Hexaferrite materials displaying ultra-high coercivity and sub-terahertz ferromagnetic resonance frequencies

Single-domain Sr1-x/12Cax/12Fe12-xAlxO19 (x = 4-6) particles are synthesized by a simple citrate auto-combustion method. The room temperature coercivity of the materials rises with aluminum content from 21.3 kOe (x = 4) to a maximum of 36 kOe (x = 5.5). This value is the highest among ferrite materials to date. Moreover, the magnetic alignment of the particles leads to further coercivity improvement up to 40 kOe. Due to large magnetic anisotropy the samples demonstrate sub-terahertz electromagnetic wave absorption by natural (zero-field) ferromagnetic resonance (NFMR). The absorption lines shift with aluminum substitution from 160 GHz (x = 4) to 250 GHz (x = 5.5), which is the record NFMR frequency known for a magnetic material. This research paves the way for development low-cost materials with extremely high coercivity and sub-terahertz NFMR.