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 (DOS) by an effective exchange field (H0) from the GdN. The splitting increases linearly with applied field (Hext) enabling the relative sign of H0 and Hext to be determined. We show that the short NbN coherence length concentrates H0 at the NbN/GdN interface and eliminates any averaging over the GdN domain structure leading to a large zero-field splitting.