The resistance and magnetization behavior of [ferromagnetic (FM)/superconducting (SC)/ferromagnetic (FM)] trilayers of the type $(\mathrm{Co}∕\mathrm{Nb}∕\mathrm{Fe})$ 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 $\mathrm{CoO}∕\mathrm{Co}∕\mathrm{Nb}∕\mathrm{Fe}$ 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 $\mathrm{Fe}∕\mathrm{Nb}$ and $\mathrm{Co}∕\mathrm{Nb}$ 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.

• Received 7 April 2006

DOI:https://doi.org/10.1103/PhysRevB.74.094504