Field history effect of the flux gradient in superconducting niobium

Abstract

The flux density gradient in a plastically deformed niobium single crystal was measured. The profiles in increasing and decreasing field are not symmetric, and depend on the history of the magnetic field. This effect was observed up to 0.9Hc2, although it was more pronounced at lower fields. These results, as well as an observed asymmetry of minor hysteresis loops, cannot be understood by the critical state model. We suggest that they are related to the creation of dislocations in the flux line lattice.