| With fast development of science and technology, the absorbing materials have caught wide attention in the world. First of all, modern military detection technology have attracted the interest of the researchers, especially the radar absorbing materials (RAM). At the same time, with the increasing of frequency for microwave communication, the problem of the electromagnetic interference and electromagnetic radiation is serious. The requirement for microwave absorber is arising. Snoek limit is the product of permeability and natural resonance frequency. The metal-insulator granular films characterized with high saturation magnetization and un-axis shape anisotropy field are potential for high natural resonance frequency and high permeability.Anisotropic absorbent attracts mostly attention. Nanometer metal-insulators magnetic films have strong shape anisotropy, controlled in-plane anisotropic field, amorphous or nanocrystalline phase, high saturation magnetization strength (Ms), high complex permeability (μ) and relatively high volume resistivity (p). The adjustments of the structure and composition expect to break through Snoek limit. Absorption spectrum of the magnetic granular films corresponds to the radar low frequency band, which are exactly the weakness of traditional radar absorbing material. Therefore, nano metal-insulators magnetic granular films were widely researched. The μ of the granular films are dozens times as high as that of the traditional absorbing material, indicating that these films have good attenuation characteristics and p cannot significantly decrease. Films with high volume resistivity (p) can depress the eddy current effect and meet the impedance matching characteristic.Aiming at the above problems, the films structure was designed in the paper. Magnetic co-sputtering method was used and the granular films and composition graded granular films were preparated. The microstructure, static magnetic, high-frequency properties were studied. The research contents and the experimental results were summarized as follows,1. A series of FeCoB-CoFe2O4granular films deposited on Kapton polyimide were prepared by tuning films thickness at room temperature with magnetic co-sputtering method. In order to effectively scattered magnetic particle and tune the microstructure, the granular layer in the FeCoB-CoFe2O4films were prepared to replace the traditional continuous magnetic layer.2. XRD results showed that the films were amorphous or nanocrystal phase. The dopant of B elements inhibit the growth of the grain. AFM result showed that the film had relatively small surface roughness. With the increase of films thickness, both Hce and p decreased. Obvious in-plane uniaxial magnetic anisotropy was observed in the films prepared at low Ar pressure. Good soft magnetic properties (Ms-999.2emu/cmj, Hce-2.79Oe), relatively high p (-0.3ma-cm) and μ (μ’i=115.3, μmax"=411.4) were obtained in the films with Ar pressure of0.3Pa and films thickness of250nm.3. Compositionally graded granular films structure was designed,and the graded change direction was the films thickness direction,. Compositionally graded FeCoB-CoFe2O4granular films with obvious in-plane anisotropy and god soft magnetic properties were prepared on Kapton successfully by the optimization design of the composition and layer numbers. This film maintained high Ms-962.7emu/cm3. At the same time, the p was up to0.6mΩ·cm.4. The saturation magnetization of the granular films with CoFe2O4as the insulator were increased when the p did not decrease obviously. Composition graded granular films structure broke through the conductive percolation threshold limitation of the conventional granular films, and high p, large Ms were obtained simultaneously and the impedance matching and attenuation characteristic demands were satisfied hopefully. The composition graded granular films design provides a novel method and greatly promote the granular films as a kind of new low frequency radar absorbing material to be actually applied in practice. |
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