Effects of La substitution on micromorphology, static magnetic properties and low ferromagnetic resonance linewidth of self-biased M-type Sr hexaferrites for high frequency application

M-type Sr (SrM) hexaferrites have been widely applied in millimetre/microwave ferrite devices such as circulators and isolators. SrM hexaferrites must exhibit a high remanence ratio Mr/Ms, a high anisotropy field Ha, and a low ferromagnetic resonance (FMR) linewidth ΔH to further develop millimetre/microwave equipment and devices with high frequencies, miniaturisation, and planarisation. However, it is difficult for M-type hexaferrite to simultaneously meet the requirements of high Mr/Ms, high Ha, and low ΔH, which are key to realising high-frequency self-biased ferrite devices. In this work, Ca–La–Co-substituted SrM ferrite was prepared, and the influence of La substitution on the micromorphology, static magnetic properties, and ΔH values of SrM hexaferrites is discussed in detail. The results demonstrated that as the La substitution content x increased from 0.0 to 0.5, the saturation magnetisation 4πMs and coercivity Hc initially increased, and then decreased. The Ha values of all samples were higher than 18 kOe. The FMR linewidth ΔH initially increases, and then decreased after reaching a maximum value at x = 0.2. The independent grain approximation theory was used to analyse the changing trend of the FMR linewidth. The results showed that the crystalline anisotropy linewidth ΔHa accounted for a larger proportion of the FMR linewidth than the porosity linewidth ΔHp. The sample with a La substitution content x of 0.3, exhibited the best performance (4πMs = 3915 Gs, 4πMr = 3387 Gs, Hc = 1818 Oe, Mr/Ms = 0.865, large Ha = 23.99 kOe, and ΔH = 1548 Oe at 58.8 GHz). The materials prepared in this research have significant potential for application in high-frequency self-biased devices and microwave engineering.