effective pinning potential is slightly nonparabolic, which follows from the magnetic-field-dependent Labusch parameter alpha. At the equilibrium (upon field cooling), alpha(H) is nonmonotonic, but it is monotonic at a finite gradient of the vortex density. Combined with the observation of a fishtail magnetization in standard dc measurements, this result implies that the fishtail appears as a result of magnetic relaxation. The functional form of M(H) curves is determined by the nonmonotonic pinning potential, implying the importance of vortex collective effects. The values of j(c)(2 K) similar or equal to 1.22 x 10(6) A/cm(2) provide an upper theoretical 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.