Magnetization-dependent T$_{c }$ shift in F/S/F trilayers with strong ferromagnets

Hybrid systems combining ferromagnetic (F) and superconducting (S) metals in contact exhibit a wide range of fascinating behaviors. Several experimental groups have used weak ferromagnetic alloys in F/S experiments to enhance the penetration of Cooper pairs into the ferromagnet. In an F/S/F trilayer structure, a difference in the critical temperature T$_{c}$, based on the mutual orientation of the outer ferromagnets, has been reported [1] in CuNi/Nb/CuNi. Systems with strong ferromagnets, on the other hand, present new challenges, due to the very different density of states and Fermi velocity for the two different spin bands. Using the strong ferromagnets Ni and NiFe (Permalloy) in F/S/F exchange-biased spin valves [2], we observe that the T$_{c}$ for the parallel (P) orientation is lower than that of the anti-parallel (AP) case, i.e. T$_{c}$ (P) $<$ T$_{c}$ (AP). These results are consistent with theoretical expectations, but opposite to recent experimental work from another group where an inverse spin switch has been reported in a NiFe/Nb/NiFe structure [3]. [1] J. Y. Gu et al, Phys. Rev. Lett. 89, 267001 (2002). [2] I. C. Moraru et al., submitted for publication (2005). [3] A. Yu. Rusanov et al., cond-mat/0509156 (2005).