Cobalt-based Nanomaterials: Controlled Synthesis, Microstructure And Their Magnetism

With the continuous miniaturization of the basic unit of spintronics devices, the observation and interpretation of their physical properties when their geometrical size is close to the physical limit（carrier mean free path） has become an urgent task. Magnetic particle systems existing exchange bias and with the size of ferromagnetic nanoparticles close to or below the critical size for superparamagnetism provide an ideal experimental model for the exploration of the novel physical behaviors. This thesis focuses on the controlled preparations and comprehensive characterization of cobalt-based magnetic nanoparticles systems, including the exchange bias of anti-ferromagnetic materials and ferromagnetic nanoparticles with size below the superparamagnetic critical size and the influences of exchange bias on the magnetism of them. The difference between the exchange bias in the ferromagnetic/antiferromagnetic nanoparticles with the size of the ferromagnetic nanoparticles smaller than the superparamagnetic critical size and that of ferromagnetic nanoparticle size bigger than the superparamagnetic critical size is further explored. Meanwhile, the influences of microstructures of the ferromagnetic/antiferromagnetic interface structure on the exchange bias are also studied. This work is believed to contribute not only several smart functional 3D building blocks of low-dimensional magnetic structures, but also useful experimental and theoretical knowledge for developing spintronic devices, high-density magnetic recording media, high-performance permanent magnetic materials, flux amplifiers of magnetic resonance imaging. The main results of this thesis are as follows:（1） The influences of exchange bias caused by oxygen vacancies on the magnetic properties of branched c-Co O nanoparticles. In this study, branched nanostructure c-Co O nanoparticles with different morphological size and concentration of oxygen vacancies were synthesized by high temperature organic liquid method. Branched nanostructure c-Co O nanoparticles have a clear ferromagnetic behavior at room temperature and exchange bias at low temperature. The room temperature ferromagnetism should originate from oxygen vacancies and be determined by the concentration of oxygen vacancies: the exchange bias field increases with the increasing of the concentration of oxygen vacancies, resulting in a stronger ferromagnetism. The concentration of oxygen vacancies can cause the transition between the negative and positive exchange bias.（2） The exchange bias of h-Co O nanotetrapods randomly doped with h-Co nanoparticles with size smaller than superparamagnetic critical size its influences on magnetism. In this part, we have demonstrated a controlled fabrication of well-defined h-Co/h-Co O nanotetrapods using a one-pot thermal decomposition method for the first time. Individual h-Co/h-Co O nanotetrapods are formed by four nanopods branching out at tetrahedral angles from a central region, to which multiple tiny Co nanoparticles are randomly interconnected or embedded into. The h-Co/h-Co O nanotetrapods surprisingly show a strong room temperature ferromagnetism. There exists a weak exchange bias in the h-Co/h-Co O nanotetrapods at low temperature. The strong room temperature ferromagnetic behavior of the h-Co/h-Co O nanotetrapods should originate from beating the superparamagnetic limit of the Co nanoparticles decorated on the Co O nanotetrapod skeleton with exchange bias.（3） Exchange bias in the antiferromagnetic material Co O-Co₃O₄ 3D flowers（Co O-Co₃O₄ 3DFs） system with Co nanoparticles with the size smaller and bigger than the superparamagnetic critical size embedded into. In this work, the jujube-cake-type hierarchically nanoporous Co/Co O-Co₃O₄ 3DFs and uniform nanoporous Co/Co O-Co₃O₄ 3DFs were synthesized by annealing the different precursor Co（OH）2 prepared by co-deposition under the same condition. The jujube-cake-type hierarchically nanoporous Co/Co O-Co₃O₄3 DFs are made of Co O-Co₃O₄ petal-like hierarchically nanoporous nanosheets skeleton with 31 nm Co nanoparticles embedded into. The uniform nanoporous Co/Co O-Co₃O₄ 3DFs consist of Co O-Co₃O₄ petal-like uniform nanoporous nanosheets skeleton with 5 nm of Co nanoparticles randomly embedded into. The interfaces between the Co and Co O or Co and Co₃O₄（Co/Co O and Co/Co₃O₄） of the two samples are formed by Co nanoparticles randomly contacted with Co O-Co₃O₄ skeleton. The change with the size of the ferromagnetic nanoparticles which are smaller or bigger than the superparamagnetic critical size of exchange bias field is different: when the size of the ferromagnetic nanoparticles is larger than the superparamagnetic critical size, the exchange bias field increases with decreasing the size of the ferromagnetic nanoparticles; while the size of the ferromagnetic nanoparticles is smaller than the superparamagnetic critical size, the exchange bias field decrease with decreasing the size of the ferromagnetic nanoparticles.（4） The influences of the microstructure of the interfaces on exchange bias in cobalt /cobalt oxide heterostructure system. In this work, we fabricated micro-flowers made of cobalt/cobalt oxide heterostructure nanosheets by annealing precursor Co（OH）2 at various temperatures, achieving the controlled nanostructure of the cobalt/cobalt oxide heterostructure interfaces. The studies of exchange bias in different heterostructural interfaces illustrate that the exchange bias field can be also tailored by changing the anisotropy constant of the antiferromagnetic materials and nanostructure of the interfaces beside the traditional way of changing the size of ferromagnetic nanoparticles. Our results provide an alternative way to design novel exchange-bias-related devices.