| During the past few decades, the low temperature dynamics of magnetic nanoparticles and nanoclusters have been intensively studied. A number of studies have been reported that when the dipole-dipole interaction among magnetic nanoparticles becomes strong enough, spin-glass phases may appear. Furthermore, aging and memory effect, as inherent characteristics of the spin-glass dynamics, have been observed in frozen magnetic liquids and discontinuous insulator/metal multilayers. On the other hand, a Monte Carlo simulation study showed that the dipole-dipole interaction cannot result in a spin-glass-like phase. Furthermore, the magnetic fluids studied previously have been composed of oxide, nitride or alloy nanoparticles, which may have a spin-glass shell or spin–glass-like surface spin configuration. One may ask what the origin of the spin-glass phase in low temperature interacting nanoparticle system is. In order to clarify this issue, we have investigated the nature of the low temperature phase in different magnetic nanoparticle systems. In this thesis, magnetic nanoclusters in granular films and chemical synthesized magnetic nanoparticles have been employed to study the issues. In chapter 3, magnetic granular films with Co nanoclusters embedded in a metallic Ag matrix have been prepared first. As a typical characteristic of spin glass, the memory effect can be observed for this series of samples. However, except for the dipole-dipole interaction among Co nanoclusters, a RKKY indirect interaction has been proven to be present in this metallic system, which would complicate the issue. Therefore, in chapter 4, in order to avoid the RKKY interaction, Co nanoclusters are prepared in an insulating SiO2 host. Interestingly, even without the existence of RKKY interaction, the memory effect can be still observed for all CoSiO2 and FeSiO2 samples. In chapter 5, well and poorly crystallized iron oxide nanoshells (or hollow nanoparticles) have been successfully fabricated using the Kirkendall effect in pure oxygen and in air environment with the same Fe nanoparticles. Interestingly, spin-glass-like behavior can be found only in poorly crystallized nanoshells and not in well crystallized nanoshells. In chapter 6, one of our cooperation research works about dipolar interaction induced self-assembly magnetic nanoparticles is shown. Finally, the conclusion and future work are presented in chapter 7. |
