Critical current, magnetization relaxation and activation energies for YBa2Cu3O7 and YBa2Cu4O8 films

Abstract

The supercurrent js and the dynamical relaxation rate Q ≡ dlnjs/dln(dBe/dt) of YBa2Cu3O7 and YBa2Cu4O8 films of about 100 nm thickness have been measured by means of a torque magnetometer for temperatures up to Tc and fields up to Be = 7 T. From these data the true critical current jc(T, Be) and the activation energy function U(j, T, Be) for thermally activated flux motion have been calculated. For fields smaller than about 2 T, both samples are in the single vortex regime. The obtained U(j, T = 0, Be) dependence is rather well described by the collective pinning interpolation formula.