Title: Competitive coexistence of ferromagnetism and metal--insulator transition of VO$_2$ nanoparticles

Abstract: We investigated the magnetic and electric properties of nanometer-sized vanadium dioxide (VO$_2$) particles. VO$_2$ nanoparticles were formed by milling VO$_2$ powder. We measured the magnetic field dependence of the magnetization of the VO$_2$ powder and nanoparticles. The VO$_2$ powder did not exhibit ferromagnetism, whereas the VO$_2$ nanoparticles exhibited ferromagnetism. In addition, we fabricated samples by bridging between electrodes with the VO$_2$ nanoparticles, and the temperature dependence of their resistance was measured. Metal-insulator transitions (MITs) were observed, and the temperature range where the MIT occurred was wider than that in a typical bulk VO$_2$. The VO$_2$ nanoparticles exhibited these properties of ferromagnetism and MIT possibly because of the surface and size effects of the VO$_2$ nanoparticles. These results indicate the first observation of the competitive coexistence of ferromagnetism and MIT of VO$_2$ nanoparticles.