Large interfacial exchange fields in a thick superconducting film coupled to a spin-filter tunnel barrier

The differential conductance of NbN/GdN/TiN superconductor/ferromagnetic insulator/normal-metal junctions, with a thick NbN layer shows a large zero-field voltage offset interpreted as a spin-filtered Zeeman splitting of the NbN density of states by an effective exchange field (H-0) from the GdN. The splitting increases linearly, with applied field (H-ext) enabling the relative sign of H-0 and H-ext to be determined. We show that the short NbN coherence length concentrates H-0 at the NbN/GdN interface and eliminates any averaging over the GdN domain structure leading to a large zero-field splitting.