Effect of La-Al Co-substitution on the microstructure and magnetic properties of M-type barium hexaferrite

For self‑biased circulators, oriented Ba 1-x La x Fe 12-x Al x O 19 (x = 0.1–0.5) hexaferrites were fabricated by solid‑state reaction. Partial substitution of Ba 2+ by La 3+ and Fe 3+ by Al 3+ progressively contracts the lattice, increases grain size, and induces local lattice distortion, evidenced by Raman peak shifts and broadening. Magnetic properties evolve in two distinct trends: remanence and coercivity decrease monotonically; saturation magnetization M s , anisotropy constant K 1 , and anisotropy field H a increase to a peak at x = 0.4 ( M s = 78.91 emu/g, H a = 12.94 kOe) and then decline. The non‑monotonic M s arises from competition between magnetic dilution by Al 3+ and La 3+ ‑induced lattice contraction that enhances superexchange interactions. The M r / M s decrease is dominated by the weakening of exchange coupling due to non-magnetic Al 3+ substitution, with additional contributions from grain growth-induced multi-domain formation. The enhanced H a originates from lattice contraction strengthening spin‑orbit coupling at the 2b site, with the decline at x > 0.4 correlated with Raman‑detected local disorder accumulation. Through a finely-resolved composition gradient (∆x = 0.1), this study decouples the competing mechanisms and identifies the optimal composition at x = 0.40, underscoring the value of fine compositional tuning for precise property optimization. This work provides a systematic understanding of the competing mechanisms that determine the magnetic performance, offering guidance for composition design of oriented hexaferrites for self‑biased circulator applications.