Microwave Absorption and Magnetic Properties of M-Type Hexagonal Ferrite Ba0.95Ca0.05Fe12−xCoxO19 (0 ≤ X ≤ 0.4) at 1–18 GHz

In order to improve the microwave-absorption performance of barium ferrite and broaden its microwave-absorption band, BaFe₁₂O₁₉, Ba_0.95Ca_0.05Fe₁₂O₁₉, and Ba_0.95Ca_0.05Fe_12-xCo_xO₁₉ (x = 0.1, 0.2, 0.3 and 0.4, respectively) hexaferrites were synthesized by the solid-state reaction method, and the influence of Co ion substitution on the phase composition, microstructure, magnetic properties, and microwave-absorption ability of the ferrites in this system was studied. Introducing minor Co ions (x < 0.2) facilitated sintering and grain growth. At x ≥ 0.2, XRD revealed the emergence of the Co₂X phase alongside the BaM phase. Increasing Co ion concentration and the secondary X-phase led to slight reductions in saturation magnetization (69 to 63.5 emu/g) and substantial decline in coercivity (2107.02 to 111.21 Oe), attributed to grain size growth and Co₂X's soft magnetic nature. Notably, Co₂X incorporation significantly enhanced the microwave absorption and provided a tunable absorption band from the Ku to the C band. For a sample with a thickness of 2.0 mm and a doping level of x = 0.2, a minimum reflection loss of -59.5 dB was achieved at 8.92 GHz, with an effective absorption bandwidth of 3.31 GHz (7.07-10.38 GHz). The simple preparation method and good performance make Ba_0.95Ca_0.05Fe_12-xCo_xO₁₉ (x = 0.1, 0.2, 0.3 and 0.4, respectively) hexaferrites promising microwave-absorbing materials.