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Recently, room temperature ferromagnetism (RTFM) has been reported to be found in many kinds of oxide semiconductor nanoparticles. The origin of the magnetic moment is believed to be defects because RTFM strongly depends on particle size. In this study, we have focused on the antiferromagnetic insulator Cr2O3 due to the interesting core-sell structure (RTFM/antiferromagnetism). Furthermore, RTFM found in oxide nanoparticles is expected to depend on the sample preparation condition. Here, the effect of sample preparation condition on RTFM was investigated. Samples were nanoparticles prepared by milling oxide powders in Ar or air circumstances. Both samples indicated hysteresis at room temperature. At higher temperatures than the Néel temperature (308 K), hysteresis loops were found. A significant difference was observed in the saturation magnetization. The sample prepared in air has four times smaller saturation magnetization in 60 min milling time due to self-compensation of defects with air.

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.