Hidden oxygen reshapes spin selectivity at a spintronic interface
A single layer of oxygen can reshape the spin-dependent electronic response and orbital character at an MgO/Fe interface. This is important because the MgO/Fe stack is a key building block of magnetic tunnel junctions, the nanoscale structures used in spintronic devices such as magnetic memories and sensors. In these junctions, electrons tunnel from the ferromagnetic Fe electrode through crystalline MgO. Because MgO allows some Fe electronic states to tunnel more efficiently than others, the resulting current becomes strongly spin selective. The efficiency of this filtering depends sensitively on the buried Fe/MgO interface, where the electronic structures of the metal and oxide meet. Yet oxygen at this hidden boundary has long been difficult to control, and even harder to detect once buried beneath MgO.
In this work, we turn this hidden oxygen from an uncontrolled complication into a measurable and tunable property of the interface. By carefully adjusting the oxygen supply during growth, we reproducibly prepare three characteristic interface states: oxygen-free, partially oxidized, and fully oxygen-intercalated. Importantly, the MgO films remain crystalline in all three cases, allowing us to reveal how the buried interface affects the electronic properties of the system.
The first clue comes from momentum microscopy, which maps the electronic band structure in momentum space. Measurements of the bare Fe(100) substrate reveal distinct features in momentum space that are sensitive to adsorbed oxygen (Fig. 1a). After MgO growth, these fingerprints evolve systematically as the oxygen supply is increased (Fig. 1b–d), allowing the state of the buried interface to be identified. Separate thickness-dependent measurements show that these characteristic interface signatures persist and remain detectable even through MgO films up to eight monolayers thick. The electronic signature of the hidden interface can therefore still be read through an insulating oxide barrier.
Figure 1: Momentum microscopy identifies oxygen-sensitive fingerprints at the MgO/Fe interface. (a) Reference maps of clean and oxygen-covered Fe(100) reveal characteristic features associated with interfacial oxygen. (b–d) After MgO growth, these fingerprints evolve systematically as the oxygen supply is increased, allowing the buried interface to be identified.
The key spectroscopic measurements were carried out at the NanoESCA beamline of Elettra, where momentum-resolved and spin-resolved photoemission provide direct access to the spin-dependent electronic structure. These measurements were complemented by electron diffraction, X-ray and Auger spectroscopy, and scanning tunnelling microscopy, linking the electronic fingerprints to the structure and chemistry of the interface.
The spin-resolved measurements reveal why controlling interfacial oxidation at the atomic scale matters. At the oxygen-free MgO/Fe interface (Fig. 2a), the minority-spin spectral weight near the Fermi level is strongly suppressed, consistent with the spin-filtering action expected for crystalline MgO. When a complete oxygen layer is formed at the interface (Fig. 2b), this spin contrast is substantially reduced. The measurements therefore directly demonstrate how a single buried atomic layer modifies the spin-dependent electronic structure relevant for electron tunnelling.
Figure 2: Spin-resolved momentum maps show how interfacial oxygen modifies spin filtering. (a) The oxygen-free MgO/Fe interface shows strong spin contrast. (b) Introducing a complete interfacial oxygen layer substantially reduces this contrast.
These results establish two important advances. First, they provide a practical route for preparing MgO/Fe interfaces with well-defined oxygen content. Second, they show that momentum microscopy can access electronic and spin-dependent information from interfaces buried beneath ultrathin insulating films. More broadly, the work established interfacial oxidation as a controllable design parameter for oxide–ferromagnet junctions, paving the way for more reproducible and precisely tunable spintronic heterostructures.
This research was conducted by the following research team:
David Maximilian Janas1, Mira Sophie Arndt1, Jonah Elias Nitschke1, Lasse Sternemann¹, Valentin Mischke1, Vitaliy Feyer2, Iulia Cojocariu2, Daniel Baranowski2, Alessandro Sala3, Andreas Windischbacher4, Peter Puschnig4, Jan Dreiser5, Stefano Ponzoni1, Giovanni Zamborlini1,4 and Mirko Cinchetti1
1 TU Dortmund University, Department of Physics, Dortmund, Germany
2 Peter Grünberg Institute (PGI-6), Forschungszentrum Jülich GmbH, Jülich, Germany
3 CNR – Istituto Officina dei Materiali (IOM), Basovizza, Trieste, Italy
4 Karl-Franzens-Universität Graz, Institut für Physik, NAWI Graz, Graz, Austria
5 Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland
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Reference
D. M. Janas et al., "Spin-Selective Interface Engineering in Oxide–Ferromagnetic Junctions via Atomic-Scale Oxygen Control", Advanced Science 13, e23165 (2026); DOI: 10.1002/advs.202523165 .
