Control of Ferromagnetism of Vanadium Oxide Thin Films by Oxidation States

Abstract

Vanadium oxide (VOx) 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 VOx, it is mostly associated with impurities or defects, and pure VOx has rarely been reported as ferromagnetic. The research presents clear evidence of ferromagnetism in the VOx thin films, exhibiting a saturation magnetization of ≈13 kA m−1 at 300 K. The 20-nm thick VOx thin films via reactive sputtering from a metallic vanadium target in various oxygen atmospheres is fabricated. The oxidation states of ferromagnetic VOx films show an ill-defined stoichiometry of V2O3+p, where p = 0.05, 0.23, 0.49, with predominantly disordered microstructures. The ferromagnetic nature of these VOx films is confirmed through a strong antiferromagnetic exchange coupling with the neighboring ferromagnetic layer in the VOx/Co bilayers, in which the spin configurations of the Co layer is influenced strongly due to the additional anisotropy introduced by VOx layer. The present study highlights the potential of VOx as an emerging functional magnetic material with tunability by oxidation states for modern spintronic applications.