Impurity pinning in epitaxial YBa/sub 2/(Cu/sub 1-x/TM/sub x/)/sub 3/O/sub 7- delta / (TM=Zn, Ni)-thin films

The dependence of resistive transitions, thermal activation energies, and critical current densities of YBa/sub 2/(Cu/sub 1-x/Zn/sub x/)/sub x/O/sub 7- delta / and YBa/sub 2/(Cu/sub 1-x/Ni/sub x/)/sub 3/O/sub 7-x/- thin films on B, T, and the angle Theta between the magnetic field and the c-axis is studied. A decrease of the critical temperature T/sub c/ with increasing doping concentration x is observed. For small x an increase of the activation energy is found, with a maximum around 0.2%. This can be explained by a model assuming local pinning potentials due to the impurity atoms, which begin to overlap for higher concentrations. The films exhibit higher j/sub c/ values for small dopant concentrations than without doping. If the angle Theta is changed, a different j/sub c/(B, T, Theta ) behavior is found than in high-quality undoped films, with additional structures in the curves.<>