Defects’ Effects On The Magnetism Of Some Transition Metal Oxides

Transition metal oxides are kinds of the most important magnetic materials which are applied in storages, sensors, spinvalues and et al. Antiferromagnetic materials are one of the three ordered magnetic materials and are crucial in giant magnetoresistance devices and tunnel magnetoresistance devices, which make the studies of the properties of antiferromagnetism important and valuable both in fundamental physics and in applications. People have made a good effort in doping and nanorization of materials, which are promised to have potential applications in the future. In the present thesis, we focused on the defects’effect on the magnetism of some transition metal oxides, which will be discussed below.1. Magnetism of doped bulk CuO.We synthesized 4% Mg, Mn, Ni and Zn doped CuO ceramics by traditional ceramic process. M-H and M-T results show ferromagnetism of Mn doped CuO samples below its Curie temperature 80 K; the ferromagnetism is assigned to double exchange interaction between magnetic cations. Mg doped CuO samples show weak ferromagnetism at 5 K, which may come from orbital defects. Ni and Zn doped CuO samples are antiferromagnetic below CuO’s Neel temperature, the only effect is the variation of magnetizations.2. The synthesis and magnetism of Mn doped CuO nanotubes and nanoparticlesWe synthesized 3% Mn doped CuO nanotubes using AAO. The nanotubes are 100 nm in diameter and micrometers in length. The thickness of the tube wall is about 10 nm. M-H result shows ferromagnetism of the nanotubes at 5 K. We synthesized 3%, 6%,9%,12% and 15% Mn doped CuO nanoparticles during hydrothermal process. The particle size is about 100 nm. Only 15% Mn doped sample show ferromagnetism at 5 K, which may be due to the size effect.3. The synthesis and magnetism of Mn doped CuO nanowiresWe synthesized Mn doped CuO nanowires using Cu-Mn alloy slices by thermal evaporation. The nanowires are less than 100 nm in diameter and more than 10 μm in length. Clear lattice fringes indicated the high crystallinity of the nanowire. M-H and M-T results show that there are both Mn3O4 and Mn-CuO nanowires in the samples and Mn-CuO nanowires show ferromagnetism below 80 K.4. The synthesis of Ni-Co-O solid solutions and the studies of variations of lattice constants and Neel temperatureWe synthesized Ni-Co-O solid solutions by traditional ceramic process. XRD and refinement results show that the lattice constant depends linearly on the Ni concentration. M-T results show the antiferromagnetic nature of all the samples and the Neel temperature of the samples increases with the increasing Ni concentration.5. The synthesis and magnetism of Al-Cr-O solid solutionsWe synthesized Al-Cr-O solid solutions by traditional ceramic process. XRD and refinement results show that the lattice constants depends linearly on the concentration of Al atoms. M-T results show the antiferromagnetic nature of the samples with Al concentration less than 60% and the Neel temperature decreases with increasing Al concentration. Weak ferromagnetism was observed in the samples with Al concentration more than 70%, which may be due to the Cr-Cr interactions or Cr-defects interactions.6.The synthesis, characterization, growth mechanism and magnetism of single-crystalline Cr2O3 nanowires/nanobeltsWe synthesized single crystalline Cr2O3 nanowires/nanobelts using Cr foils with the assistant of CrCl3 in a short time. SEM results show that the nanostructures are several tens to hundred nms in width and several tens μms in length. HRTEM results show high crystallinity of the nanostructures and we can find the growth is along [001]. The rapid corrosion of Cr foil in the atmosphere containing both O and Cl is found to be crucial to the growth. M-T results show the antiferromagnetic nature of the samples. A decrease of Neel temperature of the nanostructures was observed, which is due to the surface effect of the nanowires/nanobelts.7. The synthesis and exchange bias effect of Al doped G2O3/CO granular systemsWe synthesized Al doped O2O3/CO granular systems in one step using sol-gel method combining autocombustion. All the samples possess porous structures. Both of the coercivities and exchange bias fields decrease with the increasing temperature. 10%Al doped Cr2O3/Co sample has both the highest coercivities and exchange bias fields at all the temperatures, which can be understood in domain state models.