###### Abstract  
Mn-Ti, Zn-Ti, Zn-Zr substituted M-type Sr-hexaferrites (SrM), SrFe\({}_{12-2}\)M\({}_{12}\)M\({}_{22}\)O\({}_{19}\) (\(0\leq x\leq 2.0\), M\({}_{1}\) = Mn or Zn; M\({}_{2}\) = 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 (\(x\leq 1.0\)) in the M-type phase compared with Mn-Ti and Zn-Ti substitutions. However, the ferromagnetic resonance frequency (\(f_{FMR}\)) effectively decreased with increasing \(x\) in SrFe\({}_{12-2}\)Zr\({}_{2}\)Zr\({}_{2}\)O\({}_{19}\) (Zn-ZrSrM) (\(0\leq x\leq 1.0\)) and the electromagnetic wave (EM) absorption frequency also varied according to the shift in \(f_{FMR}\) in the 7-18 GHz range. This is attributed to a gradual decrease in the magnetocrystalline anisotropy of Zn-ZrSrM (\(0\leq x\leq 1.0\)) 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 \(<\)\(-45\) dB. The sample with \(x=0.8\) showed a broad Ku band (12-18 GHz) absorption performance satisfying RL \(<\)\(-19\) dB at \(11\leq\)\(f\leq 18\) GHz.  
materials [17; 18; 19]. In the research, high EM absorbing performances with a relatively broad absorption bandwidth could be achieved through controlling the composite structures and its complex permittivity and permeability properties. The EM wave absorption of insulating hexaferrites is mostly dependent on the magnetic loss mechanism, which is closely related to the imaginary part of the permeability (\(\mu^{\prime\prime}\)). The \(\mu^{\prime\prime}\) of hexaferrites increases at the ferromagnetic resonance (FMR) frequency (\(f_{FMR}\)), which is related to the magnetic anisotropy field (\(H_{ani}\)) by the equation below [20], where \(\gamma\) is the gyromagnetic ratio and \(\mu_{0}\) is the permeability of vacuum.  
\[f_{FMR}=\frac{\gamma\mu_{0}}{2\pi}H_{ani} \tag{1}\]  
SrM has a high \(f_{FMR}\) of \(\sim\)50 GHz, which is too high for hexaferrites to function as EM absorbers in the commercial radar frequency range, such as the X-band (8-12 GHz) and Ku-band (12-18 GHz). It has been reported that Co-Ti co-substitution effectively decreases the \(f_{FMR}\) from 50 to a few gigahertz for a substitution \(x\) range of \(0\leq x\leq 1.5\) for SrFe\({}_{12-2x}\)Co\({}_{x}\)Ti\({}_{x}\)O\({}_{19}\)[5]. In our previous research [7], the EM absorption properties of Co-Ti-substituted SrM were investigated. The \(f_{FMR}\) of Co-Ti substituted M-type Sr-hexaferrites (\(1.1\leq x\leq 1.3\)) changed gradually from 6 to 15 GHz, which included the X-band (8-12 GHz) of interest in terms of radar applications. It was demonstrated that the EM absorption area can vary according to the gradual shift of the \(f_{FMR}\). As cobalt is an expensive element, it is important to find other substitution elements that are more price competitive. The EM absorption properties of Ti-Mn [8; 9; 10] and Mn-Zr [11] substituted M-type hexaferrites have been reported. However, systematic studies with varying substitution amounts and correlations among composition, magnetic properties, and EM absorption properties are rare. In particular, to the best of our knowledge, the EM absorption properties of Zn-Zr substituted M-type hexaferrites have not yet been reported. In this study, Zn-Ti, Mn-Ti, and Zn-Zr co-substituted SrM with the chemical formula SrFe\({}_{12-2x}\)M\({}_{1x}\)M\({}_{2x}\)O\({}_{19}\) (\(0\leq x\leq 2.0\), M\({}_{1}\) = Mn or Zn; M\({}_{2}\) = Ti or Zr) were synthesized, and their crystalline structures, microstructures, high-frequency permeability, permittivity, and EM absorption properties were systematically studied.