Orthorhombic Ti₂O₃: A Polymorph-Dependent Narrow-Bandgap Ferromagnetic Oxide

Magnetic semiconductors are highly sought in spintronics, which allow not only the control of charge carriers like in traditional electronics, but also the control of spin states. However, almost all known magnetic semiconductors are featured with bandgaps larger than 1 eV, which limits their applications in long-wavelength regimes. In this work, the discovery of orthorhombic-structured Ti₂O₃ films is reported as a unique narrow-bandgap (≈0.1 eV) ferromagnetic oxide semiconductor. In contrast, the well-known corundum-structured Ti₂O₃ polymorph has an antiferromagnetic ground state. This comprehensive study on epitaxial Ti₂O₃ thin films reveals strong correlations between structure, electrical, and magnetic properties. The new orthorhombic Ti₂O₃ polymorph is found to be n-type with a very high electron concentration, while the bulk-type trigonal-structured Ti₂O₃ is p-type. More interestingly, in contrast to the antiferromagnetic ground state of trigonal bulk Ti₂O₃, unexpected ferromagnetism with a transition temperature well above room temperature is observed in the orthorhombic Ti₂O₃, which is confirmed by X-ray magnetic circular dichroism measurements. Using first-principles calculations, the ferromagnetism is attributed to a particular type of oxygen vacancies in the orthorhombic Ti₂O₃. The room-temperature ferromagnetism observed in orthorhombic-structured Ti₂O₃, demonstrates a new route toward controlling magnetism in epitaxial oxide films through selective stabilization of polymorph phases.