1 Introduction

M-type hexagonal ferrites have been widely used in household appliances, cars, communication equipment, medical facility, military industry, and aviation owing to the abundance of raw materials, low cost, excellent chemical stability, relatively high remanence, and large coercivity [1,2,3]. Much research in recent years has been made to meet the increasing demand of market and the improvement of magnetic property requirements by adding or replacing some elements. For example, interests have been focused on the substitution of Sr2+ by La3+, Nd3+, Sm3+, Ca2+ etc, Fe3+ by Co2+, Cr3+, Al3+, Bi3+, Zn2+ [4,5,6,7,8,9,10,11], etc., and various cation combinations such as La–Co, La–Zn, La–Cu, Mn–Zn, Nd–Co [12,13,14,15,16,17,18,19,20], as reported. The change of the intrinsic magnetic properties of the substituted ferrites is related with the variation of the atomic magnetic moments and especially the magneto-crystalline anisotropy field [21,22,23,24].

It is reported that appropriate La–Co–Ca co-substitution can dramatically improve the magnetic properties in the Sr1−x−yLaxCayFe11.7−zCozO19 (0.39 ≤ x ≤ 0.45, 0.2 ≤ y ≤ 0.25, 0.3 ≤ z ≤ 0.35) ferrites [25,26,27,28]. La–Co substituted strontium ferrites with La/Co ratio ranging from 1.2 to 1.67 attract interests due to the fact that the powders are of single M-type phase and could improve magnetic properties [13, 24]. At present, most of commercial hexagonal ferrites with excellent magnetic properties are the La–Co–Ca substituted strontium ferrites. Calcium and strontium have the similar electronic configuration due to belonging to the same principal family in periodic table. Moreover, CaCO3 has lower costs and richer reserves, compared with SrCO3. It has been reported that the substitution of non-magnetic Zn2+ ions for partial Fe3+ ions at the down spin sites can enhance the saturation magnetization of the ferrites [11, 14, 30]. Therefore, in this work, an approach of Zn2+ substitution on the basis of La–Co–Ca co-substituted strontium ferrites was employed to further improve the magnetization, which so far has been rarely reported. Thus, we have prepared Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 ferrites through ceramic process. The effect of Zn2+ substitution on the structure and magnetic properties of Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 hexagonal ferrites has been investigated systematically.

2 Experimental

M-type hexagonal ferrite Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 (x = 0, 0.05, 0.1, 0.15, 0.2) were prepared via a solid-state reaction method. In this study, the raw materials, SrCO3 (97%, purity), Fe2O3 (98.4%, purity), La2O3 (99%, purity), Co2O3 (99%, purity), CaCO3 (99%, purity), and ZnO (99%, purity) were accurately weighed according to the chemical composition of Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19. The ratio of Fe and (CoZn) to (LaSrCa) was set at 10.6. And then, they were mixed together using wet ball milling in water for 6 h with a constant rotation velocity of 100 rpm and a ball to powder mass ratio of 12.5:1. The as-milled powders were dried and pre-calcined at 1250 °C for 2 h in air. As-calcined samples were pulverized again and then wet-milled with proper additives (CaCO3, SrCO3, SiO2, and H3BO3) in water for 16 h to get powders with average particle size about 0.8 μm. The finely milled slurry was pressed into disk-shaped green pellets with Φ 35 × 15 mm under 200 MPa in the magnetic field of 1.2 T. Finally, the green compacts were sintered at 1190 °C for 1 h in air atmosphere.

The phase structures of the pre-calcined powders were detected by X-ray diffraction (DX-2700) with Cu Kα radiation (λ = 0.15418 nm). The morphology of the sintered compacts were observed by field emission scanning electron microscopy (FEI Inspect F50). The Fe3+ sublattice occupation of the powders were investigated by 57Fe Mössbauer spectroscopy (MS-500) at room temperature, using a 57Co/Pd source. Magnetic properties of the sintered magnets were measured by a permanent magnetic measuring system (AMT-4).

3 Results and discussion

3.1 X-ray diffraction analysis

The X-ray diffraction patterns for the as-calcined magnetic powders with different Zn contents sintered at 1250 °C for 2 h in air are shown in Fig. 1. It was found from Fig. 1 that all the diffraction patterns were consistent with the standard pattern of SrFe12O19 (PDF: 80-1198), which suggested that all the magnetic powders possessed a single hexagonal ferrite phase with no observation of any impurity phases.

Fig. 1
figure 1

X-ray diffraction patterns of Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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The lattice constants a and c are calculated from the values of dhkl corresponding to (107) and (114) peaks according to the following formula [7]:

$$d_{hkl} = \left( {\frac{4}{3} \cdot \frac{{h^{2} + hk + k^{2} }}{{a^{2} }} + \frac{{l^{2} }}{{c^{2} }}} \right)^{{ - \frac{1}{2}}}$$
(1)

where dhkl is the interplanar crystal spacing, and h, k and l are the Miller indices.

Figure 2 shows the variations of the lattice constants a and c according to the doping content x. It can be seen that the lattice parameter c increased continuously with increase of x from 0 to 0.2, however, the values of lattice constant a had hardly changed compared with the lattice constant c. The variation in lattice parameter c could be explained on the basis of differences in ionic radii of Zn2+ (0.74 Å) and Fe3+ (0.645 Å). In addition, Zn2+ ions substitution led to the conversion of Fe3+ (0.645 Å) to Fe2+ (0.80 Å) to satisfy the total charge neutrality, which contributed to the lattice expansion [29].

Fig. 2
figure 2

Variation of lattice parameters as a function of Zn content in Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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3.2 Mössbauer spectroscopy

The sublattice occupation of Fe3+ ions can be determined from Mössbauer spectroscopy. 24 Fe3+ ions of the M-type hexagonal ferrites are distributed into five different sites: three octahedral sites (12k, 2a and 4f2), one tetrahedral site (4f1) and one bipyramidal site (2b). Because of the coupling by super exchange interactions through the O2− ions, 2a, 2b and 12k are arranged parallel to the c-axis, while 4f1 and 4f2 are antiparallel arrangement. The fitted room temperature 57Fe Mössbauer spectra of Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 are shown in Fig. 3. And the hyperfine parameters are presented in Table 1. The spectra for all studied samples (Fig. 3) exhibited the splitting of the 12k component. A new component was developed, obtaining the hyperfine field values of about 400 kOe and isomer shift and quadrupole splitting less than that of 12k component (Table 1), which was assigned to 12k sites perturbed by the presence of other ions in neighboring sites, and named 12k1. Lee’s study demonstrated that Zn2+ ions preferred to occupy 4f1 site [30]. However, as reported by Du, Zn2+ ions should occupy 4f2 site [31]. The areas of Mössbauer spectra sextets of Zn-doped Sr-hexaferrite series suggested that the site areas of both 4f1 and 4f2 decreased, compared with un-doped sample. These results indicated that Zn2+ ions occupied both 4f1 and 4f2 sites. However, the site area of 4f1 decreases lightly with increasing Zn content x, while 4f2 reduces monotonically as presented in Table 1, which showed that Zn2+ ions would preferentially occupy 4f2 site. This result might be explained that the 4f2 site is close to the La3+ ions, which is conducive to the reduction of electrostatic energy [31]. The splitting of the 12k component was induced by local cancellation of the 12k–4f2 super exchange interactions, which was due to substitution of non-magnetic ions or reduction of net magnetic moment [32, 33]. For Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 (x = 0), as La3+ ions replaced Sr2+ ions, Fe3+ (5 μB) ions would be reduced to Fe2+ (4 μB) ions, to satisfy the total charge neutrality. Fe2+ ions was present in the close vicinity of the 12k site [5] and the La–Co cations preferred to occupy the 4f2, 12k, and 2b sites [12], which resulted in weakening of the 12k–4f2 super exchange interactions. On the other hand, Zn2+ (0 μB) ions preferentially occupied the 4f2 site, which led to further declining of the 12k–4f2 super exchange interactions, so that the splitting of the 12k component became more serious.

Fig. 3
figure 3

Room temperature Mössbauer spectra of Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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Table 1 The isomer shift (δ), quadruple splitting (QS), hyperfine field values (Hhf), and percentage relative area (RA), and sites of Fe3+ ions for Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 derived from Mössbauer spectra recorded at room temperature
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3.3 Electron microscopy

Figure 4 shows the FE-SEM micrographs of the Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 magnets with different Zn-substituted amount (x) at 1190 °C in air. It can be seen from the images that the sintered magnets have formed hexagonal or irregular structure and the particles were distributed relatively homogeneously. The mean particle sizes of the sintered magnets were not changed obviously with increase of Zn content. However, the radius–thickness ratio of the particle increased, compared with the magnet without Zn-doped.

Fig. 4
figure 4

FE-SEM micrographs of Sr-hexaferrites with ax = 0, bx = 0.05, cx = 0.1, dx = 0.15, ex = 0.2

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3.4 Magnetic properties

The room temperature hysteresis curves of Sr-hexaferrites doped with different amount of Zn content (x) are shown in Fig. 5. The remanence (Br), magnetic induction coercivity (Hcb), intrinsic coercivity (Hcj) and maximum energy product [(BH)max] of the magnets were determined from the obtained hysteresis curves. Then, the variations of Br, Hc and (BH)max of the magnets would be analyzed in detail.

Fig. 5
figure 5

Room temperature hysteresis curves of Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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Figure 6 shows the effect of Zn content (x) on the remanence (Br) of the hexaferrite Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 magnets. It can be seen that Br of the sintered magnets increased from 434.9 mT (at x = 0) to 440.6 mT (at x = 0.1), then decreased with the Zn content (x) shifting from x = 0.1 to x = 0.2, and reached to 434.6 mT (BH)max. The variation of Br can be attributed to the substitution of Zn2+ ions into the down-spin sites of Fe3+ ions (4f1 and 4f2), which has been determined according to the above Mössbauer analysis. As the substitution of Zn2+ ions for the Fe3+ ions at 4f1 and 4f2 sites, the net magnetic moment of the magnets at x = 0.05 and 0.1 would increase, which led to increase in Ms due to the net magnetic moment being proportional to the magnetization, so Br of the magnets increased. Furthermore, as shown in Fig. 6, the Br decreased when x is above 0.1, which can be explained by the slight decrease of the net magnetic moment and the reduction of the Fe3+–O–Fe3+ super exchange interaction due to the substitution of Zn2+ ions for Fe3+ ions [15]. These results indicated that the optimal amount of the Zn2+ addition could be beneficial to the improvement of the Br, which is consistent with the previous reports [11, 14, 17].

Fig. 6
figure 6

The influence of the Zn content on Br values of Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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Figure 7 depicts the change of the magnetic induction coercivity (Hcb) and intrinsic coercivity (Hcj) with Zn content (x) of Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 magnets. It can be seen clearly that Hcb and Hcj of the magnets continuously decreased with the increase of x from 0 to 0.2. The values of Hcb decreased from 326.2 kA/m (at x = 0) to 178.6 kA/m (at x = 0.2), and the values of Hcj dropped from 439.3 kA/m (at x = 0) to 187.9 kA/m (at x = 0.2), respectively. It is well known that the coercivity of the magnets depends on their magneto-crystalline anisotropy and microstructure. According to the literature [1], the coercivity could be expressed by the following equation:

$$Hc = \alpha Ha - \frac{{N\left( {B_{r} + J_{s }^{0} } \right)}}{{\mu_{0} }}$$
(2)

where α is the grain size factor, Ha is the magneto-crystalline anisotropy field, N is the grain demagnetization factor, \(J_{s }^{0}\) is the saturation of the magnet. The factor α increases with the decreasing grain size; the factor N increases when the grain shape becomes more platelet. As shown in Fig. 4, the FESEM micrographs exhibited that the particle size of the magnets did not change basically with increasing Zn contents, while the grain morphology became more platelet shaped, which demonstrated that the factor α remained constant and N increased. On the other hand, Mössbauer spectra indicated that Zn2+ ions occupied both 4f1 and 4f2 sites. The substitution of non-magnetic Zn2+ ions not only eliminates the contribution of magnetic Fe3+ ions to the magneto-crystalline anisotropy, but also weakens the super exchange interaction [27], which results in the reduction of the magneto-crystalline anisotropy. Therefore, the coercivity of the magnets monotonously decreased with the increase of Zn content (x).

Fig. 7
figure 7

The influence of the Zn content on Hcb and Hcj values of Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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The maximum energy product [(BH)max] of the Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 magnets with different Zn content (x) are shown in Fig. 8. It can be seen that (BH)max had the similar feature as that of Br, which increased firstly and then decreased, but it reached the maximum value of 36.88 kJ/m3 at x = 0.05 due to the inconsistency between the variation trends of the values of Br and Hcj. Since (BH)max of the magnets was estimated by the product between the coercivity and remanent magnetization, and is considered as a comparative indication of the hysteresis area, the values of Br and Hcj will have their influence on it.

Fig. 8
figure 8

The influence of the Zn content on (BH)max values of Sr-hexaferrites (x = 0, 0.05, 0.1, 0.15 and 0.2)

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4 Conclusions

Sr0.1La0.45Ca0.45Fe11.7−xZnxCo0.3O19 (x = 0, 0.05, 0.1, 0.15 and 0.2) ferrite series were successfully prepared and systematically investigated in the present study. Ferrites with different Zn substitutions were all composed of single M-type phase, the lattice constant c increased and the particle shape became more platelet shaped with increasing Zn substitution. The Mössbauer spectra for all samples demonstrated the splitting of the 12k component, and indicated that Zn2+ ions occupied both 4f1 and 4f2 sites, whereas Zn2+ ions would prefer to occupying 4f2 sites with increasing Zn substitution. Therefore, Zn substitution could slightly improve the remanence of the magnets at Zn content x ≤0.1, while the intrinsic coercive force of the ferrites continuously decreased with Zn substitution. In addition, Zn substitution reduced the maximum energy product when Zn content x was larger than 0.05. Thus, it is recommended that the substitution amount of Zn is about 0.05.