Spin Valve Effect in all-oxide Superconductor/Ferromagnet Junctions*

Due to their antagonistic properties, the interaction between ferromagnets and superconductors yields interesting behaviours such as the so-called superconducting spin valve effect. This is observed in junctions made of two ferromagnetic layers sandwiching a superconductor. The term spin-valve denotes the electrical resistance change observed upon switching the relative magnetizations in the ferromagnetic layers, from parallel to antiparallel alignment, by the application of magnetic field pulses. Here we demonstrate spin-valve effects in vertical junctions made of the d-wave superconductor YBa₂Cu₃O₇ and a half-metal La_0.7Ca_0.3MnO₃. Below the superconducting critical temperature, the junctions show a hysteretic magnetoresistance whose sign changes depending on the temperature and the injected current. These effects are discussed in terms of triplet Cooper pairs flow when the two ferromagnets are aligned.