Magnetic relaxation over the Bean-Livingston surface barrier.

Abstract

Magnetic relaxation over the Bean-Livingston surface barrier is considered in high-temperature superconductors at fields H>${\mathit{H}}_{\mathit{p}}$ (where ${\mathit{H}}_{\mathit{p}}$>${\mathit{H}}_{\mathit{c}1}$ is the first field for flux penetration) using the Clem model for a critical state with a surface barrier. The relaxation rates for vortex entry and exit are expressed through the basic thermodynamic characteristics of a superconductor. For the flux exit the magnetization M(t) depends logarithmically on time whereas for the case of entry M(lnt) appears to be a strongly nonlinear function with downward curvature, as has been found experimentally. The initial relaxation rate, dM/d lnt, proves to be much larger for flux entry than for exit, in contrast to the case of conventional bulk creep. The competing interplay between this surface relaxation and the usual bulk one, which results in a crossover in the M(lnt) curves, is discussed.