Room-temperature ferromagnetism in metal–insulator transition nanoparticles of VO₂

There have been many reports on room-temperature ferromagnetism (RTFM), which is often observed in pure semiconductor nanoparticles without transition metal elements. Because RTFM tends to be found in semiconducting thin films or nanoparticles, the origin of ferromagnetism is believed to result from the specific surface structure or defects on the surface. However, there is no direct evidence for such surface phenomena, and the relationship between RTFM and physical properties reported in the bulk state must be investigated. If RTFM is independent of the bulk properties, then it is concluded to be a surface phenomenon. In this study, VO2 nanoparticles with metal–insulator transitions (MITs) are evaluated. Magnetic field dependences of the magnetization of VO2 nanoparticles confirmed with MITs indicate a small amount of hysteresis before and after the transition temperature. This indicates that RTFM is independent of the particle's core properties, and ferromagnetic order is concluded to occur only on the surface.