Mössbauer and magnetic properties of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂

Abstract

We have studied the crystal structure and magnetic properties of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ using X-ray diffraction (XRD), a vibrating sample magnetometer (VSM) and a Mössbauer spectrometer. We analyzed the XRD patterns to apply FullProof Riedvelt refinement. From the XRD analysis, we found the crystal structure of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ is hexagonal(R-3m). The temperature-dependent magnetic properties of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ were measured by a VSM. The hysteresis loops were measured at several temperatures. From the zero-field-cooled–field-cooled (ZFC–FC) experiment, the spin re-orientation temperature (T_s) of this sample was 209 K. In the results of the Mössbauer spectrometer experiment from 4.2 to 295 K, the magnetic hyperfine fields (H_hf) decreased with respect to increase in temperature. The isomer shift values were between 0.1 and 0.4 mm/s, and represented the ion states of the sample, which were the balanced Fe³⁺ state at all temperatures. H_hf and quadrupole splitting values showed abrupt change at 209 K, we deduced that this is occurred because of spin re-orientation.

1 Introduction

Hexaferrite is a multiferroic material that is classified into M, U, W, Y, X and Z according to the chemical formula. Among them, the Y-type hexaferrite has the chemical structure of (Ba, Sr)₂Me₂Fe₁₂O₂₂ (Me = a bivalent transition metal) [1, 2]. The Y-type hexaferrite has attracted considerable attention owing to its great magnetic properties. Recently, studies on the reavlation of its magnetoelectric (ME) effect at room temperature are being done actively. In previous work, a study result on the ME effect expressed at room temperature were presented. [3, 4]. Some of Y-type hexaferrite (Ba₂Mg₂Fe₁₂O₂₂, Ba_0.5Sr_1.5Ni₂(Fe_0.97Al_0.03)₁₂O₂₂) samples were reported to exhibit the ME effect at room temperature [5, 6]. These reports have led to research on substitution and doping for many elements, especially for transition metal. It is expected that the substitution and doping lead to various property changes. In the case of Ba₂Zn₂Fe₁₂O₂₂, the nonlinear ME effect was reported at microwave frequencies [7]. Additionally, a study on Mg doped Ba₂Zn₂Fe₁₂O₁₂ reported that the Mg doped Y-type hexaferrite has a helical spin structure at low temperature [14].

In this study, we synthesized the Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ by a solid state reaction method. We then evaluated the magnetic properties of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ using the VSM and Mössbauer spectrometer. The purpose of this study was to observe the change of the magnetic properties by doping Mg ions to Ba₂Co₂Fe₁₂O₂₂.

2 Experiments and discussion

Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ was synthesized by solid-state reaction method. High-purity powders of BaCO₃(99.98%), CoO(99.99%), MgO(99.999%), Fe₂O₃(99%) were used as the starting materials of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂. This sample was finely ground in agate mortar for 1 h to create a mixture. And then this mixture was calcinated at 1000 °C. After grinding this sample again for 1 h, it was palletized into a cylindrical shape. The palletized sample was sintered at 1100 °C for 10 h in air. To remove BaFe₂O₄, frequently generated secondary phase of barium hexaferrite, we carried out final sintering at 1150 °C for 10 h in air. The crystal structure was measured by XRD with CuKα radiation (λ = 1.5406 Å). The XRD results were analyzed by Rietveld refinement with FullProf. The magnetic properties of the sample were measured by VSM. We measured the M–H hysteresis loop at several temperatures as well as M–T curves (ZFC–FC) from 4.2 to 295 K. To obtain the Mössbauer spectra using Mössbauer spectroscopies with a moving ⁵⁷Co source, we performed Mössbauer experiments from 4.2 to 295 K.

By analyzing the XRD data, we found that the crystal structure of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ was hexagonal and it belonged to the R-3m group as shown in Fig. 1. The lattice constants were obtained a₀ = 5.864 Å and c₀ = 43.531 Å. This is smaller than Ba₂Co₂Fe₁₂O₂₂ but the ratio between c₀ and a₀ (c₀/a₀) is remained 7.423 [8]. These decreases were caused by the ionic radius of Mg²⁺ (0.72 Å) being smaller than that of Co²⁺ (0.745 Å).

Fig. 1

XRD patterns of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂

We measured the M–H hysteresis loops at several temperatures as shown in Fig. 2. The coercivity (H_c) of the sample decreased with respect to increasing the temperature. This is because the magnetic planar anisotropy was decreased [13]. We measured the temperature dependence of magnetization (ZFC–FC) by VSM. This was measured under external magnetic field (100 Oe) from 4.2 to 295 K. Figure 3 shows the result of the ZFC–FC measurement. The ZFC curve (red curve) rose to 209 K with respect to the temperature and had the biggest magnetization at 209 K. After 209 K, it slowly decreases up to 295 K owing to the spin re-orientation. Under 209 K, the spin structure is helical, and becomes a collinear ferrimagnetic after 209 K [9]. Referring to the T_s of Ba₂Co₂Fe₁₂O₂₂ of 215 K [10] and Ba₂Co_1.5Mg_0.5Fe₁₂O₂₂ at 203 K [11, 12], we decided that the T_s of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ is 209 K. From these results, it can be observed that T_s decreases as the doped Mg increases. The reason of this decrease was predicted that Mg ions substitution at Co ions induced the decrease of the magnetic anisotropy [15]. In general, as the magnetic anisotropy decreases, the energy is required to change the spin structure. This is because the tendency of the spin to align in one direction decreases. Therefore, spin re-orientation occurs at a lower temperature.

Fig. 2

a Temperature dependence hysteresis loops and b H_c–T, graph for Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂

Fig. 3

Zero-field-cooled – field-cooled (ZFC–FC) curve Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂

Figure 4 shows the Mössbauer spectra from 4.2 to 295 K. We fitted these spectra using the least squares method with six Lorentzian sextets of Fe sites (3b_VI, 6c^*_IV, 6c_VI, 18h_VI, 6c_IV, and 3a_VI). The parameters of Mössbauer are listed in Table 1. Figure 5 shows that H_hf sites decreased with respect to increase in temperature because the super exchange interaction of the linkage of the Fe³⁺–O²⁻–Fe³⁺ was getting weak [12]. For all the 209 K, the H_hf curves exhibited a change in the slopes; this is because the spin re-orientation occurred at 209 K. For all temperature values and Fe sites, the isomer shift (δ) values were maintained between 0.1 and 0.4 mm/s. We confirmed that the ion states of Fe were in the balanced Fe³⁺ state. We represent the quadrupole splitting values for 4.2–295 K in Fig. 6. Below 209 K temperature, although the temperature was changed, the quadrupole splitting values were stable. However, abrupt changes occurred through the 209 K. The quadrupole splitting is a value that changes with the gradient of the magnetic field; this abrupt change can be explained by the change in the structure of the magnetic hyperfine field. In general, the Y-type hexaferrite under the T_s has the helical spin structure. After T_s, the structure changes to a collinear ferrimagnetic spin structure. The helical and collinear ferrimagnetic spin structure created different magnetic structure. Therefore, through the spin re-orientation temperature, the quadrupole splitting graph showed abrupt changes and we confirmed that the T_s is 209 K.

Fig. 4

Mössbauer spectra of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ from 4.2 to 295 K

Table 1 Mössbauer parameters of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ from 4.2 to 295 K

Fig. 5

Magnetic hyperfine field (H_hf) of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ from 4.2 to 295 K

Fig. 6

Electric quadrupole splitting (ΔE_Q) of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ from 4.2 to 295 K

3 Conclusion

In conclusion, we studied Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ using a XRD, a VSM and a Mössbauer spectrometer. We defined the crystal structure of Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ as hexagonal through the XRD analysis. By measuring the zero-field-cooled and field-cooled from 4.2 to 295 K, we determined that the T_s is 209 K. The spin structure changed from helical magnetic to collinear ferrimagnetic at T_s. Using the Mössbauer spectrometer, we evaluated the temperature dependence of the magnetic hyperfine field, the isomer shift and the electric quadrupole splitting. The magnetic hyperfine fields decreased as the temperature increased because the super exchange interactions between Fe³⁺ and O²⁻ ions weakened. The isomer shift values were maintained between 0.1 and 0.4 mm/s, and the Fe ions in Ba₂Co_1.7Mg_0.3Fe₁₂O₂₂ existed in the balanced Fe³⁺ state. The electric quadrupole splitting values exhibited abrupt changes at 209 K; we deduced that this was caused by spin re-orientation.