Achieving low frequency electromagnetic wave absorption by gyromagnetic ferrite

Abstract

Under the action of static bias magnetic field, the magnetized ferrite has a permeability tensor which can be adjusted by the applied magnetic field. In this paper, the absorption properties of bulk gyromagnetic ferrites under different magnetized conditions are studied and the great potential of gyromagnetic ferrite in achieving low frequency electromagnetic wave absorption is demonstrated. Full wave electromagnetic simulations are performed based on the finite element method (FEM). A floquet port is adopted at the top boundary of the unit cell to simulate a normally incident plane wave. The unit cell boundary conditions are used in the <i>x</i>-<i>y</i> plane to simulate a periodic structure. Orthogonality magnetization in plane is utilized to solve the polarization selectivity in the condition of transverse magnetic field. The influence on absorption capacity of discrete ferrite array structure and the coupling effect of ferrite elements with different sizes are also studied in consideration of the size effect. The simulation results show that a thin bulk gyromagnetic ferrite layer whose thickness is only 4 mm can possess frequency as low as 0.48 GHz and reflectivity below –10 dB. Gyromagnetic ferrite presents different absorption properties under longitudinal magnetization and transversal magnetization, and different polarization directions in transversal magnetization as well. When longitudinal bias magnetic field <i>H</i>₀ = 200 Oe, the bandwidth of the reflectivity below –10 dB ranges from 0.48 to 1.84 GHz. The resonant absorption frequency can be regulated by adjusting bias magnetic field and the size of ferrite element. In general, a large bias magnetic field leads to a high resonant frequency due to the ferromagnetic resonance frequency positively associated with the applied magnetic field, but a ferrite array consisting of larger size elements provides a lower resonant frequency for the size resonance negatively associated with the size. By introducing the coupling between elements with different sizes, the reflection bandwidth below –10 dB can be effectively extended to above 80% of the sum of the bandwidth possessed by single unit cell, especially 105.7% under transversal bias magnetic field 700 Oe. And the broadening effect is effective in both longitudinal and transverse magnetized state but it will be weaker when the two absorption peaks are closer. To further understand the absorption mechanism of the two-element absorber, the distribution of the electric field, magnetic field and power loss density are examined. The results prove that the two peaks at the lower frequency exactly originates from <inline-formula><tex-math id="M1">\begin{document}$ \Delta R = 0$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191229_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191229_M1.png"/></alternatives></inline-formula> and the higher frequency originates from <inline-formula><tex-math id="M2">\begin{document}$ \Delta R = 4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191229_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191229_M2.png"/></alternatives></inline-formula>, and therefore the widened absorption is contributed by the coupled multiple resonances provided by the elements with different sizes.