Magnetic and morphological properties of Ca substituted M-type hexaferrite powders synthesized by the molten salt method

Permanent magnets have been widely used in various electro-magnetic devices, such as motors, absorbers, and magnetic sensors, and the demand in the market has been increasing yearly. Among them, M-type hexaferrite has the largest demand and supply in the permanent magnet market since it was developed in the 1950s. M-type hexaferrite has been used in various applications due to its excellent magnetic properties, good corrosion resistance, and low cost.¹ Thus, numerous research studies have been carried out to enhance the magnetic properties such as saturation magnetization and coercivity by adding substitutional elements. However, as the demand for magnetic sensors and high-frequency absorbers increases, the morphological control of M-type hexaferrite powder is required to improve their alignment characteristics and to simplify the manufacturing processes.

There are various methods for synthesizing hexaferrite powder, such as the conventional ceramic process, the sol–gel method,² and the co-precipitation process.³ These methods generally require a high-temperature reaction to dissolve replacement elements in a hexagonal lattice to enhance the magnetic properties. However, the high-temperature reaction could make it difficult to control the morphology of the hexaferrite powder.

The most common doping elements for M-type hexaferrite are La^3+7,8 and Co²⁺⁹ which render it excellent magnetic properties. Recently, for the further enhancement of the magnetic properties of M-type hexaferrite, the effect of various doping elements, such as Ti²⁺¹⁰ Cr³⁺¹¹ Mn³⁺¹² and Ce³⁺¹³ has been investigated. In particular, due to the low melting temperature of Mn oxides, the addition of Mn effectively reduced the size of powder, resulting in a sharp increase in the coercivity. However, other elements except Mn are not cost-effective and are difficult to be dissolved in hexagonal lattices even at high temperature. In addition, the high-temperature reaction causes rapid particle coarsening and the grain growth of hexaferrite powder, which deteriorates the coercivity of the synthesized powder.

The molten salt method^4–6 is known to have the advantage of lowering the reaction temperature and facilitating the morphology control by the liquid-phase reaction during the ferrite synthesis phase. In this study, M-type hexaferrite powders were synthesized using the molten salt method to reduce the reaction temperature and to control the size of the powders. Furthermore, the Ca element was substituted to control the morphology and aspect ratio (t/d) of hexaferrite powders. The magnetic properties and morphologies of the synthesized powders were analyzed according to the Ca content and calcination conditions.

In this experiment, M-type hexaferrite powders were synthesized by the molten salt method using strontium carbonate (SrCO₃, 97.0%, ∼1 µm, Daejung, Korea), calcium carbonate (CaCO₃, 99.0%, ∼1 µm, Sigma-Aldrich, Germany), α-hematite (Fe₂O₃, 99.9%, ∼0.3 µm, Kojundo, Japan), and sodium chloride (NaCl, 99.0%, Duksan Pure Chemical, Korea). The mole ratios of these precursor powders were controlled using the following equation, and the mixing was done by ball milling for 24 h to form a homogeneous mixture:

The mixed raw materials were calcined in an alumina crucible using a box furnace. During the calcination process, the heating rate was controlled to 10 °C/min and the blowing rate of O₂ was 3 l/min. Calcination was conducted at various temperatures above the melting point of NaCl (T_m = 801 °C). The calcined powders were furnace-cooled to room temperature and then washed more than five times with distilled water using a sonicator to remove residual NaCl. The washed M-type hexaferrite powders were dried at 60 °C in an oven for 24 h.

The magnetic properties of the synthesized powders were measured using a vibrating sample magnetometer (VSM, LakeShore 7410) under 25 kOe. The microstructures and morphologies of the synthesized powders were analyzed using an x-ray diffractometer (XRD, Rigaku, D/MAX-2500) with Cu Kα radiation, a scanning electron microscope (SEM, Tescan MIRA3), and a transmission electron microscope (TEM, JEOL, JEM-2100F). The compositional analysis of the powders was carried out using an x-ray fluorescence spectrometer (XRF, Rigaku, ZSX Primus II).

In this experiment, SrCO₃, CaCO₃, Fe₂O₃, and NaCl were homogeneously mixed according to the stoichiometric ratio and calcined at various temperatures to synthesize Sr_1−xCa_xFe₁₂O₁₉ (x = 0.00–0.20) powders. Figure 1 shows the XRD diffraction patterns of the Sr_0.90Ca_0.10Fe₁₂O₁₉ powders synthesized under various calcination conditions. As shown in the patterns, the synthesized powders have a hexagonal structure without additional impurity peaks such as Fe₂O₃ or CaFe₂O₄. This result confirms that a Ca element was successfully substituted with a Sr element to form a stable hexagonal structure in all calcination conditions.

Figure 2 shows the magnetic properties of the synthesized Sr_0.90Ca_0.10Fe₁₂O₁₉ powders under different calcination conditions. As shown in Fig. 2, the magnetic saturation value is similar in all samples due to the identical compositions without any impurities. However, the highest coercivity value was obtained in the sample calcined at 950 °C for 1 h, and the coercivity decreased gradually with increasing calcination temperature. In general, the coercivity is dependent on the morphology and size of the powder. This indicates that the calcination temperature plays an important role in determining the coercivity of the powder because the thermal energy is an important factor in leading surface and diffusion reactions.

The SEM analysis was carried out to observe changes in the morphology of the synthesized powders. Figure 3 shows the SEM images of Sr_0.90Ca_0.10Fe₁₂O₁₉ powders under various calcination conditions. As shown in this figure, Sr_0.90Ca_0.10Fe₁₂O₁₉ powders have a hexagonal plate-like shape with an average size of 1.0–2.0 µm when the calcination temperature is below 950 °C. However, as the calcination temperature increases above 1050 °C, the morphology of the synthesized powders becomes irregular and the particle size starts to increase. As a result, when the calcination temperature is 950 °C, the morphology of the Sr_0.90Ca_0.10Fe₁₂O₁₉ powders still has a hexagonal plate-like shape, and this morphological characteristic becomes a factor that improves the coercivity value.

The above SEM images suggest that the morphological change in the Sr_0.90Ca_0.10Fe₁₂O₁₉ powders could be due to the different growth mechanisms depending on the calcination temperature. In the powder calcined below 950 °C, the growth mechanism of the synthesized powder should be an interfacial reaction mechanism, which reduces the growth rate due to the reaction between the molten salt and the crystal surface.¹⁴ Thus, it is possible to maintain the hexagonal shape, which is the crystalline structure of the M-type ferrite. However, as the calcination temperature increases, the atomic diffusion rate increases and dominates the growth rate of the powder.¹⁴ Thus, the powder changes from the hexagonal shape into an irregular shape, as shown in Figs. 3(c) and 3(d). In addition, as shown in Fig. 3(d), necking between the powder particles can be observed. Therefore, the control of the calcination temperature is critical to improve the morphology and magnetic properties of the synthesized powders.

Given that the magnetic properties are excellent and the shape of the synthesized powders remains hexagonal, the 950 °C calcination condition was selected to observe the changes in the magnetic and morphological characteristics according to the Ca content. In order to observe the characteristics depending on the composition change, the Ca element was substituted from 0.00 to 0.20 and calcination was carried out at 950 °C for 1 h. Figure 4 shows the XRD diffraction patterns of Sr_1−xCa_xFe₁₂O₁₉ powders synthesized at 950 °C for 1 h. As shown in Fig. 4, no noticeable peak shift of the XRD patterns is observed in all compositions although the Ca content is changed. It is considered that the lattice parameters are dominantly determined by Fe and O in the M-type hexaferrite crystal structure. Thus, the XRF analysis is carried out to confirm Ca element substitution in the hexaferrite structure. Table I shows the changes in the Ca content of the Sr_1−xCa_xFe₁₂O₁₉ powders. This clearly shows that Ca was substituted in the hexaferrite lattice.

As shown in Figs. 4(d) and 4(e), additional phases, such as CaFe₂O₄ and Fe₂O₃, are observed in the XRD patterns, indicating that the reaction was not completed. This indicates that when the substitutional ratio of the Ca element exceeds 0.15, it is difficult for the Ca element to be completely substituted into the hexaferrite lattice under the calcination condition of 950 °C for 1 h. Due to these additional phases, the magnetic saturation decreased drastically under conditions (d) and (e) in Fig. 5.

Figure 5 shows the magnetic properties of the synthesized Sr_1−xCa_xFe₁₂O₁₉ powders calcined at 950 °C for 1 h. In powder containing the Ca element less than 0.15, the change in the magnetic saturation should be due to the change in the spin structure inside the hexaferrite lattice. As the c/a ratio decreases with Ca element substitution, the Fe³⁺–O⁻²–Fe³⁺ distance on the c-axis is known to decrease.¹⁵ Thus, the magnetic saturation values of the powder decreased with an increase in the Ca content. On the other hand, previous studies showed that the coercivity was strongly dependent on the grain size, not the Ca content.¹⁶ Thus, it is required to investigate the morphological change in the powder to understand the magnetic properties.

Figure 6 shows the SEM and TEM micrographs of Sr_1−xCa_xFe₁₂O₁₉ powders synthesized at 950 °C for 1 h. As shown in Figs. 6(a)–6(e), the size of the powder is decreased and the shape changes to a thin hexagonal-plate shape as the Ca content increases. It is noteworthy that the t/d ratio of the hexagonal-plate particles sharply decreases when the Ca content reaches 0.1. In order to clarify the above changes, the size distributions of the powders are shown in Fig. 7(a). As shown in this figure, the hexaferrite powders without the Ca content have an average thickness of 1 µm. As the Ca content increases, the average thickness of the synthesized powders reduces to 0.4–0.6 µm. However, the t/d ratio of the synthesized powders is converged, as shown in Fig. 7(e). In order to understand the above morphological changes, the TEM analysis was carried out, and the TEM micrograph and the corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 6(f). As shown in this figure, the powder has a single crystalline structure and the direction of the (0001) plane is parallel to the thickness direction of the hexagonal-plate powder. This should indicate that the Ca element substitution lowers the interface energy between the molten salt and the (0001) plane of the hexagonal structure.¹⁷ This low surface energy can enhance the development of thin hexagonal-plate shaped powder with a low t/d ratio. In addition, the low surface energy could make the interfacial reaction mechanism more dominant rather than the diffusion mechanism, reducing the powder size. Therefore, it is clear that the morphology changes of the synthesized Sr_1−xCa_xFe₁₂O₁₉ powders were successfully achieved by the Ca element substitution into the hexagonal lattice.

As the technology advances, M-type ferrite powder and magnetic sensors with excellent properties are required. This hexagonal plate-like ferrite powder is able to achieve better alignment than irregular shaped powder, which is an important factor in manufacturing a magnetic sensor with high performance. In addition, the shape-controlled M-type ferrite powder can be used as an electromagnetic wave absorbing material in the high frequency band.

In this study, Ca element substituted M-type strontium ferrite powders were synthesized by the molten salt method under various calcination conditions. It was observed that the morphology of SrFe₁₂O₁₉ powders changed from hexagonal plate-like to irregular shapes as the calcination temperature increased above 1050 °C for 1 h. As a result, SrFe₁₂O₁₉ powders synthesized at 950 °C for 1 h showed excellent magnetic properties. For the further improvement of the magnetic properties and the control of the powder morphology, Ca element was substituted in M-type strontium ferrite. Thin hexagonal plate-like powders with a low t/d ratio were found to be formed as the Ca content increased, resulting in an increase in the coercivity. As a result, single-crystalline Sr_0.90Ca_0.10Fe₁₂O₁₉ powders with an average thickness of 0.4 µm were successfully obtained by calcining at 950 °C for 1 h. The microstructural analysis confirmed that the morphological change was due to that the substitution of the Ca element enhanced the interfacial reaction mechanism.

This work was supported by the Technology Development Program No. [S2521249] through the Ministry of SMEs and Startups (MSS, Korea) and the Basic Science Research Program No. [NRF-2019M3D1A2104158] through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.

The data that support the findings of this study are available from the corresponding author upon reasonable request.