Synthesis, Characterization, and Electromagnetic Wave Absorbing Properties of M₁²⁺–M₂⁴⁺ Substituted M-Type Sr-Hexaferrites

Mn–Ti, Zn–Ti, Zn–Zr substituted M-type Sr-hexaferrites (SrM), SrFe12−2xM1xM2xO19 (0 ≤ x ≤ 2.0, M1 = Mn or Zn; M2 = Ti or Zr) were synthesized, and their solubility, crystalline structure, and high-frequency properties were studied. Zn–Zr substitution caused a relatively large lattice parameter change and resulted in lower solubility (x ≤ 1.0) in the M-type phase compared with Mn–Ti and Zn–Ti substitutions. However, the ferromagnetic resonance frequency (fFMR) effectively decreased with increasing x in SrFe12−2xZnxZrxO19 (Zn–Zr:SrM) (0 ≤ x ≤ 1.0) and the electromagnetic wave (EM) absorption frequency also varied according to the shift in fFMR in the 7–18 GHz range. This is attributed to a gradual decrease in the magnetocrystalline anisotropy of Zn–Zr:SrM (0 ≤ x ≤ 1.0) with an increase in x. Zn–Zr:SrM (x = 0.9)–epoxy(10 wt%) composites exhibited a high EM absorption in the X-band (8–12 GHz) with the lowest reflection loss of x = 0.8 showed a broad Ku band (12–18 GHz) absorption performance satisfying RL f ≤ 18 GHz