Samples of ceramics based on barium ferrite (FB) and sodium potassium niobate (KNN) are obtained and studied. The elemental composition of the obtained composite is analyzed. A study of pyroelectric properties and magnetic hysteresis loops showed that the composite based on barium ferrite and sodium potassium niobate ceramics obtained in this work had magnetic and ferroelectric properties.
Ferroelectromagnetic substances have been studied since the middle of the 20th century. Their main feature is a combination of the properties of ferroelectrics and ferro- or antiferroelectromagnets . The practical applications and prospects for using ferroelectromagnets are due to their properties. In crystalline ferroelectrics–antiferroelectromagnets (of which one of the best known and most studied is bismuth ferrite BiFeO3 ), there is a strong connection between the electric (polarization), magnetic, and elastic subsystems, as a result of which we can control the acoustic characteristics of the crystal. The resonant magnetoelectric effect in ferroelectric substances, which manifests as a shift of the magnetic resonance line under the action of an electric field, allows them to be used at ultrahigh frequencies and in creating electrically controlled modulators, switches, filters, phase shifters, and power sensors . The combination of their radar-absorbent properties, high values of the real part of the dielectric constant, and high dielectric losses allow such composites to be used in radioelectronic devices that operate in the microwave (microwave) range.
Bismuth ferrite is modified with various additives to improve its multiferriidal properties, [4, 5]. The most promising of these for practical applications are composite materials in which there is a bulk combination of ferroelectric and magnetic materials. Such composites are currently created in two forms: layered composites, which are samples with alternating layers of ferroelectric and magnetic materials [6, 7], and polymer magnetic substances, which include ferroelectric ceramics . By controlling the composition of composite magnetodielectrics, we can create samples with the values of dielectric and magnetic permeability needed for practice.
Examples of ceramics based on barium ferrite (FB) and sodium-potassium niobate (KNN) were obtained and investigated in this work. Initial ceramics FB and KNN were synthesized using standard technologies. Samples of FB + KNN were sintered at T = 1100°C. The initial components were taken in percentages of FB 20, KNN 80, and 50/50 vol %. Pure KNN sintered at the same temperature was used as the control sample. At the first stage, we studied the structure and control of the elemental composition on a JEOL 6510LV scanning electron microscope.
In KNN ceramics, the grains have a cubic shape with dimensions of around 2.5 μm. Adding barium ferrite to KNN produced hexagonal grains with sizes of up to 2 μm in the ceramic structure. The dimensions of cubic grains at the same time fell to 0.5–1.0 μm and their distribution became denser.
The elemental composition was determined via energy-dispersive analysis (on an Oxford INCA Energy 350 unit (Oxford Instruments) in the secondary electron mode with an accelerating voltage of 15 kV. During each experiment, spectra were measured and processed from the surfaces of the samples, and on chips at individual points totaled over a rectangular area and over a grid. Table 1 shows the results from measuring the molar concentrations of elements for the KNN + FB (50/50%) composite, obtained at different points on the surface and the lateral shear of the sample. We can see there is a wide scatter in the ratio of the elements included the composite. Only oxygen and iron atoms were present throughout the volume.
To clarify the distribution of elements, we performed a statistical analysis of the elemental composition at 170 points along the grid on the lateral cleavage of the sample (fields 1–3), in places corresponding to different grain structures (Fig. 1). The areas containing only FB (field 1) and the KNN + FB composite (fields 2 and 3) were analyzed separately. The analysis showed that in the region with a predominance of FB (field 1), the oxygen content is inversely proportional to the content of iron, while its content was uniform in the other two regions (fields 2 and 3). For a more detailed analysis, diagrams of the distribution of (K + Na)/Nb for KNN and Ba / Fe for FB were constructed by assuming that KNN and FB materials do not chemically react with one another, (Fig. 2). According to the chemical formula, the ratio (K + Na) /Nb for KNN should be 1/1 (curve 1 in Fig. 2a). As we can see, only some points correspond to the chemical formula, while the rest lie near curve 2 (Fig. 2a) corresponding to the ratio (K + Na)/Nb 2/3. This was apparently due to the redistribution of potassium atoms over the volume because of its volatility. Even though elements Na and Nb are present only at separate points in region 1, K is distributed evenly.
Barium ferrite can exist in different Ba/Fe ratios . The curves corresponding to compounds BaFe12O19 (curve 1), BaFe2O4 (curve 2) and Ba3Fe4O9 (curve 3) are superposed on the Ba/Fe diagram (Fig. 2b). As can be seen, these compounds, if present in pure form, are at separate points. There is generally a mixture of them. The presence of compound BaFe12O19 in the samples of ceramic composite KNN + FB obtained in this work is demonstrated by the magnetic hysteresis loops observed in them (Figure 3a), since only this compound  has magnetic properties. Magnetic hysteresis loops were measured on a LaskeShote model 7404 vibration magnetometer in fields of up to 16 kE at room temperature.
The temperature dependences of the dielectric permeability (Fig. 3b) and the pyroelectric response were examined to confirm the existence of ferroelectric properties. The pyroelectric response was measured dynamically using rectangular modulation of a heat flow , the source of which was an IR laser, at a modulation frequency of 10 Hz on the sides corresponding to the positive (+Ps) and negative (−Ps) ends of the polarization vector.
The maximum on the temperature dependence of the dielectric permeability was observed for all samples. Adding KNN 20% FB to ceramics only raised the maximum temperature and increased the dielectric losses at high temperatures (curve 1 in Fig. 3b), compared to pure KNN ceramics (curve 3 in Fig. 3b).
At the same time, the 80/20 KNN + FB composite had no direct current conductivity. To study the pyroelectric response, the sample was polarized in an electric field of 1 kV/mm. Adding barium ferrite to the composition of the KNN ceramics did not alter a specific feature of KNN ceramics, in which the pyroelectric response is asymmetric: i.e., it is much stronger on the side corresponding to −Ps than on the one corresponding to +Ps, but it led to a 250% increase in the pyroresponse.
Our study of the magnetic hysteresis loop (Fig. 3a), the temperature dependence of dielectric permeability (Fig. 3b), and the pyroelectric response indicates the obtained samples of the ceramic composite of barium ferrite and sodium-potassium niobate were ferroelectromagnets; i.e., they had both magnetic and ferroelectric properties. At the same time, since the specific properties of KNN ceramics do not allow uniform polarization of a sample, the authors plan to continue research in this direction with other ferroactive compositions.
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This work was performed as part of State Task no. 3.8032.2017/BCh from the RF Ministry of Education and Science.
Translated by M. Drozdova
This article relates to the special issue “Ordering in Minerals and Alloys” (2020, vol. 84, no. 9).
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Malyshkina, O.V., Shishkov, G.S., Ivanova, A.I. et al. Composite Magnetoelectrics Based on Ceramics of Sodium Potassium Niobate and Barium Ferrite. Bull. Russ. Acad. Sci. Phys. 84, 1422–1424 (2020). https://doi.org/10.3103/S1062873820110167