Zero to π transition in superconductor-ferromagnet-superconductor junctions

We report a systematic study of Nb/ferromagnet (FM)/Nb trilayer structures in which the FM layer is one of the strong ferromagnets Co, Fe, Ni, and Ni80Fe20(Py). Accurate control of the FM layer thickness has enabled detailed studies of the magnetic and transport properties in the superconducting state. In all cases, we estimate the thickness of the magnetic dead layer and the exchange energies of the ferromagnetic layers; in doing so, we demonstrate inconsistencies between the exchange energies derived elsewhere from superconductor (S)/FM bilayer experiments and from S/FM/S junction measurements compared to their bulk Curie temperatures, which may hint at further complexity in the underlying physics. We show results in support of a recent publication [J. W. A Robinson , Phys. Rev. Lett. 97, 177003 (2006)], focus in detail on a single 0-pi phase transition, and show evidence for the appearance of a second harmonic in the current-phase relation at the minimum of the critical current.