Evidence For A Pi Junction In Nb/Ni0.96v0.04/Nb Trilayers Revealed By Superfluid Density Measurements

We report measurements of the superfluid density, lambda(-2)(T), in ferromagnet-on-superconductor (F/S) bilayers and S/F/S' trilayers comprising Nb with Ni, Py, CoFe, and NiV ferromagnets. Bilayers provide information about F/S interface transparency and the T dependence of lambda(-2) that inform interpretation of trilayer data. The Houzet-Meyer theory accounts well for the measured dependence of lambda(-2)(0) and T-c of F/S bilayers on thickness of F layer, d(F), except that lambda(-2)(0) is slightly under expectations for CoFe/Nb bilayers. For Nb/F/Nb' trilayers, we are able to extract T-c and and lambda(-2) for both Nb layers when F is thick enough to weaken interlayer coupling. The lower "T-c " is actually a crossover identified by onset of superfluid in the lower-T-c Nb layer. For Nb/NiV/Nb' trilayers, lambda(-2)(0) versus d(F) for both Nb layers has a minimum followed by a recovery, suggestive of a pi junction.