Room Temperature Ferromagnetism Of Nonmagnetic Materials Induced By Defects

In decades, many groups found the unexpected room temperature ferromagnetism in various non-magnetic nano-scale materials. These discoveries have being challenged our conventional understanding of ferromagnetism completely whose source is the d or f electrons of metals or metal ions in typical magnetic materials. The observed room temperature ferromagnetism might have direct relationship to defects, valence change of ions, or carriers; most researchers thought it is the defects that can dominate ferromagnetic orders both calculated ly and experimentally. However, there are two questions:firstly, whether ferromagnetism would induced by defects; secondly, if so, what relationship between ferromagnetism and defects is. In order to further solve these questions, we study origin of room temperature ferromagnetism by doping non-magneitc elements; and futher focus our attention on the relationship between surface defects and ferromagnetism.A. Room Temperature Ferromagnetism Is Induced by Defects.1. Defects play an important role in introducing room temperature ferromagnetism.Zr doped ZnO nanoparticles with room temperature ferromagnetism are prepared by sol-gel method and we study the influence of doping concentrations to structural and magnetic properties. The more doping concentrations are in doping samples, the less oxygen vacancies are, the less saturation magnetization is. These results can be well explained by the BMP model. So we get a conclusion that oxygen vacancies play an important role in introducing ferromagnetism for oxides. That is confirmed further by Zn doped ZrO2nanoparticles prepared by the sol-gel method.2. There are a few defects inducing room temperature ferromagnetism.Cu doped ZnO thin films with room temperature ferromagnetism are prepared by sol-gel spin-coating method. Oxygen vacancies have been proved as an important role in introducing ferromagnetism. What’s more, transport properties are studied. Ferromagnetic and transport properties should be induced by differnent oxygen vacancies, there is few evidence of interplay between them. B. Surface Defects is the Main Origin of Room Temperature Ferromagnetism.1. Surface defects play an important role in room temperature ferromagnetism.Cr2O3particles with room temperature ferromagnetism were prepared from thermal decomposition. The bigger lattice distortion is, the more saturation magnetization is. The sample has cores with original antiferromagnetic phase admixed with superparamagnetic shells (owing to the valence increase Cr4+state). Spin-flop transition is tremendously uncommon through the exchange field of the superparamagnetic shells. In this system, surface defects play an important role in room temperature ferromagnetism.2. Surface defects controlled by epitaxy growth is the main origin of room temperature ferromagnetism.PbTiO3heteroepitaxial thin films with room temperature ferromagnetism on (100) SrTiO3substrate were produced by hydrothermal epitaxy. As the growth time increased, the bigger the crystal size is, the better cystal is, the less saturation magnetization is. The hydrothermal epitaxy can get better crystals where most of defects exsit on the surface. These results suggested that the observed ferromagnetism is related to surface defects.3. Controllable surface defects is the main origin of room temperature ferromagnetism.NaCl particles with room temperature ferromagnetism synthesized by breaking for different times. It is found to enhance the strength of the ferromagnetic component with decreasing the samples’ size. Breaking progress does not change the samples’ body, but drastically reduces the size of the samples. These results suggested that the observed ferromagnetism is related to surface defects. The first-principle calculation results show the conclusion:ferromagnetism originates from the long range ferromagnetic interaction between the surface Cl vacancies. The results of NaCl particles synthesized by recrystallization confirms that surface defects controlled is the main origin of room temperature ferromagnetism.