C -axis textured M-type hexaferrite with tailored magnetic properties and narrow FMR linewidth for Ka-band self-biased circulator applications

C-axis textured BaZn0.3Ti0.3Fe11.4O19 M-type hexaferrite was synthesized by solid-state reaction under magnetic field alignment, and its crystallographic site occupation, static and dynamic magnetic properties, and self-biased circulator performance were systematically investigated. At this intermediate substitution level, Rietveld refinement and Raman spectroscopy independently reveal that Zn2+–Ti4+ dopants are distributed across all five Fe3+ sublattices, with the 12k site accommodating the largest individual fraction—marking the onset of a multi-site substitution regime distinct from the selective 4f1/4f2 occupation dominant at lower doping levels. The saturation magnetization reaches 70.01 emu/g with a remanence ratio of 0.84, while the anisotropy field is reduced to 9871 Oe, reflecting a balance between the 4f1/4f2-driven anisotropy reduction and partial compensation by the 12k site. A narrow ferromagnetic resonance linewidth of 428 Oe is obtained at 58 GHz, and surface-integral analysis of the position-dependent demagnetizing factor demonstrates that finite-size-induced broadening contributes less than 9% to the measured linewidth. A slotted-structure self-biased circulator designed with the measured parameters exhibits a center frequency of 31.7 GHz and a 15-dB bandwidth of 2.3 GHz at the Ka-band. These results demonstrate that the intermediate-doping regime, wherein the 12k site becomes the primary dopant recipient, provides an effective route for simultaneously achieving a reduced anisotropy field and a narrow linewidth in substituted M-type hexaferrites for self-biased millimeter-wave circulator applications.