Hidden non-relativistic spin splitting in layered Ruddlesden–Popper compounds

Altermagnets have emerged as a new class of magnetic materials that host non-relativistic spin split bands in spite of their fully compensated antiferromagnetic order. They combine the advantages of ferromagnets with those of conventional antiferromagnets, thus offering great potential for high-frequency, high-density spintronics applications. Neverthless, the stringent symmetry requirements of altermagnetism limits the materials supporting this physics to a relatively small set of possibilities. In this work, we theoretically demonstrate that altermagnetic states are hidden in a well-known family of layered compounds, i.e., Ruddlesden-Popper oxides. We show that the spin splittings exist in individual layers, but are masked in bulk or in slabs with even numbers of pervoskite layers, due to symmetry relations among the layers. We further demonstrate that the hidden altermagnetism in even layer number slabs can be made apparent by applying an electric field, breaking inversion symmetry. Moreover, the altermagnetic properties in these systems can be engineered by tuning the oxygen stoichiometry, where equatorial oxygen vacancies enhance spin splittings and apical oxygen vacancies lead to an insulator-to-metal transition. Our findings therefore not only broadens materials platforms by uncovering the emergent altermagnetic states in a well-known family of materials, but also provides strategies to engineer altermagnetic properties for spintronic applications.