Research On Microwave Damping Mechanisms Of FeCo-based Nanometer Grade Magnetic Films

With the development of economy and modern science and technology, more and more electronic devices have come into people's life and have a more intensive impact on us. These electronic productions, such as the computer, mobile, and WiFi networks, bring much convenience and fun to us. However, unavoidably, electromagnetic radiation generated by them exists everywhere and becomes more and more intensive, and would be harmful to the human health while people using them. Moreover, the interference among the productions is more and more serious. The electromagnetic pollutant is named the forth public hazard in our daily life. In addition, in the military area, in order to improve the surviving ability and lethality of the aerocraft, reducing of the radar cross section (RCS) to avoid exposure to the radar detection system is very important. The optimization of shape of the aerocraft can effectively diminish the RCS to some extent, but the flexibility will be seriously confined. Additionally, the RCS of some part of the aerocraft can not be reduced by the optimal design of structure. Therefore, the research on light and thin microwave absorbing materials, with intensive absorb properties in wide absorb frequency band, is very important and necessary to satisfy the requirements of reducing undesirable electromagnetic radiation.So far, with potential values for electromagnetic applications, ferromagnetic thin films attract more interest of researchers, because of their combined properties of high saturation magnetization, controllable in-plane anisotropy field, high permeability in gigahertz range and more interesting physical phenomena. The most important and essential work in this research is to get a better understanding of the damping mechanisms of the materials in the microwave frequency band. Against the above background, based on the microstructure of films and in the framework of Landau-Lifshitz equation, this dissertation, by the ferromagnetic resonance (FMR) and magnetic permeability spetra measurements, systematically studied the damping mechanisms of the multi-layer films with the structure of [ferromagnetic(FM)/ non-magnetic insulator (NMI)] and the single-layer films possessing multiple magnetic phases with a more complex structure. Our main research results are shown as following:1. By magnetron sputtering method, we prepared three series of Fe65Co35/SiO2 multi-layer films with different thickness of FM and NMI layer and number of [FM/NMI] layers. The results show that all of the samples possess excellent soft magnetic properties, such as tiny coercivities and certain in-plane unixial anisotropies. The relationships between their static magnetic properties, the microwave permeability and the thickness of FM, NMI layer, the number of [FM/NMI] layers are displayed and reasonably explained in the view of the microstructure of films. The relationship between the polar angleθH and FMR field HRES, line-widthΔHRES measured from the FMR, are successfully fitted by the theory. From the perfect fitting, we can distinguish the different origins of the line-width and their respective weights of contribution. The research on several [FM/NMI] layers film is rarely reported, and our tentative study obtains a well-explained result and is in a forward place in this area. We also find that there are a lot of new regulations and abundant physical nature, which is worth making further efforts to investigate theoretically and experimentally.2. By magnetron sputtering method, we prepared a series of FeCo-SiO2 sample named S1 which within multiple magnetic phases by triple-target alternately sputtering and FeCoB sample series S2 by annealing after composite-target sputtering. The results also show that all of the samples possess excellent soft magnetic properties, such as small coercivities and certain in-plane unixial anisotropies. We investigated the sample series S1 by FMR and microwave permeability spectra measurement, and S2 by microwave permeability spectra measurement, finding that double resonance peaks existed in the FMR and permeability spectra for all the samples. The existing of double resonance peaks can be attributed to two distinct magnetic nanocrystalline phases, FeCo and Fe, existing in these films. In order to further argue that the film is composed of such phases, the results of the FMR and microwave permeability spectra have been fitted, the calculated curves are in good correspondence with the measurement curves. More surprisingly, a remarkable positive shift in resonance peak in higher frequency is observed with the increase in annealing temperature. The phenomena are definitely anomalous and should not happen, which is totally different from what we usually know. Generally, as the annealing temperature increases, the inhomogeneities in the film become more intensive, the damping factor will increase, and then the resonance peaks will shift to the lower frequency. From the further investigation, the phenomena are resulted from the dispersion effects of the internal stray fields (IFS) which come from the inhomogeneities of the composition and grain size of multiple magnetic phases. By using the IFS theory, we calculated the microwave permeability of the films in a different way, and the fitting to the experimental results is very well. The fitting parameters we used also have a tendency corresponding with the reality. In conclusion, this dissertation studied the films possessing complex structure of multiple magnetic phases, and didn't avoid or just make a qualitative analysis of the positive shift of resonance peaks. By the multiple magnetic phases and IFS model, both of which are based on microstructure of the films, we made a perfect quantitative analysis of the double resonance peaks and the positive shift. Our research is an important complementarity to the existing research on damping mechanisms of ferromagnetic film. We highlighted the in-negligible contribution of the relaxations from inhomogeneities in the film to extrinsic damping, which has considerable significance to the research on the microwave absorbing materials with intensive absorbing properties in wide frequency band.