Current-dependent crossover in the flux dynamics of MgB2 thin films

Abstract

We have investigated the dissipative behaviour of MgB2 textured thin films at different perpendicular magnetic fields H and bias currents J, to study activation energy dependencies U(J) and U(H). For J < J*, where J* is a magnetic-field–dependent crossover current, activation energies are weakly current dependent, U(J) ≈ const, while, for J > J*, U(J) ∼ – ln J. At J < J*, the magnetic-energy dependencies are logarithmic, U(H) ∼ – ln H, while for J > J*, U(H) ∼ H–1, up to μ0H ≈ 5 T. The J*(H) curve separates two zones with different vortex dynamics. We discuss the J*(H) curve in terms of a current-driven transition from a thermally activated flux flow to a flux creep regime. For pronounced disorder, the data suggest the influence of the activation energy distribution on the U(J) and U(H) dependencies.