Site-controlled multi-ion substitution enabling low-loss and high-permittivity microwave ferrites

Modern wireless communication and radar systems urgently require low-loss and high-permittivity yttrium iron garnet (YIG) ferrite applied for highly efficient and integrated microwave circulators, isolators, filters, etc. However, it is challenging to achieve high dielectric constant, low dielectric loss, and narrow ferromagnetic resonance (FMR) linewidth simultaneously. Here, we synthesized Bi-Ca-Zr co-substituted YIG ferrite and comprehensively investigated the effect of multi-ion substitution on the polycrystalline microstructure and microwave electromagnetic properties. The introduction of Bi³⁺ ions at the crystallographic dodecahedral sites enhances the electronic polarization of single Fe³⁺ ions and the super-exchange interaction between them. The substitution of Zr⁴⁺ ions for the Fe³⁺ ions at the octahedral sites suppress the FMR linewidth broadening caused by magnetocrystalline anisotropy. Moreover, multi-ion substitution constitutes a competition between liquid phase sintering and grain boundary pinning and influences the densification and grain growth processes, resulting in a non-uniform and dense microstructure composed of crystallites with a bimodal size distribution. This distinctive morphology further contributes to FMR linewidth reduction and permittivity increase. The optimized Bi-Ca-Zr co-substituted YIG ferrite exhibits a narrow FMR linewidth of 33 Oe, high permittivity of 27, and high Curie temperature of 200 °C, making it a promising candidate for next-generation microwave devices.