| As is well-known, a bottleneck is formed in the application of weak ferromagnetism fordilute magnetic semiconductors. Therefore, it is essential to improve the ferromagnetism ofthese materials. Meanwhile, ferromagnetism has been observed in defective undoped oxides,and this phenomenon has received great attention because of potential new applications inspintronics devices. For device applications, however, it is a prerequisite to increase the Curietemperature (TC) well in excess of room temperature. Among oxide systems, TiO2is apromising candidate for potential spintronics applications owing to its prominent opticaltransmission in the visible and near-infrared regions, high n-type carrier mobility and high TC.According to the above information, porous TiO2thin films were deposited by DC-reactivemagnetron sputtering on porous anodic alumina substrates in a home-made system.Morphology of the films was implemented by using a field-emission scanning electronmicroscope (FE-SEM, Hitachi S-4800) and scanning probe microscope (SPM). Thecrystallographic structure was examined using X-ray diffractometer (XRD) with Cu Kradiation at a scan rate of1°/min and2θ scanning from20°to75°. The composition of theporous TiO2thin films with porous anodic alumina (PAA) substrate was analyzed by EDX.The photoluminescence (PL) spectrum was measured using a visible-ultravioletspectrophotometer (Hitachi UV-3010) with a deuterium lamp as the excitation light source atroom temperature. Magnetic measurements were carried out using a physical propertymeasurement system (PPMS-6700) at room-temperature.(1) It can be seen that a highly ordered porous structure was observed by using aFE-SEM and SPM. All diffraction peaks of the prepared sample from the X-ray diffractionpattern confirm the tetragonal anatase phase of TiO2, and not show any impurity phases. Thecorresponding EDS spectrum shows that only Al, O and Ti elements were detected. No othermagnetic impurity elements are detected in the detection limit.(2) Remarkable room-temperature ferromagnetism and magnetic anisotropy ware foundusing a physical properties measurement system (PPMS-6700) with the magnetic fieldperpendicular and parallel to the porous TiO2thin films at room temperature. Thismeasurement indicates that the out-of-plane saturation magnetization is about126emu/cm3.The magnetization depends on special porous structure and decreases rapidly afterair-annealing.(3) The generation of oxygen vacancies in porous TiO2thin films is further confirmed by photoluminescence (PL) measurement using a visible-ultraviolet spectrophotometer (HitachiUV-3010) with a deuterium lamp as the excitation light source at room temperature. Ascompared with the as-grown sample, a broad defect emission in visible region around520-540nm became weaker after air-annealing, which is probably caused by the decrease in theconcentration of the singly ionized oxygen vacancies (F+centers). This trend is similar to thedecrease trend of the saturation magnetization of the annealed samples. It is further confirmedthat the origin of ferromagnetism of non-magnetic oxides is due to the singly ionized oxygenvacancies (F+centers).(4) In addition, TiO2thin films and powder was prepared by other ways, and thesesamples had not showed obvious ferromagnetic at room temperature in the scope of thetesting instrument accuracy. Remarkable room-temperature ferromagnetism in the porousTiO2thin films are associated with the special porous structure and the singly ionized oxygenvacancies.(5) Measurement data suggest that the origin of ferromagnetism may be due to exchangeinteractions between singly ionized oxygen vacancies directly (F+-F+), or by mediation ofconduction electrons (F+-conduction electrons-F+) in the porous TiO2thin films. The resultsreveal that it is a promising candidate for spintronic devices. |
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