Magnetic coupling in superposed type-II superconducting films

A study has been made of the magnetic-coupling properties of vortex arrays in a dc superconducting transformer composed of two oxygen-doped aluminum films separated by a thin insulating layer. By doping to the appropriate level, very small depinning currents have been achieved, making possible an investigation of the vortex-coupling characteristics in a low-current regime free of heating instabilities. Measurements of the system's $V-I$ characteristics have been obtained at currents more than an order of magnitude greater than the current at which the two vortex lattices cease to move at the same velocity. A simple periodic-coupling-force model has been used to generate theoretical $V-I$ curves which are found to compare well with the experimental data. Values of the magnetic interaction force coupling the two vortex arrays have been determined as a function of temperature and intervortex spacing, and are compared with theory.