Effect of flux line lattice history on the critical current in niobium

Abstract

The critical current I_c in a plastically deformed Nb single crystal is shown to vary with the method of formation of the flux line lattice (FLL). The largest critical currents are observed when the FLL is formed by cooling the sample from above T_c(H). Passing a current through the sample during the cooling process, in the same direction as it is subsequently swept, increases I_c by up to 60%. By slowly cycling the current at zero voltage, decreases in I_c can be induced with the V‐I characteristic asymptotically approaching the characteristic after flux flow. The size of the decrease is found to depend on the number of cycles, and especially for negative current excursions, on the amplitude of these excursions. It is proposed that optimum pinning of the FLL to the underlying pin structure occurs, for thermodynamic reasons, at the moment the FLL is formed. Subsequent changes in the local value of the FLL parameter, either due to changes in magnetic induction or changes in FLL defect structure, lead to decreases in the number of effective pins, and thus to decreases in the critical current.