  We consider a microscopic theory of F/S/F trilayers with metallic or
insulating ferromagnets. The trilayer with metallic ferromagnets is controlled
by the formation of non local pair correlations among the two ferromagnets
which do not exist with insulating ferromagnets. The difference between the
insulating and ferromagnetic models can be understood from lowest order
diagrams. Metallic ferromagnets are controlled by non local pair correlations
and the superconducting gap is larger if the ferromagnetic electrodes have a
parallel spin orientation. Insulating ferromagnets are controlled by pair
breaking and the superconducting gap is smaller if the ferromagnetic electrodes
have a parallel spin orientation. The same behavior is found in the presence of
disorder in the microscopic phase variables and also in the presence of a
partial spin polarization of the ferromagnets. The different behaviors of the
metallic and insulating trilayers may be probed in experiments.
