Influence of external magnetic field geometry on flux and current distribution in superconducting thin films

Flux density distributions in superconducting thin films exposed to external magnetic fields can be studied very suitable by magneto-optics. In conventionally applied homogeneous magnetic fields, which are oriented perpendicular to the film surface, the influence of the macroscopic sample geometry dominates the flux B-z(x, y) as well as the induced current distribution j(x,y) in the superconductor. We have studied flux and current distribution in different kinds of inhomogeneous field geometries (radial or axial symmetric). In this case flux and current distribution depend on the symmetry of the magnetic field and the ratio of field gradients b(z)(y)/b(z)(x), with b(z)(x) = (partial derivativeB/partial derivativez)(x) and b(z)(y) = (partial derivativeB/partial derivative(z). In radial symmetric fields (b(z)(y)/b(z)(x) approximate to 1) the domination of sample geometry can be observed in analogy to the homogeneous case. The magneto-optically observed current discontinuity lines can be understood within the Bean model and indicate an isotropic critical current distribution j(c)(x, y). This could be confirmed by calculating the current distribution from the magneto-optically detected flux distribution. A strong difference occurs by changing to an axial symmetric field geometry (b(z)(y)/b(z)(x) not equal 1). In this case, the field induced current distribution is strongly anisotropic, even in the fully penetrated state. This could not be explained by the Bean model. As a result the j(c)(x, y)-distribution in an intrinsic inplane-isotropic superconductor can be anisotropically affected by the magnetic surroundings. This should be considered in the conception of high-current HTS-devices. (C) 2004 Elsevier B.V. All rights reserved.