Magnetic and Structural Properties of Barium Hexaferrite BaFe12O19 from Various Growth Techniques

Barium hexaferrite powder samples with grains in the μm-range were obtained from solid-state sintering, and crystals with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes, respectively. Carbonate and borate fluxes provide the largest and structurally best crystals at significantly lower growth temperatures of 1533 K compared to flux-free synthesis (1623 K). The maximum synthesis temperature can be further reduced by the application of PbO-containing fluxes (down to 1223 K upon use of 80 at % PbO), however, Pb-substituted crystals Ba1–xPbxFe12O19 with Pb contents in the range of 0.23(2) ≤ x ≤ 0.80(2) form, depending on growth temperature and flux PbO content. The degree of Pb-substitution has only a minor influence on unit cell and magnetic parameters, although the values for Curie temperature, saturation magnetization, as well as the coercivity of these samples are significantly reduced in comparison with those from samples obtained from the other fluxes. Due to the lowest level of impurities, the samples from carbonate flux show superior quality compared to materials obtained using other methods.


Introduction
The increase in electromagnetic pollution due to the rapid development of gigahertz (GHz) electronic systems and telecommunication has resulted in a growing and intense interest in electromagnetic absorber technology. Electromagnetic interference (EMI) can cause severe interruption of electronically controlled systems. It can produce device malfunctions, generate false images, increase clutter on radar, and reduce performance because of system-to-system coupling. These by cooling at a rate of 4.5 K/h to 1173 K to homogenize the starting materials. The system was then allowed to naturally cool to room temperature. The spontaneously obtained crystals with sizes of up to 7 mm were separated from the solidified melt by leaching in hot nitric acid to remove side phases. For solid-state sintering, mixtures of Fe 2 O 3 and BaCO 3 were heated to 1623 K and held for 3 h. Table 1. Initial molar ratios of charge compositions (at %) and maximum temperatures of the different crystal growth experiments. Resulting compositions originate from full structure refinements based on single crystal X-ray diffraction intensity data.

Characterization
To investigate the composition, structure, and properties of the samples, they were examined by the following methods: The chemical compositions of the grown samples were determined with use of an electron scanning microscope with an energy dispersive spectrometer for elemental analysis (Jeol JSM7001F/Oxford INCA X-max 80, Tokyo, Japan). X-ray powder diffraction analysis was carried out with filtered Cu Kα radiation (RigakuUltima IV, Tokyo, Japan). To determine the crystallographic details of the Pb-substituted samples, single crystal X-ray diffraction intensities were collected with monochromatic Mo Kα radiation (NONIUS κ-CCD). This information is presented in the supplementary material. Magnetic properties were examined on a differential scanning calorimeter for determination of T C (Netzsch 449C Jupiter, Netzsch, Selb, Germany) and a vibrating sample magnetometer (VSM LakeShore 7407, Lake Shore Cryotronics Inc., Westerville, OH, USA).

Solid-State Sintering
Barium hexaferrite crystals were grown in the absence of a flux as well as from Na 2 CO 3 , BaB 2 O 4 , and PbO fluxes to analyze the crystal quality, size, and the resulting magnetic properties. In the case of the PbO flux, a significant substitution of Ba by Pb depending on the growth temperature and flux concentration was observed. Table 1 presents the initial charge compositions, general compositions according to single crystal X-ray diffraction structure refinements, as well as the maximum temperature of the crystal growth process. The obtained chemical compositions were additionally validated using energy dispersive X-ray spectroscopy (EDX) analysis: For all crystals, the derived Fe/(Ba + Pb) ratio was equal to 12 ± 0.2. For the crystals obtained from PbO flux, the Pb content resulted in x = 0.21, 0.45, and 0.81 for a general formula Ba 1-x Pb x Fe 12 O 19 , compared to 0.23, 0.44, and 0.80, respectively, from refinements of X-ray diffraction data obtained on single crystals.

Solid-State Sintering
The optimal temperature of the solid-state synthesis of Ba hexaferrite, which ensures a stable formation and crystal growth, was earlier established experimentally to be 1623 K [31]. Figure 1 shows the crystals obtained at this temperature from mixtures of barium carbonate and iron(III) oxide to range within the µm-size. Within this study, these are the smallest crystallites. While the highest crystal growth temperature was necessary, this underlining the effect of higher diffusion at already Materials 2017, 10, 578 4 of 11 lower temperatures in a flux. Powder X-ray diffraction (PXRD) patterns proof the purity of the product. Unit cell parameters are virtually identical to those reported in the literature (Table 2). Figure 2 presents the powder XRD patterns of the experimental samples and the literature data [32].

Carbonate and Borate Flux Growth
Crystal growth experiments from Na2CO3 and BaB2O4 fluxes were carried out at 1533 K, thus 90 K below the solid-state sintering experiment. Under these conditions, Na2CO3 eventually decomposes to form Na2O. Still, crystal sizes and visual quality were significantly improved (see Figure 1, second and third row).
The most complete analysis of parameters in the BaO-Fe2O3-Na2O system, ensuring the growth of barium hexaferrite, was carried out in 1961 by R. Gambino and F. Leonhard [24]. The optimal ratio of "crystal/solvent" was derived to 73.7/26.3 at %, very close to our initial ratio in the crystal growth experiment. SEM images ( Figure 1) indicate a good surface morphology of the obtained crystals easily approaching the mm-size range, which represent the largest crystals obtained within this study.
The general possibility of barium hexaferrite crystal growth using BaB2O4 as a flux has been established more recently [27]. Experimentally, it was found that a mixture with 30 at % BaB2O4 provides a homogeneous melt at 1533 K. The obtained crystals are only slightly smaller than those from carbonate flux and exhibit a similarly good surface morphology with an apparent low surface defect density.

Carbonate and Borate Flux Growth
Crystal growth experiments from Na 2 CO 3 and BaB 2 O 4 fluxes were carried out at 1533 K, thus 90 K below the solid-state sintering experiment. Under these conditions, Na 2 CO 3 eventually decomposes to form Na 2 O. Still, crystal sizes and visual quality were significantly improved (see Figure 1, second and third row).
The most complete analysis of parameters in the BaO-Fe 2 O 3 -Na 2 O system, ensuring the growth of barium hexaferrite, was carried out in 1961 by R. Gambino and F. Leonhard [24]. The optimal ratio of "crystal/solvent" was derived to 73.7/26.3 at %, very close to our initial ratio in the crystal growth experiment. SEM images ( Figure 1) indicate a good surface morphology of the obtained crystals easily approaching the mm-size range, which represent the largest crystals obtained within this study.
The general possibility of barium hexaferrite crystal growth using BaB 2 O 4 as a flux has been established more recently [27]. Experimentally, it was found that a mixture with 30 at % BaB 2 O 4 provides a homogeneous melt at 1533 K. The obtained crystals are only slightly smaller than those from carbonate flux and exhibit a similarly good surface morphology with an apparent low surface defect density.

PbO Flux Growth
Barium hexaferrite crystals previously have been grown from PbO flux at 1473 K [26]. To investigate the optimal temperature for the growth process applying PbO flux, experiments were carried out with different flux contents of 60, 70, and 80 at % PbO. The mixtures were apparently molten at 1473, 1298, and 1223 K, respectively, and then rapidly cooled to room temperature, during which crystallization occurred. In all experiments, faceted hexagonal crystals of several µm were obtained, however, the amount of crystals and their size decreased with increasing PbO concentration ( Figure 3). According to PXRD analysis, the samples are single phase and of magnetoplumbite-type structure ( Figure 2). Materials 2017, 10, 578 6 of 11

PbO Flux Growth
Barium hexaferrite crystals previously have been grown from PbO flux at 1473 K [26]. To investigate the optimal temperature for the growth process applying PbO flux, experiments were carried out with different flux contents of 60, 70, and 80 at % PbO. The mixtures were apparently molten at 1473, 1298, and 1223 K, respectively, and then rapidly cooled to room temperature, during which crystallization occurred. In all experiments, faceted hexagonal crystals of several m were obtained, however, the amount of crystals and their size decreased with increasing PbO concentration ( Figure 3). According to PXRD analysis, the samples are single phase and of magnetoplumbite-type structure (Figure 2). As was earlier observed [26], the PbO flux causes the growth of crystals with partial substitution of Ba by Pb, resulting in a general composition of Ba1-xPbxFe12O19. Unit cell determination from powder diffraction ( Table 2) already indicates a composition dependence of the lead content x on the flux composition and growth temperature. Due to the larger ionic radius of Ba 2+ compared with Pb 2+ (r(Ba 2+ ) = 1.61 Å; r(Pb 2+ ) = 1.49 Å with CN = 12 [37]), a decreasing unit cell with increasing x is expected. However, upon lower PbO concentrations in the flux (60 at %) the unit cell is even very slightly enlarged, but almost does not change with increasing Pb content. Due to the rigid iron oxide framework with larger Ba 2+ ions as spacers, the unit cell generally hardly changes upon partial substitution by Pb 2+ . However, the pure lead compound has a significant about 1.5% smaller unit cell [36]. In order to obtain a more reliable Pb content incorporated in the crystals, we have undertaken full single crystal structure determinations (see below). One result is the increasing Pb content with increasing Pb concentration in the flux from Ba0.77(2)Pb0.23Fe12O19 at 60 at % PbO over Ba0.56(2)Pb0.44Fe12O19 at 70 at % PbO to Ba0.20(3)Pb0.80Fe12O19 at 80 at % PbO. Earlier, an approximate composition of Ba0.9Pb0.1Fe12O19 was discussed for a PbO flux proportion of 50 at % at 1473 K [26].
Single crystal X-ray diffraction data result in compositions Ba1-xPbxFe12O19 with no indication for any Ba/Pb order. Close inspection of the refinement results revealed a large flat displacement ellipsoid for the mixed occupied Ba/Pb site (Figure 4). Such enlarged displacement parameters for Pb were earlier observed for pure PbFe12O19 [36]. With respect to an apparently stereochemically active electron lone pair of Pb 2+ , a displacement of this ion from the ideal 2d site (invariant 2/3, 1/3, 1/4) to the 12j site (x, y, 1/4) within the space group P63/mmc is discussed, at which Pb is equally distributed over six split-positions resulting in an occupancy of 1/6 ( Figure 5). Due to the partial occupation of the central ideal 2d site by Ba, these split-positions are not sufficiently separated to resolve them from the ideal position and refine them independently. The split-positions are located within the hexagonal ab-plane and thus lead to the observed enlargement of the displacement ellipsoid with the observed shape if modeled with one site at the ideal position for Ba and Pb. Further information on the crystal structure refinements can be found in the supplementary material. As was earlier observed [26], the PbO flux causes the growth of crystals with partial substitution of Ba by Pb, resulting in a general composition of Ba 1-x Pb x Fe 12 O 19 . Unit cell determination from powder diffraction ( Table 2) already indicates a composition dependence of the lead content x on the flux composition and growth temperature. Due to the larger ionic radius of Ba 2+ compared with Pb 2+ (r(Ba 2+ ) = 1.61 Å; r(Pb 2+ ) = 1.49 Å with CN = 12 [37]), a decreasing unit cell with increasing x is expected. However, upon lower PbO concentrations in the flux (60 at %) the unit cell is even very slightly enlarged, but almost does not change with increasing Pb content. Due to the rigid iron oxide framework with larger Ba 2+ ions as spacers, the unit cell generally hardly changes upon partial substitution by Pb 2+ . However, the pure lead compound has a significant about 1.5% smaller unit cell [36]. In order to obtain a more reliable Pb content incorporated in the crystals, we have undertaken full single crystal structure determinations (see below). One result is the increasing Pb content with increasing Pb concentration in the flux from Ba 0.77(2) Pb 0. 23  Single crystal X-ray diffraction data result in compositions Ba 1-x Pb x Fe 12 O 19 with no indication for any Ba/Pb order. Close inspection of the refinement results revealed a large flat displacement ellipsoid for the mixed occupied Ba/Pb site (Figure 4). Such enlarged displacement parameters for Pb were earlier observed for pure PbFe 12 O 19 [36]. With respect to an apparently stereochemically active electron lone pair of Pb 2+ , a displacement of this ion from the ideal 2d site (invariant 2/3, 1/3, 1/4) to the 12j site (x, y, 1/4) within the space group P6 3 /mmc is discussed, at which Pb is equally distributed over six split-positions resulting in an occupancy of 1/6 ( Figure 5). Due to the partial occupation of the central ideal 2d site by Ba, these split-positions are not sufficiently separated to resolve them from the ideal position and refine them independently. The split-positions are located within the hexagonal ab-plane and thus lead to the observed enlargement of the displacement ellipsoid with the observed shape if modeled with one site at the ideal position for Ba and Pb. Further information on the crystal structure refinements can be found in the supplementary material.

Magnetic Properties
Magnetic hysteresis loops taken at room temperature for all discussed samples are presented in Figures 6 and 7. Important magnetic parameters such as Curie temperature (derived from DSC (differential scanning calorimetry) measurements), saturation magnetization, and coercivity are also summarized in Table 2. Literature data on magnetic characteristics of BaFe12O19 vary within comparatively large ranges [34−36] and apparently depend on the applied synthesis technique, however, they are consistent with our data. As observed earlier, the saturation magnetization of fluxgrown samples is higher than those of powder-sintered materials [38]. The coercivity of powdersintered samples differs significantly from those grown from flux, although the Ms values are very close. This observation can be explained since the coercivity depends on the grain sizes ( Figure 1).

Magnetic Properties
Magnetic hysteresis loops taken at room temperature for all discussed samples are presented in Figures 6 and 7. Important magnetic parameters such as Curie temperature (derived from DSC (differential scanning calorimetry) measurements), saturation magnetization, and coercivity are also summarized in Table 2. Literature data on magnetic characteristics of BaFe12O19 vary within comparatively large ranges [34−36] and apparently depend on the applied synthesis technique, however, they are consistent with our data. As observed earlier, the saturation magnetization of fluxgrown samples is higher than those of powder-sintered materials [38]. The coercivity of powdersintered samples differs significantly from those grown from flux, although the Ms values are very close. This observation can be explained since the coercivity depends on the grain sizes ( Figure 1).

Magnetic Properties
Magnetic hysteresis loops taken at room temperature for all discussed samples are presented in Figures 6 and 7. Important magnetic parameters such as Curie temperature (derived from DSC (differential scanning calorimetry) measurements), saturation magnetization, and coercivity are also summarized in Table 2. Literature data on magnetic characteristics of BaFe 12 O 19 vary within comparatively large ranges [34−36] and apparently depend on the applied synthesis technique, however, they are consistent with our data. As observed earlier, the saturation magnetization of flux-grown samples is higher than those of powder-sintered materials [38]. The coercivity of powder-sintered samples differs significantly from those grown from flux, although the M s values are very close. This observation can be explained since the coercivity depends on the grain sizes ( Figure 1).  The samples grown from Na2CO3 (Na2O, respectively) and BaB2O4 fluxes show the highest values of Hc, which are in agreement with earlier derived numbers for materials obtained from such fluxes [35] and are likely due to the low impurity levels. As a result of partial substitution of Ba by Pb, all samples grown from PbO flux exhibit reduced values of all magnetic properties. However, there is hardly any significant dependence of magnetic properties on the Pb content achieved in our study (Figure 7).

Conclusions
Barium hexaferrite samples BaFe12O19 with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes as well as m-size powder-sintered samples were investigated. Fluxes are able to reduce the growth temperature compared to solid-state sintering, but may supply impurities or additional challenges due to evaporation. The Na2CO3 flux method provides the largest crystals and can be used to obtain materials of superior quality, since coercivity and magnetization suffer from a  The samples grown from Na2CO3 (Na2O, respectively) and BaB2O4 fluxes show the highest values of Hc, which are in agreement with earlier derived numbers for materials obtained from such fluxes [35] and are likely due to the low impurity levels. As a result of partial substitution of Ba by Pb, all samples grown from PbO flux exhibit reduced values of all magnetic properties. However, there is hardly any significant dependence of magnetic properties on the Pb content achieved in our study (Figure 7).

Conclusions
Barium hexaferrite samples BaFe12O19 with sizes up to 5 mm grown from PbO, Na2CO3, and BaB2O4 fluxes as well as m-size powder-sintered samples were investigated. Fluxes are able to reduce the growth temperature compared to solid-state sintering, but may supply impurities or additional challenges due to evaporation. The Na2CO3 flux method provides the largest crystals and can be used to obtain materials of superior quality, since coercivity and magnetization suffer from a The samples grown from Na 2 CO 3 (Na 2 O, respectively) and BaB 2 O 4 fluxes show the highest values of H c , which are in agreement with earlier derived numbers for materials obtained from such fluxes [35] and are likely due to the low impurity levels. As a result of partial substitution of Ba by Pb, all samples grown from PbO flux exhibit reduced values of all magnetic properties. However, there is hardly any significant dependence of magnetic properties on the Pb content achieved in our study (Figure 7).

Conclusions
Barium hexaferrite samples BaFe 12 O 19 with sizes up to 5 mm grown from PbO, Na 2 CO 3 , and BaB 2 O 4 fluxes as well as µm-size powder-sintered samples were investigated. Fluxes are able to reduce the growth temperature compared to solid-state sintering, but may supply impurities or additional challenges due to evaporation. The Na 2 CO 3 flux method provides the largest crystals and can be used to obtain materials of superior quality, since coercivity and magnetization suffer from a strong dependence on impurity values. This study shows the possibility to control the magnetic properties of BaFe 12 O 19 in a wide range by using different synthesis techniques.
For PbO-containing fluxes, the lowest melting temperature of 1223 K was observed for a solution with 80 at % PbO, which is significantly lower than the melting temperature of pure barium hexaferrite (1853 K) [21]. However, the Pb content increases with flux Pb content. According to magnetization measurements, the substitution by Pb reduces the values for Curie temperature, saturation magnetization, and coercivity, but these important material characteristics do not significantly depend on the degree of substitution.
From the materials grown in this study, the largest single crystals obtained from Na 2 O flux are optimal for the production of spherical isolator elements. The crystals showed diameters up to 7 mm and thicknesses up to 5 mm. BaB 2 O 4 and PbO fluxes can be used for the production of single crystals with large aspect ratios for flat/plane attenuator manufacturing, since crystal diameters ranged up to 5 mm, while typically exhibiting thicknesses below 1 mm. However, lead oxide shows a high evaporation rate at the applied growth temperatures, and thus presents environmental issues during its use as a flux as well as in the final material lead content. The solid-state sintering synthesis is ideally suited for the cost-effective production of powder for absorbing coatings.