Magnetism,Exchange Bias Effect In TiO₂/SnO₂ Diluted Magnetic Semiconductors And Ferrite Related Composites

The magnetic coupling properties in diluted magnetic semiconductors (DMS) and limited magnetic systems are one of the foreland fields of spintronics. In which, the study on magnetism and exchange bias effect in oxide-based DMSs and oxide-related composites becomes particularly important, because it lay the foundations of developing practical applications of spintronic devices. In this thesis, our studies focus on the transition metal (Mn, V) doped TiO₂/SnO₂ diluted magnetic semiconductors samples and a granular system of ferrite embedded an antiferromagnetic NiO matrix. The relationship between the microstructure and the magnetic properties is established. The main contents of this thesis can be summarized as follows:1. The Mn-doped TiO₂ polycrystalline bulk samples were synthesized by standard solid state reaction method, and then effects of doping content, sintering temperature and atmosphere on the magnetism have been studied. As for the Ti_1-xMn_xO₂ (x=0.02) samples, the results show that the magnetic properties are strongly dependent on the sintering temperature and atmosphere. For samples sintered in air, the magnetization initially increase with the increase of sintering temperature up to 600oC and thereafter it decrease. While the magnetization of samples sintered in argon atmosphere decreases monotonically with the increase of sintering temperature. Furthermore, for samples sintered at 600oC in air, the magnetic susceptibility exhibits a dominant Curie-Weiss behavior and no magnetic transition is observed over the temperature range from 10K to 300K. In contrast, for samples sintered in argon atmosphere, besides the magnetic transition near 45 K perhaps caused by Mn3O4, another magnetic transition appears near room temperature. Additionally, in the framework of bound magnetic polaron theory, it shows that the existence of large number of oxygen vacancies and magnetic ions at the grain boundaries of Ti_1-xMn_xO₂ particles play critical roles in activation of the ferromagnetism.2. The exchange bias effect in Mn doped TiO₂ bulk samples have been studied. For the Ti_1-xMn_xO₂(x=0.04) samples sintered at 450oC, both horizontal and vertical exchange bias effect is observed when the samples were cooled below 50K under 1T. The observed exchange bias effect can be interpreted considering that the magnetic coupling between ferromagnetic Ti_1-xMn_xO₂ and antiferromagnetic Mn₂O₃ (Ti_1-xMn_xO)Mn₂O₄. For the Ti_1-xMn_xO₂(x=0.04) samples sintered at 600oC, horizontal exchange bias effect is clearly observed after field cooled below 60 K, which is attributed to the exchange coupling between antiferromagnetic Mn₂O₃ with ferrimagnetic Mn3O4 and ferromagnetic Ti_1-xMn_xO₂. This exchange bias behavior also provides strong support for the RTFM in Ti_1-xMn_xO₂.3. Polycrystalline nanoparticles with nominal composition Ti_1-xV_xO₂ (0≦x≦0.16) were synthesized by a sol-gel technique in air, and then the samples were postannealed in argon atmosphere at different temperatures from 500oC to 850oC. The XRD and Raman spectroscopy analysis show that the Vanadium ions can be incorporated into the TiO₂ lattice. Furthermore, the microstructure shows that the samples transform from anatase phase to rutile phase of TiO₂ as the annealing temperature increases to 700oC. Meanwhile, room temperature ferromagnetism is enhanced after postannealing in argon atmosphere and the ferromagnetism increases with annealing temperatures. A plausible explanation for the enhancement of ferromagnetism with annealing temperatures is presented in terms of bound magnetic polaron model.4. The Mn-doped SnO₂ nanoparticles were synthesized by chemical co-precipitation method, the effect of doping contents and annealing temperature on the magnetism has been investigated. The results show that both a correct doping content and appropriate sintering temperature are crucial for the activation of room temperature ferromagnetism. No ferromagnetism is observed for samples sintered at 800oC, irrespective of the doping content. In contrast, the samples sintered at low temperature (Ts=450oC) can exhibit room temperature ferromagnetism when the doping content is below 0.05. Furthermore, the ferromagnetism decreases with the increase of annealing temperature. The results indicate that the ferromagnetism in Sn_1-xMn_xO₂ nanoparticles is highly correlated to the surface structural defects.5. A granular system composed of ferrimagnetic (Ferri) NiFe₂O₄ nanoparticles embedded in an antiferromagnetic (AFM) NiO matrix has been synthesized by a high-temperature phase precipitation method from Fe-doped NiO matrix. Magnetic studies show that, exchange bias effect can be observed below 250 K in this system. For the samples annealed at 600oC, exchange bias field (HEB) can be as large as 3050 Oe and the enhanced coercivity (â–³HC) reach 2150 Oe at 10 K. In addition, the accompanied magnetization shift can be reached 10%. This exchange bias effect can be explained in terms of the existence of frozen spins at the Ferri/AFM interface. For the samples annealed at 750oC, only horizontal exchange bias effect is observed, the corresponding exchange bias field (HEB) is about 260 Oe at 10K.