Microwave absorbing properties of Co-substituted Ni2W hexaferrites in Ka-band frequencies (26.5-40 GHz)

Magnetic (static, high-frequency) and microwave-absorbing properties of Co-substituted Ni₂W hexaferrites are investigated in the frequency range of Ka-band (26.5-40 GHz). The powders of composition BaNi_2-XCo_XFe₁₆O₂₇ are prepared by calcining at 1250°C, where the substitution ratio x was varied from 0.0 to 1.4. With the calcined powders, the composite specimens are prepared using the silicon rubber as a matrix material. Vibrating sample magnetometer measurement indicates that the coercivity of the specimens of BaNi_2-XCo_XFe₁₆O₂₇ decreases with Co substitution, which is attributed to a reduction of crystal anisotropy field by changing the easy-axis of magnetization from c-axis to basal plane. The complex permeability spectrum shows the decrease of resonance frequency with Co substitution. The specimens with composition of x=0.2 and x=0.4 show the superior microwave absorbing properties in Ka-band. For the specimen of x=0.4, the bandwidth (with respect to -20-dB reflection loss) is 26.5-32 GHz at a matching thickness of 0.9 mm. The Co substituted Ni₂W-hexaferrites can, therefore, be suggested as a good absorbing material for the electromagnetic radiation in the frequency range of Ka-band.