Tunable and broad-band electromagnetic wave absorption using W-type Hexaferrites in 1-40 GHz range

W-type Sr-hexaferrites, Sr(0.75)Ca(0.25)Zn(2-x)CoxFe(16)O(27) (0.0 < x < 2.0), were synthesized, and their magnetic and electromagnetic (EM) wave absorption properties were investigated. As x increased from 0.0 to 2.0, the saturation magnetization value increased and reached a maximum at x = 1.25, followed by a decrease, while the magnetic anisotropic field (Hani) decreased and showed a minimum at x = 1.0, then increased again. The variation in Hani could be controlled by adjusting the value of x, which also changed the ferromagnetic resonance (FMR) frequency and enabled control over the frequency band of EM wave absorption. It is demonstrated that the EM wave absorption is primarily controlled by imaginary part of permeability (& mu;") induced by FMR. Composite of the W-type hexaferrite-epoxy (10 wt%) with x = 0.75-2.0 showed a broad-band EM wave absorption in the 1-18 GHz range, while the composites with x = 0-0.4 showed it in the 26.5-40 GHz. The x = 1.25 sample satisfied RL < - 10 dB in the 7-17 GHz range with a thickness of 2.1 mm, while x = 0.3 sample satisfied RL < - 10 dB in the 27-39 GHz with a thickness of 0.8 mm. Sr(0.75)Ca(0.25)Zn(2-x)CoxFe(16)O(27) is a very promising material for tunable wideband EM wave absorbers in 1-40 GHz range.