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

Abstract

By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current js and the dynamic magnetic-moment relaxation of YBa2Cu3O7 and YBa2Cu4O8 films of typically 100 nm thickness at temperatures between 2 K and Tc in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q≡d ln js/d ln (dBe/dt) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q≈0.06 in YBa2Cu3O7 and 0.04 in YBa2Cu4O8. At high fields (Be=μ0He≈ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q≊1 is observed when the irreversibility line is approached.

By means of the generalized inversion scheme (GIS), the js(T, Be) and Q (T, Be) data are used to determined the current dependent activation energy U (j, T, Be) for thermally activted flux creep. Although the GIS does not make any a priori assumptions about the explicit functional dependences on T and j, the U(j, T=0, Be) function derived from the experimental data by means of the GIS can remarkably well be described with the collective-creep interpolation formula U(j)=(Uc/μ)[(jc/j)μ-1] with μ≈0.6 for currents j>0.15jc (T=0, Be) where Jc(T=0, Be) is the critical current at T=0, and wi Uc depending on Be. At lower current densities U(j, T=0, Be) does not diverge as j-0.6 but shifts gradually to a weaker ln(jc/j) dependence. At low temperatures the current and relaxation data cannot be explained in terms of a thermally activated flux-motion model. Quantum creep has an influence up to ∼13 K.