Magnetization-induced resistance-switching effects in La_0.67 Sr_0.33MnO₃/YBa₂Cu₃O_7-δ bi- and trilayers

We have studied the influence of the magnetization on the superconducting transition temperature (Tc) in bi- and trilayers consisting of the half-metallic ferromagnet La_0.67Sr_0.33MnO₃ and the high-temperature superconductor YBa₂Cu₃O_7-δ (YBCO). We have made use of tilted epitaxial growth in order to achieve contacts between the two materials that are partly in the crystallographic ab plane of the YBCO. As a result of uniaxial magnetic anisotropy in the tilted structures, we observe sharp magnetization-switching behavior. At temperatures close to T_c, the magnetization-switching induces resistance jumps in trilayers, resulting in a magnetization dependence of T_c. In bilayers, this switching effect can be observed as well, provided that the interface to the ferromagnetic layer is considerably rough. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor. A simple model describes the observed behavior well. We find no evidence that the switching behavior is caused by a so-called superconducting spin switch, nor by accumulation of spin-polarized electrons. Observation of magnetic coupling of the ferromagnetic layers, through the superconductor, supports the idea of field-induced resistance switching.