Hexagonal strontium ferrite has found wide application in the fabrication of permanent magnets and active media of UHF electronics and photonics. The performance of hexagonal strontium ferrites depends largely on their synthesis method. Hexagonal polycrystalline isotropic SrFe₁₂O₁₉ ferrite specimens have been synthesized using fast electron beam radiation-thermal sintering (RTS) in a PLU-6 electron accelerator of G.I. Budker Institute of Nuclear Physics of the Russian Academy of Sciences. The RTS temperature has been varied from 1200 to 1400 °C, and the RTS time, from 10 to 90 min. The phase composition and lattice parameters of the specimens have been studied using X-ray diffraction on a DRON-8 diffractometer in CoK_α1 radiation. The density of the specimens has been measured following Archimedean law on a UW620H electronic balance with a liquid density measurement attachment. The magnetic hysteresis loop parameters of the specimens have been studied with a VM-07 vibromagnetometer.

The specimens has proven to be single-phase with a P63/mmc space group (No. 194), corresponding to a hexagonal ferrite structure. For the first time, changes in the magnetic hysteresis loop parameters of hexagonal SrFe₁₂O₁₉ strontium ferrite specimens synthesized with fast electron beam RTS have been studied. We show that the width and intensity of the magnetic hysteresis loop decrease noticeably with an increase in the RTS temperature from 1200 to 1350–1400 °C. The magnetization coercive force _jH_c decline to the minimum values at RTS temperatures T_sint ≥1250 °C, probably due to recrystallization in the ferrite lattice: the combined impact of RTS temperature and radiation-induced diffusion triggers intense grain growth, significantly reducing the number of domain boundaries and, hence, causing a drop in _jH_c. The high energy efficiency of RTS in ferrite ceramics synthesis has been confirmed for the example of SrFe₁₂O₁₉.

radiation-thermal sintering, polycrystalline hexagonal isotropic ferrite SrFe₁₂O₁₉, X-ray diffraction, fast electrons, electron accelerator, density, lattice magnetic hysteresis loop parameters, magnetization, coercive force

M-type hexagonal SrFe₁₂O₁₉ strontium ferrite has a crystalline structure that is isomorphic to that of magnetoplumbite (PbFe₁₂O₁₉) [1]. Due to the difference in the ionic radii of barium and strontium, a substitution of the former for the latter in the hexagonal ferrite lattice increases the magnetic crystallographic anisotropy constant K by 10 % [2]. Furthermore, SrМ hexaferrites have better processability as compared to BaFe₁₂O₁₉ hexaferrites [2]. Due to its combination of magnetic properties and performance, SrМ ferrite has found wide application in the fabrication of permanent magnets [1, 2], active media of millimeter-wavelength UHF electronics devices [3], and THz photonics devices [4]. In science and engineering, there is still great interest in the development of UHF electronics devices based on thick hexagonal ferrites films [5, 6]. Today, the main industrial synthesis process for the above materials is the ceramic technology [1, 2, 5]. Typical process diagram of polycrystalline hexagonal SrFe₁₂O₁₉ ferrites is shown in Fig. 1. Along with the commonly known advantages, the ceramic technology has a number of drawbacks. One of the main drawbacks is the high energy consumption. Alternative SrFe₁₂O₁₉ hexaferrite technology can be radiation-thermal sintering (RTS) which has been well tested for a wide range of ferrites [7–9], including BaМ [7]. The difference of the RTS process from the conventional ceramic one is that the thermal sintering of raw billets is replaced for fast electron beam sintering in an electron accelerator [7–9]. The aim of this work is to explore the possibility of synthesis of high-quality isotropic polycrystalline SrFe₁₂O₁₉ hexaferrites using the RTS technology and to study the magnetic properties of synthesized hexaferrite specimens.

Figure 1.

Typical synthesis process route of isotropic polycrystalline hexagonal ferrites using ceramic technology

The iron-containing raw material for the preparation of the test specimens was MM-2 Grade Fe₂O₃ iron oxide (TU6-09-4816–80, impurity contents (wt.%) 0.01 SiO₂, 0,01 Al₂O₃, 0,01 Cr₂O₃, 0,01 Na₂O). The strontium containing raw material was Ch Grade (99.0% purity) crystalline strontium carbonate from Isfara Chemical Factory (Tajikistan). The raw billets were prepared in accordance with the diagram presented in Fig. 1. The preparation route included all conventional operations except thermal sintering. The final operation was replaced for RTS. The replacement of conventional sintering for fast electron beam RTS was chosen due to the far lower energy consumption and higher sintering quality delivered by fast electron beam RTS [7–9]. For RTS, along with the high-temperature treatment, radiation-enhanced diffusion contributes significantly to the process. The combination of the above factors delivers sintering at lower temperatures and in a shifter time [7–9]. Radiation-thermal sintering of the specimens was carried out in a fast electron beam of an ILU-6 electron accelerator used for radiation technologies (maximum fast electron energy 2.5 MeV) at the G.I. Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences [10]. Fast electron beam sintering of hexagonal ferrites was carried out in a cell for ferrite ceramic sintering designed at the Chair of Electronics Materials of the MISIS Research and Technology University Table 1 summarizes data on the SrМ specimens.

X-ray structural analysis of the test specimens was carried out on a DRON-8 X-ray diffractometer (NPP Burevestnik JSC, St. Petersburg, Russia) in CoK_α1 radiation (λ = 0.178897 nm) with Bragg-Brentano focusing by two Soller slots at room temperature.

The density of the specimens was measured using Archimedean law on a UW620H electronic balance with a liquid density measurement attachment and calculated using the following formula:

$\rho ={\rho }_0\left(\frac{W_a}{W_a-W_l}\right),$ (1)

where ρ is the specimen density (g/cm³) ρ₀ is the known liquid density (g/cm³), W_a is the specimen weight in air (g), and W_l is the liquid sample weight (g).

The X-ray density of the ferrites was calculated based on X-ray analysis data using the following formula:

${\rho }_{X-\mathrm{ray}}=\frac{2M_A}{V{N}_A}=\frac{2M_A\cdot 2}{c{a}^{2}3\sqrt{3}{N}_A},$ (2)

where M_A is the molecular weight of hexaferrite, c and a are the lattice parameters of hexaferrite, N_A is the Avogadro number, and V is the unit cell volume. The average density proved to be 5.1 g/cm³.

The hysteresis loops of the 1.0−3.0 mm diam. spherical sintered hexaferrite specimens were recorded using a VM-07 vibromagnetometer.

Analysis of the X-ray diffraction spectra showed that the synthesized specimens were single-phase (space group P6₃/mmc (No. 194), i.e., hexagonal ferrites. Figure 2 shows RTS-synthesized SrFe₁₂O₁₉ hexagonal ferrite specimen density as a function of (a) sintering time at T = 1300 °C and (b) of RTS temperature for sintering time 30 and 60 min.

Figure 2.

SrFe₁₂O₁₉ hexaferrite specimen density as a function of (a) RTS time at 1300 °C and (b) RTS temperature

The density of SrFe₁₂O₁₉ decreases with an increase in the RTS time at 1300 °C (Fig. 2a) and increases with the RTS temperature (Fig. 2b). It can be seen from Fig. 2 that the optimum RTS mode for SrFe₁₂O₁₉ is 1250–1300 °C for 30–60 min.

The decrease in the polycrystal density for RTS beginning from a certain time seems to be accounted for by the fact that, upon reaching the optimum density of the material, the energy of fast electrons starts to exert the contrary impact, i.e., lattice degradation, making the polycrystal structure looser [11–14].

Figure 3 shows magnetic hysteresis loops for the specimens synthesized with 60 min RTS.

Figure 3.

Magnetic hysteresis loops for polycrystalline SrFe₁₂O₁₉ hexagonal isotropic ferrite specimens synthesized using RTS at different temperatures: (a) 1200 °C, (b) 1250 °C, (c) 1300 °C, and (d) 1400 °C. Sintering time 60 min.

Figure 4 shows magnetic hysteresis loops for the specimens synthesized with RTS at 1300 °C for different time.

Figure 4.

Magnetic hysteresis loops for polycrystalline SrFe₁₂O₁₉ hexagonal isotropic ferrite specimens synthesized using RTS at different time: (a) 10 min, (b) 30 min, (c) 60 min, and (d) 90 min. Sintering temperature 1300 °C

It can be seen from Figs 3 and 4 that the intensity and width of the magnetic hysteresis loops decrease with an increase in the sintering temperature and time.

Figure 5 shows specific saturation magnetization σ_s and specific remanent magnetization of the RTS SrFe₁₂O₁₉ specimens as a function of (a and b) RTS temperature for 60 min sintering time and (c and d) sintering time for 1300 °C RTS temperature. Figures 6, 7, 8, 9 show coercive force _jH_c, magnetic anisotropy field H_k, magnetic hysteresis loop area S_loop, and magnetic hysteresis loop rectangularity, respectively, as a function of RTS temperature and time.

Figure 5.

(a and c) Specific saturation magnetization σ_s and (b and d) specific remanent magnetization of RTS SrFe₁₂O₁₉ specimens as a function of (a and b) RTS temperature and (c and d) RTS time

Figure 6.

Magnetization coercive force of SrFe₁₂O₁₉ specimens as a function of (a) RTS temperature and (b) RTS time

Figure 7.

Magnetic anisotropy of SrFe₁₂O₁₉ specimens as a function of (a) RTS temperature and (b) RTS time

Figure 8.

Magnetic energy (magnetic hysteresis loop area) of SrFe₁₂O₁₉ specimens as a function of (a) RTS temperature and (b) RTS time

Figure 9.

Magnetic hysteresis loop rectangularity coefficient of SrFe₁₂O₁₉ specimens as a function of (a) RTS temperature and (b) RTS time

As can be seen from Figs 5, 6, 7, 8, 9, the magnetic hysteresis parameters of the SrFe₁₂O₁₉ specimens change in a similar manner during RTS with an increase in the RTS temperature and time. RTS temperature determines the shape and parameters of the hysteresis loop. With an increase in the RTS temperature and time, all magnetic hysteresis loop parameters of the test specimens decline in comparison with their parameters for the lowest RTS temperature and shortest time.

The RTS temperature growing from 1200 to 1350–1400 °C (sintering time 60 min), the specific saturation magnetization σ_s declines by 32 %; the RTS time increasing from 10 to 90 min (sintering temperature 1300 °C), σ_s drops by 29.5 %. The specific remanent magnetization σ_r for the same RTS modes drops by 93 and 91 %, respectively; the magnetization coercive force _jH_c declines by 96 % in both cases; the magnetic field anisotropy H_k decreases by 81 and 82 %, respectively; the magnetic energy (magnetic hysteresis loop area) drops by 79 and 80 %, respectively; the magnetic hysteresis loop rectangularity coefficient drops by 88 and 90 %, respectively. Similar changes, though less intense, were observed [15] for RTS-synthesized hexagonal BaFe₁₂O₁₉ ferrites.

It was shown that the RTS technology allows ferrite sintering in several decades of minutes. The predominant factor was the RTS temperature, differing for each type of ceramics. The physical mechanism of ceramic sintering process acceleration by fast electrons is not yet clear [16–23]. Most researchers believe that the predominant mechanism of radiation acceleration of mass transport in ferrite ceramics is surface recombination. Irradiation of ceramics billets induces electron excitation in grains and powder compacts. Excited electrons tend to localize at grain boundaries and recombine, releasing energy. This results in the formation of temperature gradients, causing thermal diffusion flows which significantly accelerate solid state reactions of ferrite synthesis.

The results reported herein show that, for the RTS modes used, the highest SrFe₁₂O₁₉ hexaferrite density was obtained for sintering temperature T_sint = 1200÷1300 °C and sintering time t_sint = 60 min, and at sintering temperature T_sint = 1300÷1400 °C and sintering time t_sint = 30 min. However, at RTS temperature ≥1350 °C, even for a sintering time of 30 min, the magnetization drops by 30 % and the coercive force, by 96 %. Thus, strontium hexaferrite, in fact, becomes low-magnetization and low-coercive force hexaferrite, i.e., a magnetically soft material. This material cannot be used for the fabrication of high-quality permanent magnets. The problem of its applications in other conventional application domains of hexagonal ferrites requires further study. The experiments carried out in this work showed that the existing RTS process modes for polycrystalline SrFe₁₂O₁₉ require improvement. Yet it is obvious that the RTS temperature should be within 1250–1300 °C.

The abrupt decline of _jH_c at RTS temperatures of above ≥1250 °C can be accounted for by recrystallization in the material. Elevated RTS temperatures cause intense grain growth. The overall quantity of domain boundaries decreases abruptly, also leading to a significant decrease in the coercive force [24].

Changes in the parameters of magnetic hysteresis loop parameters in hexagonal SrFe₁₂O₁₉, strontium ferrite synthesized using fast electron beam RTS were studied for the first time.

The high energy efficiency of RTS technology of ferrite ceramics was confirmed for the example of M-type hexagonal strontium ferrite.

We showed that the RTS modes tested are not suitable for the synthesis of high-quality polycrystalline SrFe₁₂O₁₉ hexaferrites and require improvement.

The significant decline of _jH_c of the SrFe₁₂O₁₉ specimens at T_sint ≥1250 °C is accounted for by recrystallization and intense grain growth due to high temperatures and radiation-enhanced diffusion.

The authors are grateful to M.V. Korobeinikov and M.A. Mikhailenko for carrying out radiation-thermal sintering of the specimens.

The authors claim no conflict of interests.

This work was funded at the expense of project Russian Science Foundation No. 24-13-00268.

No AI tools were used in the work on the article.