Oscillatory behavior of the magnetoresistance of F/S/F trilayers

We have measured magnetoresistance of trilayers made of highly spin polarized La$_{0.7}$Ca$_{0.3}$MnO$_3$\ ferromagnets and high-T$_c$\ superconducting YBa$_2$Cu$_3$O$_{7-\delta}$\ with the current in plane (CIP) geometry. Large magnetoresistance peaks occur for antiferromagnetic (AF) alignment of the manganite layers, much resembling the GMR in metallic superlattices, and probably resulting of enhanced interface scattering in the AF configuration. We show RKKY-type oscillations in the intensity of the giant magnetoresistance (GMR) peak as a function of the thickness of the superconductor, perhaps originating in oscillatory magnetic coupling. The magnetoresistance vanishes for thickness of the superconductor in excess of 30 nm suggesting that this length could be an upper limit for the spin diffusion length.