| One of the most intriguing discoveries in the quest for oxide diluted magnetic semiconductors is the observation of room- temperature ferromagnetism in un-doped wide band-gap semiconductors. Although there are many different opinions about the origin of the room-temperature ferromagnetism, the influence of oxygen vacancy on this semiconductor is acknowledged. In the last decades, the oxygen vacancy has great influence on the the physical properties of the metal oxide, such as ferroelectricity, ferromagnetism and resistance switching behavior, which has received considerable attention. However, the room-temperature ferromagnetism is very weak. Based on the solid state physics knowledge, the oxygen vacancy easily exists on the surface of solid material. Therefore, if we want to improve the ferromagnetilsm of metal oxide, we need to find the material with large surface area. There is a larger surface-area-to-volume ratio in porous anodic alumina(PAA). Consequently, we prepared PAA thin film with different pore diameter using the different electrolyte by the two step anodization. Then we research on surface morphology, ferroelectricity, ferromagnetism and resistance switching behavior of PAA film. In order to study the properties of other porous metal oxides, pure porous Zn O, Cu2 O, Ti O2 and Ag-doped Zn O films were prepared by direct current reactive magnetron sputtering on porous anodic alumina substrates, and we discuss the physical properties. These porous metal oxides are characterized by field-emission scanning electron microscope(FE-SEM), x-ray energy dispersive spectroscopy(EDS), scanning probe microscope(SPM), ultraviolet-visible spectrophotometer(UV-vis), X-ray diffractometer(XRD), physical property measurement system(PPMS) and so on.(1) PAA thin films with different pore diameter are prepared using the different electrolyte by the two step anodization. We observe that the three PAA films all show weak ferromagnetism, accompanied by large magnetic anisotropy. The magnetization can be controlled by annealing conditions, which suggested that the ferromagnetism is attributed to the oxygen vacancies with one electron. For the ferroelectricity of PAA films prepared in oxalic acid, the films exhibit well-defined hysteresis loops. The reason is that the oxygen vacancy is a positive center, which easily trapped electrons. The oxygen vacancy with positively charge repulses the metal ions, and attracts anions. This phenomenon creates electric dipole moment, resulting in the ferroelectricity. For the I-V measurement, there is stable bipolar resistive switching behavior in PAA films prepared in oxalic acid. Due to a large number of oxygen vacancies, it is easy to form a conductive path along the inner wall of hole. Therefore, there are stable HRS and LRS in PAA thin film. The results of ferromagnetism show the magnetization in the HRS is larger than that in the LRS. The probability that the electrons are trapped by the oxygen vacancy in different resistance state is different. The electrons are easy to be trapped in HRS, andthe oxygen vacancy trapped one electron has a localized magnetic moment. As a result, it is further reveald the conclusion that the ferromagnetism related with the o xygen vacancy trapped an electron is correct. The magnetization of the PAA films can be manipulated not by the annealing conditions but applying the small electric field.(2) Pure porous Zn O films were prepared by direct current reactive magnetron sputtering on porous anodic alumina substrates. Remarkably large room-temperature ferromagnetism was observed in the films, accompanied by large magnetic anisotropy. We observe that the longer the sputtering time, the smaller the pore diameter. The saturation magnetization decreased rapidly as the sputtering time increased in the out-of-plane. Compared to other films, there is the largest surface/volume ratio in the porous film for 5 min, since the pore diameters are larger than in films produced with longer sputtering times, that is the defect concentration decreases, which can affect the magnetization. Strong evidence has been presented showing the significant ferromagnetism is attributed to oxygen vacancies with magnetic moments(OVg), which is consistent with theoretical results. Ag/Zn O-PAA/Al device has stable bipolar resistance switching behavior by I-V characterization. Oxygen annealing and vacuum annealing I-V curves of Ag/Zn O-PAA/Al devices are significantly different, so the conductive mechanisms are different. The conductive mechanisms of as-prepared device can be explained by the Schottky barrier model.(3) Porous C u2 O films have been fabricated on porous anodic alumina substrates using DC-reactive magnetron sputtering with pure Cu targets, and unexpectedly large room temperature ferromagnetism has been observed in the films. The maximum saturation magnetic moment along the out-of-plane direction was as high as 94 emu/cm3. Photoluminescence spectra show that the ferromagnetism originates with oxygen vacancies. The ferromagnetism could be adjusted by changing the concentration of oxygen vacancies through annealing in an oxygen atmosphere. These observations suggest that the origin of the ferromagnetism is due to coupling between oxygen vacancies with local magnetic moments in the porous C u2 O films, which is similar to the PAA and Zn O films.(4) Pure porous Ti O2 films were prepared by direct current reactive magnetron sputtering on porous anodic alumina substrates. In order to compare the difference of porous and dense films, Fe doped Ti O2 thin film is prepared on glass substrate by DC-reactive magnetron sputtering. Based on the result of magnetization, magnetization of porous film is about 10 times than that of dense films, which further proved that the room temperature ferromagnetism of porous film is intrinsic, not caused by magnetic pollution.(5) Porous Ag-doped Zn O films were prepared by direct current reactive magnetron sputtering on porous anodic alumina substrates. The main purpose is to research the influence of non-magnetic doping in porous film on the room temperature ferromagnetism. Compared with pure Zn O film, the ferromagnetism of Ag-doped Zn O film weakened. The reason is that there is Ag vacancy after Ag doping Zn O film, while Ag vacancy decreased the ferromagnetism. Therefore, the ferromagnetism of porous metal oxides is attributed to the oxygen vacancy, especially the oxygen vacancy trapped one electron.Based on the research on physical properties of ordered porous metal o xides, we can conclude that there are a lot of oxygen vacancies in porous metal oxide and the oxygen vacancy is the main factor that affects the physical properties of porous metal oxides. |
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