Microstructure and coercivity of Sr_1−x−x′La_xCa_x′Fe_2n−yCo_yO_α ferrites

M-type strontium hexaferrites with the chemical composition of Sr_1−x−x′La_xCa_x′Fe_2n−yCo_yO_α were prepared by ceramic process. X-ray diffraction, X-ray photoelectron spectrometer, scanning electron microscopy, vibrating sample magnetometer and permanent magnetic measuring system were employed to investigate the influence of La–Ca–Co substitution on the microstructure and intrinsic coercivity of the powder and sintered samples. The results show that both Sr-ferrite powders and sintered samples with Ca–La–Co substitution have high intrinsic coercive forces more than 500 KA/m. The acting mechanism of La–Ca–Co substitution on the high coercivity and microstructure was discussed. La–Co substitution causes not only the increase of the anisotropy, but also the decrease of the grain size, which increases the coercivity greatly. The presence of Ca together with La and Co in the substituted system helps to increase the magneto-crystalline anisotropy field. However, it accelerates the sintering and the grain growth, making the intrinsic coercivity of the sintered magnets decrease.