Flux pinning and magnetic relaxation in Ga-doped LiFeAs single crystals

Abstract

The effect of nonmagnetic Ga3+ impurities on structural and superconducting properties of LiFeAs single crystals has been studied. The out-of-plane magnetization of the doped material exhibits a remarkable double-peak structure. The presence of a low-field peak observed both in doped and undoped LiFeAs is suggestive of intrinsic structural defects, while a secondary high-field fishtail maximum, which shifts progressively with temperature, is associated with the extrinsic pinning centers created by Ga. The superconducting transition temperature of Ga doped LiFeAs is suppressed by about 4.8 K/at. %. However, a set of superconducting parameters including the superconducting transition temperature, the coherence length, the upper critical field, and the irreversibility field are not significantly reduced in a sample doped with 0.5 at. % Ga. At this Ga concentration, the critical current density of doped LiFeAs is about four times that of our undoped material in intermediate magnetic fields at 5 K. The scaling of the normalized pinning force density Fp = JcB vs the applied field normalized by Bmax (Bmax denotes the peak position of Fpmax) in the temperature range 5–13 K indicates a single type of predominant flux-pinning mechanism provided by Ga additions. Analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning model. The magnetic relaxation measurements combined with the peak position of the critical current density in the B-T phase diagram suggests an elastic–plastic transition of the vortex lattice at higher temperatures and fields. The observed vortex behavior of Ga doped LiFeAs strongly resembles that of YBa2Cu3O7 doped with nonmagnetic impurities.