Research On Microwave Absorption Mechanisms Of 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 mobile,computer,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 cannot 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 permeability in Gigahertz range, high saturation magnetization,controllable in-plane anisotropy field 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,this dissertation,by the ferromagnetic resonance(FMR) and magnetic permeability spectra measurements, systematically studied the absorption mechanisms of the multilayer films with the structure of [ferromagnetic(FM)/non-magnetic insulator(NMI)] and the granular films.Our main research results are shown as following:(1) We prepare the BaZnxCo2-xFe15.5O27ferrite film, using plasma spray technique, and then use VSM and vector network analyzer to test ferrite microwave magnetostatic and physical property. The results show that it is a kind of soft magnetic thin film,with terrific ferromagnetic properties, low coercive force. Its magnetic permeability and magnetic loss in2GHZ is significantly greater than the one of microwave magnetic powder, which shows a significant boost of two-dimensional anisotropic ferrite film in the microwave magnetic properties,and the value of further optimizing technology and in-depth research.(2) By magnetron sputtering method,we prepared three series FeCoB/SiO2multilayer films with different thickness of FM and NMI layer and number of [FM/NMI] layers.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 results show that all of the samples possess clear multilayer structure,good dispersion characteristics,intensive absorbing properties in wide frequency band.But compared with the single-layer pure metal film,the real part μ’ and the imaginary part μ" of complex permeability is reduced, while the electric resistivity maximum still show metal strong.So,it is difficult to find microwave absorbing materials relying solely on multilayer film.(3) Because the metal film,with the continuous conductivity at microwave frequency, which causes high reflection of incident electromagnetic wave.can not be used as microwave absorbing materials.To improve this, we assume that if we take some surface treatment of the metal film, the surface will become discontinuous metal films.This method is expected to form the effective medium for metal and air, with the effective permittivity of dielectric constant on the similar metal-dielectric complex, which can be reduced to the acceptable range. Considering the metal element size based on Bruggeman’s effective medium theory,we obtained the effect of element size of film on the final effective dielectric constant.By calculating.it is both the real part and the imaginary part of the effective permittivity of the discontinuous-surface metal that has been effectively controlled, we got a high absorption intensity in the L microwave band(1-2GHz).So,that implies it is a kind of microwave absorbing material,which is very promising and worth further study.