Magnetotransport properties of La_2/3Ca_1/3MnO₃/YBa₂Cu₄O₈/La_2/3Ca_1/3MnO₃ sandwiches

La_2/3Ca_1/3MnO₃(LCMO)/YBa₂Cu₄O₈(YBCO)/LCMO sandwiches are prepared by the facing-target sputtering technique. The superconducting transition temperature as a function of spacer layer thickness is described by the magnetic proximity effect in the superconductor/ferromagnet structure. The metal—semiconductor transition temperature exhibits a nonmonotonic behaviour which is strongly dependent upon the YBCO layer thickness, and is also shown from the measurement of resistance and magnetoresistance. These results imply that the magnetic spin interaction between the two LCMO layers may occur through the normal-conductive YBCO layer.