Anomalous double peak structure in superconductor/ferromagnet tunneling density of states.

We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers as a function of Ni thickness, d(F). Our thinnest samples show the usual DOS peak at +/- Delta(0), whereas intermediate-thickness samples have an anomalous "double-peak" structure. For thicker samples (d(F) > or =3.5 nm), we see an inverted DOS, which has previously only been reported in superconductor or weak-ferromagnet structures. We analyze the data using the self-consistent nonlinear Usadel equation and find that we are able to quantitatively fit the features at +/- Delta(0) -- in particular the thickness at which the inversion occurs -- only if we include a large amount of spin-orbit scattering in the model. Interestingly, we are unable to reproduce the subgap structure through the addition of any parameter(s). Therefore, the observed anomalous subgap structure represents new physics beyond that contained in the present Usadel theory.