Modifiable natural ferromagnetic resonance frequency and strong microwave absorption in BaFe12-y-2xAlySnxMnxO19 M-type hexaferrite

The site preferences of the ions in M-type BaFe12-y-2xAlySnxMnxO19 were determined from the XRD patterns refined with the Fullprof program. Due to the preferential occupation of the nonmagnetic Sn4+ ions, the doping Sn4+ and Mn2+/Mn3+ ions decreased the saturation magnetization. The doping also decreased the anisotropic field and the coercivity as well. BaFe12-y-2xAlySnxMnxO19 had strong natural ferromagnetic resonances at 15.37, 9.96, and 6.96 GHz with x = 0.7, 0.9 and 1.1, respectively. The frequency of the natural ferromagnetic reso-nance fell in the frequency range between 2 and 18 GHz was closely related to the decreased anisotropic field with the Sn4+ and Mn2+/Mn3+ doping. Accompanied with the ferromagnetic resonance, a high attenuation factor was obtained. The highest attenuation factor was 240 in x = 0.7. The minimum reflection loss was at the fre-quency with the impedance matching being close to 1, and at the thickness satisfying the quarter wavelength mechanism. The impedance matching was good around the natural resonance frequency in each compound. These led to a strong microwave absorption in the x = 0.7, 0.9 and 1.1 compounds. As the absorber's thickness was as thin as 1.8 mm, the x = 0.7 compound had an effective bandwidth of over 4.1 GHz in the range from 13.9 to higher than 18 GHz. At thin thickness of 2.5--2.9 mm, the x = 0.9 compound had a broad absorption bandwidth of 8-14.8 GHz, which covered the full X band of 8--12 GHz. By varying the doping content x, the microwave absorption band covered the range of 6.1 to 18 GHz with the absorber thickness thinner than 2.9 mm. The quarter wavelength mechanism played an important role on the strong microwave absorption.