Preparation And Magnetic Properties Of Co-doped PbPdO₂ Film

Half metal and diluted magnetic semiconductor were regarded as the potential candidates of the spintronic materials. However, the spin relaxation length of half metal with a high spin polarization is small, and the spin polarization of diluted magnetic semiconductor with a large spin relaxation length is low. As a new kind of spintronic materials, the spin gapless semiconductor was proposed to have both the high spin polarization and the large spin relaxation length. Now, the PbPdO2-based magnetic semiconductors have been proved to have the spin-gapless band structure theoretically and experimentally. It is necessary to carry out the systematical investigation on the magnetic, electric and transport properties of such kind of materials.In this dissertation, Co-doped PbPdO2 nanograin films were prepared by using the sol-gel spin-coating method. The influences of the calcination temperature, the Co-doping level and the film thickness on the film’s microstructure and magnetic properties were studied in detail. The underlying mechanism for the origin of the high-temperature ferromagnetism of the Co-doped PbPdO2 nanograin film was analyzed. And the methods to achieve the spin-gapless-related features were explored. The main research work and the conclusion are listed as follows:(1) A controllable sol-gel spin-coating method was developed to prepare the Co-doped PbPdO2 nanograin film. The single-phase film was achieved by regulating the calcination temperature. It was found that the ferromagnetism and the paramagnetism coexisted in the film and the ferromagnetism can be retained to the temperature higher than the room temperature. With the temperature increasing, the saturation magnetization exhibited an increasing tendency, which was believed to be related to the spin-gapless band structure. The analysis of the Co K-edge X-ray absorption spectrum proved that Co cluster did not exist in the single-phase Co-doped PbPdO2 film and the film’s room-temperature ferromagnetism was intrinsic.(2) The influence of the Co-doping level on the electric and magnetic properties of PbPd1-xCoxO2 (x=0-0.19) nanograin films was studied. It was found that the films had the insulator-metal transition temperatures higher than 330 K. The temperature dependence of the film’s conductivity indicated that the Co-doped PbPdO2 had a gapless band structure and the increase of the state density at the bottom of the conduction band caused by the impurity energy levels introduced by the Co doping was the physical origin which determined the variation of the electric properties. Moreover, the Co-doping level had the great influence on the film’s magnetic properties. All the PbPd1-xCoxO2 films had the superparamagnetism, the paramagnetism and the high-temperature ferromagnetism which can be kept at 380 K. Increasing the Co-doping level resulted in the weakening of the paramagnetism. For the film’s high-temperature ferromagnetism, two abnormal phenomena about the saturation magnetization can be observed. One is that the saturation magnetization decreased abruptly with the increase of the magnetic field. The other is that the saturation magnetization increased with the temperature increasing. These two facts implied that the films had the spin-gapless-related band structure. The analyses about the X-ray absorption spectra indicated that the increase of the valence state of the Pd ion caused by the existence of the Pb vacancy and the Co ions introduced by the doping were the origins of the magnetism of PbPd1-xCoxO2 films.(3) The dependence of the magnetism of the Co-doped PbPdO2 nanograin film on the film’s thickness was investigated in detail. It was found that the films with different thicknesses had both the paramagentism and the ferromagnetism which can be kept at room temperature or even higher and the ferromagnetism enhanced gradually with the increase of the film’s thickness. In the films with a small thickness, two above-mentioned abnormal phenomena can also be observed. This indicated that the films with a small thickness had the spin-gapless-related band structure. With the film’s thickness increasing, the film’s spin-gapless-related ferromagnetism changed into the regular one gradually. Based on these findings, we propose that the regulation of the film’s thickness is one of the effective methods to achieve the spin-gapless-related features within the PbPdO2-based materials.