Emergent Ultra-High Temperature Ferromagnetism in La2CoO4±x Thin Films

Searching for novel ferromagnetic oxides with high Curie temperature (TC) has been one of the main goals for oxide spintronics. The well-known perovskite cobaltate LaCoO3 is a classical ferromagnet in its thin-film form; however, it suffers from a low TC (∼ 85 K). Here we report a new type of ferromagnetic La-Co-O films with an ultrahigh TC of ∼ 820 K. They are fabricated by pulsed laser deposition from a LaCoO3 target at low oxygen partial pressures. Detailed structural analysis indicates that they crystallize in terms of the Ruddlesden–Popper phase of La2CoO4±x. In sharp contrast to the antiferromagnetism of bulk La2CoO4, the strong ferromagnetism in the La2CoO4±x thin films is firmly demonstrated by magnetometry measurements, X-ray magnetic circular dichroism characterization, and magnetotransport experiments. More importantly, density functional theory calculations indicate that the nonstoichiometric oxygen induces an antiferromagnetic-to-ferromagnetic phase transition, accompanied by the orbital reconstruction of Co 3d electrons. Thus, our study provides an attractive strategy for designing or synthesizing exotic magnetic oxides with high ordering temperatures.