Spin-Selective Interface Engineering in Oxide-Ferromagnetic Junctions via Atomic-Scale Oxygen Control

Atomic-scale control of oxide-ferromagnet interfaces is crucial for optimizing spintronic heterostructures, yet interfacial oxygen remains difficult to control and verify. Here, we deterministically tune the prototypical MgO/Fe(100) interface from oxygen-free terminations to fully intercalated oxygen layers by reactive growth under controlled O₂ exposure, while preserving epitaxy. Momentum-resolved photoemission identifies oxygen-dependent fingerprints in k-space that originate from the buried interface and persist up to a thickness of 8 layers of MgO. Insights from complementary spectroscopic methods link these k-space signatures to interfacial chemistry, structural order, work-function shifts, and an oxygen-induced interface resonance within the MgO gap that alters the tunneling response. The combined results define a calibrated growth protocol that allows reproducibly preparing and identifying three distinct terminations - oxygen-free, partially oxidized, and oxygen-intercalated - and enables post-growth conversion even in thicker films. Complementary spin-resolved experiments reveal that oxygen-free interfaces exhibit pronounced suppression of minority-spin spectral weight at the Fermi level, consistent with coherent spin filtering across crystalline MgO, whereas oxygen intercalation reduces the spin contrast at E_F. By turning interfacial oxygen from an uncontrolled variable into a measurable, adjustable parameter, our approach establishes MgO/Fe(100) as a benchmark platform for optimizing spintronic functionality in oxide/metal junctions.