| Summary: | Mn–Ti, Zn–Ti, Zn–Zr substituted M-type Sr-hexaferrites (SrM), SrFe<sub>12−2x</sub>M<sub>1<i>x</i></sub>M<sub>2<i>x</i></sub>O<sub>19</sub> (0 ≤ <i>x ≤</i> 2.0, M<sub>1</sub> = Mn or Zn; M<sub>2</sub> = 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 (<i>x ≤</i> 1.0) in the M-type phase compared with Mn–Ti and Zn–Ti substitutions. However, the ferromagnetic resonance frequency (<i>f<sub>FMR</sub></i>) effectively decreased with increasing <i>x</i> in SrFe<sub>12−2x</sub>Zn<i><sub>x</sub></i>Zr<i><sub>x</sub></i>O<sub>19</sub> (Zn–Zr:SrM) (0 ≤ <i>x ≤</i> 1.0) and the electromagnetic wave (EM) absorption frequency also varied according to the shift in <i>f<sub>FMR</sub></i> in the 7–18 GHz range. This is attributed to a gradual decrease in the magnetocrystalline anisotropy of Zn–Zr:SrM (0 ≤ <i>x ≤</i> 1.0) with an increase in <i>x</i>. Zn–Zr:SrM (<i>x</i> = 0.9)–epoxy(10 wt%) composites exhibited a high EM absorption in the X-band (8–12 GHz) with the lowest reflection loss of <−45 dB. The sample with <i>x</i> = 0.8 showed a broad Ku band (12–18 GHz) absorption performance satisfying RL <−19 dB at 11 ≤ <i>f ≤</i> 18 GHz.
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