Large magnetoresis\-tance in ferromagnet / super\-con\-ductor La$_{0.7}$Ca$_{0.3}$MnO$_3$ / YBa$_2$Cu$_3$O$_7$ trilayers

We show magnetoresistance in excess of 1000{\%} in trilayers containing highly spin polarized La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ manganite and high Tc superconducting YBa$_{2}$Cu$_{3}$O$_{7}$. This large magnetoresistance is reminiscent of the giant magnetoresistance (GMR) in metallic superlattices but with much larger values, and originates at spin imbalance due to the injection of spin polarized carriers. This result, aside from its fundamental importance, may be of interest for the design of novel spintronic devices based on F/S structures.