Ferromagnetic Order from p-Electrons in Rubidium Oxide

Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO2). The anion vacancies in the resulting phase with composition RbO1.72 provide greater structural flexibility compared to RbO2 and facilitate a Jahn−Teller-driven order−disorder transition involving the anion orientations at ∼230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below ∼50 K. A ferromagnetic cluster-glass state then forms below ∼20 K, embedded in an antiferromagnetic matrix that orders at ∼5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides.