THERMALLY ACTIVATED FLUX MOTION IN YBA2(CU1-XZNX)(3)O7-DELTA EPITAXIAL THIN-FILMS - INFLUENCE OF MAGNETIC-FIELD AND ZN DOPING

High quality c-axis oriented epitaxial YBa2(Cu1-xZnx)(3)O7-delta (x = 0, 0.02, 0.04) thin films were deposited by DC magnetron sputtering on MgO (100) substrates. The broadening of the resistive transition has been investigated in a magnetic field range from 0.03 to 20 T for H pe rpendicular to c-axis and H parallel to c-axis in order to study the i nfluence of different microstructures on the pinning properties. The d educed field dependence of the activation energy follows a power law: U-0(H) similar to H-alpha with a pronounced anisotropic behaviour. We observe crossovers of the alpha-values from alpha(1) = 0.13 to alpha(2 ) = 0.5 (planar pinning) for H perpendicular to c-axis and from alpha( 1) = 0.3 to alpha(2) = 1 (point defect pinning) for H parallel to c-ax is. For both orientations the alpha-values obtained in the high field region can be explained in the framework of collective pinning. The re sults are discussed taking account of the influence of the Zn substitu tion on the flux pinning in epitaxial YBa2Cu3O7-delta thin films. (C) 1997 Elsevier Science B.V.