Effect of Crystal Orientation on the Magnetic Resonance Properties of M-Type Hexagonal Ferrites

This paper describes the effect of crystal orientation on the electromagnetic wave absorption properties of barium M-type (Ba-M) ferrite, in which Fe3+ was substituted by (Ti0.5Mn0.5)3+. The BaFe9(Ti0.5Mn0.5)3O19 sintered samples were prepared by a conventional powder metallurgy technique, in which and the c-axes of the powder particles were aligned in a magnetic field during compaction. After sintering, toroidally shaped samples were formed whose c-axes were aligned at certain angles (θ) to the sample thickness (d), and their microwave absorption properties were investigated in the GHz frequency range. It was found that the maximum values of the complex permeability (μr=μr′−jμr″) increased with decreasing θ. The θ=0° sample exhibited the highest μr″ value, which resulted in the smallest matching thickness (dm) of 0.57 mm with a reflection loss (R.L.) less than −20 dB at 12.05 GHz. In addition, increasing the sintering time had the effect of increasing μr″; reaching a μr″ value of 6.0 after sintering for 100 h. As a result, the matching thickness decreased from 0.72 to 0.51 mm, which is 30% smaller than for an isotropic sample sintered for 10 h. Therefore, it is concluded that the degree of crystallographic alignment of the sample and the sintering time used are both important factors in obtaining good electromagnetic wave absorption properties with a smaller matching thickness, in Ba M-type microwave absorbers.