Basic Study On Dynamic Magnetic Characterizations And Resonance Mechanism Of Ferromagnetic Nanocrystalline Structure

Ferromagnetic nanocrystalline alloy, highlighted as the promising alternative of classical ferrites, has been the hotspot in microwave magnetic material research in recent years. However, the dynamic magnetization model and multi-resonance mechanism of this kind of material still remains an unsolved problem with regard to its complex microstructure, especially the nanostructure effect which makes the classical model of bulk material inadequate. Therefore, both the basic research in nanomagnetism and the design of nanocrystalline material are limited.In this dissertation, systematic studies have been carried out on phase structure and magnetic properties of iron-based nanocrystalline alloy particles. Mechanical alloying and amorphous crystallization were employed to fabricateα-Fe nanocrystalline alloys with various secondary phase structures. We have investigated the mechanism of material's magnetic characterization and multi-resonance phenomenon with our experimentally evidenced new model of dynamic magnetization and analysis methods of microwave magnetic properties from composite to intrinsic state. On the other hand, we have developed some novel aspects on the theories of spin wave and magnetic bounds relation.We obtained some new results in following aspects:1. We have introduced for the first time an effective permeability model for composite with anisotropic iron-based nanocrystalline flake inclusion. In this model, an expression of effective demagnetization factor is proposed to depict the morphology of composite, says the characteristics of ferromagnetic inclusion's shape and orientation; Maxwell-Garnett's equation and retrieving method is also developed accordingly to gain the intrinsic permeability of inclusion from composites'data, and the results are consistent with experiments.2. We have reported for the first time a non-uniform precession model for ferromagnetic nanocrystalline structure based on Aharoni's exchange model and experimental evidence. It is assumed that nanocrystalline exchange coupling and surface effect works as an effective inner field, then multi-resonance phenomenon can be systematically explained by uniform ferromagnetic and non-uniform exchange resonance. Composition/decomposition method of microwave magnetic spectrum is developed as the analysis approach to ferromagnetic particle's permeability, relating experimental results to our model.3. Considering surface anisotropy on boundary in our non-uniform precession model, we have found an exchange spin wave mode transition with nanostructure size. Character length of exchange spin wave mode is introduced to complement the mechanism of this phenomenon. This observation is important for the application of nanomagnets in microwave devices.4. We have reported several novel magnetic bounds relations with regard to nanostructure magnetic effect. These relations are compatible with existing ones, can provide clues of the alignment of inclusions in composite and have even taken into account the damping effect. Therefore, they are more close to the real ferromagnetic nanomaterials.In a summary, we have explored the interaction mechanism between mesoscopic nanostructure effect and macroscopic magnetism in this dissertation. The established dynamic magnetization model and resonance mechanism for ferromagnetic nanocrystalline alloys will help to develop the magnetization theory of this material.