Control of Ferromagnetism of Vanadium Oxide Thin Films by Oxidation States

Abstract

Vanadium oxide (VO_x) is a material of significant interest due to its metal-insulator transition (MIT) properties as well as its diverse stable antiferromagnetism depending on the valence states of V and O with distinct MIT transitions and Néel temperatures. Although several studies reported ferromagnetism in the VO_x, it is mostly associated with impurities or defects, and pure VO_x has rarely been reported as ferromagnetic. The research presents clear evidence of ferromagnetism in the VO_x thin films, exhibiting a saturation magnetization of ≈13 kA m⁻¹ at 300 K. The 20-nm thick VO_x thin films via reactive sputtering from a metallic vanadium target in various oxygen atmospheres is fabricated. The oxidation states of ferromagnetic VO_x films show an ill-defined stoichiometry of V₂O_3+p, where p = 0.05, 0.23, 0.49, with predominantly disordered microstructures. The ferromagnetic nature of these VO_x films is confirmed through a strong antiferromagnetic exchange coupling with the neighboring ferromagnetic layer in the VO_x/Co bilayers, in which the spin configurations of the Co layer is influenced strongly due to the additional anisotropy introduced by VO_x layer. The present study highlights the potential of VO_x as an emerging functional magnetic material with tunability by oxidation states for modern spintronic applications.