The Preparation And Room-Temperature Magnetic Behavior Of TiO₂Based Diluted Magnetic Semiconductors

Ferromagnetism which describes a latest discovered physical phenomenon that a nonmagnetic oxide semiconductor doped with magnetic ions behaves ferromagnetic has attracted a lot of interest rencently. To realize practical application of spintronics devices, intensive attention has been focused on doped with transition metals. However, there are a number of conflicting reports on origin of the magnetism at room temperature in the samples grown under similar conditions. For this reason, the origin of room temperature ferromagnetism (RTFM) in DMSs materials remains a very controversial topic, whether it is due to an intrinsic or an extrinsic effect. The investigation of ferromagnetism materials is singnifcant and essential not only from the point of view of fundamental physics but also for their practical applications. Therefore, an important aspect of a growth of nanostructure based DMSs materials need to be realized of intrinsic RTFM. On the other hand, the origin of the observed ferromagnetic properties is discussed. The details are shown below.1. Undoped TiO2nanoparticles are synthesized by sol-gel method. X-ray diffraction (XRD) analysis reveals that the samples have the pure anatase TiO2structure and the crystallinity becomes better with increase of the annealed temperature. Scanning electron microscopy (SEM) observation shows that the resulted particles are uniformly shaped like spheres. The vibrating sample magnetometer (VSM) measurement shows that the TiO2nanoparticles exhibit weak ferromagnetism, and the saturation magnetization is increased with increasing the annealed temperature. X-ray photoelectron spectra (XPS) analyses confirm that the binding energy of Ti2p peaks shifts to higher energy and the oxygen vacancies are formed in the as-prepared samples. Based on the above investigation, the origin of the ferromagnetism in the TiO2nanoparticles is attributed to the3d orbits of a small amount of tetravalent titanium cations which are occupied by electrons, then tetravalent titanium cations convert into trivalent or divalent titanium cations, indicating that the ferromagnetism of the samples is due to charge transfer interactions between Ti3+and Ti2+cations.2. We prepare Co, Ni doped TiO2nanoparticles by a chemical synthesis route using sol-gel and analyze the microstructural properties, surface morphology and magnetic properties. For lower Co, Ni doping concentration, no precipitations or other phases are detected by XRD and TEM. For higher Co, Ni doped samples, CoTiO3and NiTiO3clusters are found by XRD. Magnetic measurement shows that, when measured at the same applied field, the magnetization is increasing with Co, Ni concentration increasing. When Co, Ni concentration is increased, more Co, Ni ions will take up the interstitial sites and generate paramagnetic phases, which will cause the hole content to be decreased and make the remanent magnetization and coercive force weak.3. Co doped TiO2nanoparticles were grown by sol-gel. All Co-doped TiO2nanoparticles have an anatase structure.3.5mol%doped TiO2exhibit ferromagnetic behavior at room temperature and have a much larger magnetic moment. This study has shown that the substitution of Co for Ti in TiO2could result in a potential diluted magnetic semiconductor. VSM and XPS implied that the room temperature ferromagnetism does not come from Co clusters but from Co doped TiO2nanoparticles.