Magnetic switching of the superconducting transition temperature in layered ferromagnetic/superconducting hybrids: Spin switch versus stray field effects

The resistance and magnetization behavior of [ferromagnetic (FM)/superconducting (SC)/ferromagnetic (FM)] trilayers of the type (Co/Nb/Fe) was experimentally analyzed and related to their superconducting transition temperature T-c. As opposed to what is expected by proximity effect theory, T-c exhibited a minimum in case of an antiparallel relative orientation of the magnetization of the two sandwiching FM layers. Though this result is consistent with the predictions of spin-imbalance theory, additional experiments on exchange-biased CoO/Co/Nb/Fe systems revealed strong T-c changes, even for a fixed relative magnetization orientation of the FM layers pointing to stray fields, rather than magnetization orientations, as causing the observed T-c shifts. This idea is corroborated by additional measurements on Fe/Nb and Co/Nb bilayers, which clearly show T-c changes related to specific magnetic domain configurations with a T-c maximum in the magnetically saturated state of the FM layer. Complementary micromagnetic simulations, delivering also the magnitude of stray fields, support the picture of influencing T-c by controlling and switching domain configurations.