## 1 Introduction  
Hexagonal ferrites, such as BaFe\({}_{12}\)O\({}_{19}\) (BaM) and SrFe\({}_{12}\)O\({}_{19}\) (SrM), are commercially important as ceramic permanent magnets for industrial applications due to their cost-effectiveness, strong uniaxial magnetic anisotropy along the \(c\)-axis, and excellent chemical stability [1]. The BaM hexaferrite and SrM hexaferrite, which have the same crystal structure as the magnetoplumbite mineral, were discovered in the early 1950s by Philips [2; 3]. It was found that SrM hexaferrite has better magnetic properties than BaM at room temperature [4; 5]. These M-type hexaferrites are typically prepared by solid-state reaction [1] because this process is cost-effective and suitable for mass production [6]. The performance of a permanent magnet can be evaluated by its remanent flux density (\(B\),), coercivity (\(H_{c}\)), and maximum energy product (\(BH\))\({}_{max}\). These properties are extrinsic and depend on the microstructure, including sintered density, average grain size, and degree of grain alignment. However, these parameters are closely related to intrinsic properties such as magnetization saturation (\(M_{\rm S}\)) and uniaxial crystal anisotropy constant (\(K_{\rm u}\)). These properties depend on the composition of the substituted cation and their occupancy on each site within the unit cell [6; 7]. Although high values of \(B_{r}\) and (\(BH\))\({}_{max}\) can be obtained from a highly dense, small-grained sample with a microstructure that is highly textured along the easy axis, it is necessary to use a composition with higher \(M_{S}\) and \(K_{u}\) values. This is because the \(B_{r}\) value is limited to the saturation magnetic induction (\(B_{s}\)) on the \(B\)-\(H\) loop, which corresponds to the \(M_{S}\) value on the \(M\)-\(H\) curve. Additionally, the \(H_{c}\) value depends not only on the grain size but also on \(K_{u}\)[1; 8; 9]. Sometimes, the magnetic anisotropy field (\(H_{a}\)) is a more convenient representation of the strength of magnetic anisotropy along the easy axis than \(K_{u}\) because \(H_{a}\) is proportional to \(K_{u}\), as shown by the relationship \(H_{a}=2K_{u}/M_{S}\) for a spherical single domain particle [10]. Numerous studies have been conducted to improve the intrinsic magnetic properties of SrM hexaferrites through cation substitutions. Co [11], Ti [12], Al [13], and Zn [14] have been substituted for the Fe sites, while La [15], Nd [16], and Sm [16] have been substituted for the Sr sites. Among these substituents, La-Co co-substitution has been found to be very effective in improving \(H_{a}\) without significantly decreasing \(M_{s}\). However, previous reports on the effect of La-Co co-substitution for SrM on the structural parameters, including the lattice parameters and unit cell volume, as well as the intrinsic magnetic properties of \(M_{s}\) and \(H_{a}\), are not in good agreement with each other [17; 18; 19; 20; 21; 22; 23]. The \(M_{s}\) value increased gradually with increasing La-Co co-substitution (\(x\)) up to \(x=0.3\)[18], or it increased up to \(x=0.2\) and then decreased with further increasing \(x\)[19], or it decreased continuously [20]. The variation in \(H_{a}\) with \(x\) showed a common tendency; \(H_{a}\) increased gradually with increasing \(x\), although there was a significant difference in the \(H_{ci}\) values [17; 18; 19; 20; 21; 22; 23]. This difference in the intrinsic magnetic properties of La-Co co-substituted SrM is likely due to the sintering additives such as SiO\({}_{2}\)[18; 21; 24], CaCO\({}_{3}\)[18; 21], and Bi\({}_{2}\)O\({}_{3}\)[19]. According to reports, Ca can substitute for Sr sites [25], while Si [26] and Bi [27] can substitute for Fe sites in M-type hexaferrites. This suggests that the kind and amount of sintering additives can affect \(M_{S}\) and \(H_{a}\). To identify the intrinsic magnetic properties of La-Co co-substituted SrM hexaferrites without using sintering additives, it is necessary to prepare samples consisting of a single phase of M-type without any second phase.