Emergent ultra-high temperature ferromagnetism in La₂CoO_4±x thin films

Searching for novel ferromagnetic oxides with high Curie temperature (T_C) 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 T_C (~ 85 K). Here we report a new type of ferromagnetic La-Co-O films with an ultrahigh T_C 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 La₂CoO_4±x. In sharp contrast to the antiferromagnetism of bulk La₂CoO₄, the strong ferromagnetism in the La₂CoO_4±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.