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 andbiascurrentsJ, to study activation energy dependencies U(J)andU(H). For J<J∗,whereJ∗ is a magnetic-field–dependent crossover current, activation energies are weakly current dependent, U(J) ≈ const, while, for J>J∗, U(J) ∼−lnJ.AtJ<J∗, the magnetic-energy dependencies are logarithmic, U(H) ∼−lnH, while for J>J∗, U(H) ∼ H−1,uptoµ0H ≈ 5T. TheJ∗(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)andU(H) dependencies.