Activation energy in La0.7Ca0.3MnO3/YBa2Cu3O7-delta/La0.7Ca0.3MnO3 superconducting trilayers

Resistivity vs. temperature measurements on La0.7Ca0.3MnO3/YBa2Cu3O7-delta /La0.7Ca0.3MnO3 (LCMO/YBCO/LCMO) trilayers with different YBCO thickness, were performed in external magnetic field H up to 8 T. By evaluating the activation energy U from the slope of the resistivity Arrhenius plot, a strong depression of U has been observed when decreasing the YBCO layer thickness and the absolute U values appear to be reduced with respect to the values reported in literature in the case of YBCO thin films and YBCO/insulating multilayers. Moreover, a logarithmic U vs. H dependence is shown both in the case of thick and thin YBCO layers indicating the formation of a two dimensional vortex lattice. The experimental data are discussed considering the strong influence of the ferromagnetic LCMO on the superconducting YBCO properties which reduces the effective YBCO thickness more than predicted by the conventional theories