Preparation and Magnetic Properties of La-Substituted Strontium Hexaferrite by Microwave-Assisted Sol-Gel Method

Abstract

Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) hexaferrites were prepared by microwave-assisted sol-gel method. The thermal decomposition process, structural, and magnetic properties of the products were studied by thermal differential scanning calorimeter (DSC), thermogravimetry (TG), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). The phase of α-Fe₂O₃ appeared at x = 0.25 and x = 0.5. The coercivity of La³⁺-substituted strontium hexaferrites is improved to 5960.2 Oe at x = 0.25.

1 Introduction

M-strontium hexaferrites have been widely used in communication, electromagnetic wave absorbers, and magnetic recording medium for their good chemical stability, high magnetization, excellent corrosion resistivity, and low manufacturing cost [1–5]. The magnetization and intrinsic magnetic properties of SrFe₁₂O₁₉ hexaferrites result from the unfinished offset of Fe³⁺ magnetic moment and the partial substitution for Sr²⁺ sites in different crystallographic sites, respectively [6, 7]. The rare earth elements, such as La³⁺ [6, 7], usually were used to replace the Sr²⁺ of crystallographic sites to modify the coercivity. Many previous studies focused on the improvement of the magnetic characteristics by the preparation of thin films and submicron particles [8–10]. Sol-gel auto-combustion is an advanced preparation method of ultrafine power because of cheap precursor, low reaction temperature, and accurate control of material composition. Moreover, the process of calcination has great effects on magnetic properties. Microwave-assisted synthesis has been paid more attention since the first discussion in 1986 [11–14]. The heating energy in the process of microwave-assisted calcination derives from the interaction between materials and microwave, instead of originating from external sources, which can improve the diffusion coefficient, reduce calcination temperature, shorten reaction time, and endow materials with some new characteristics [15]. In this work, the structural, magnetic properties of La-substituted strontium hexaferrites have been studied by employing microwave-assisted sol-gel method.

2 Experimental

The analytically pure of Sr(NO₃)₂ ∙ 3H₂O, La(NO₃)₃ ∙ 6H₂O, Fe(NO₃)₃ ∙9H₂O and citric acid with stoichiometric amounts were dissolved in a certain amount of deionized water. After the solution was stirred homogeneously at 80 °C, the pH value was adjusted to 6 through aqua ammonia solution. The heating and stirring process continued until it formed a viscous dark-green sol. Subsequently, the sol was dried at 120 °C to be brown gel. The self-combustion process took place at 175 °C and the gray fluffy precursor formed. The precursor was calcined at 800 °C for 80 min in microwave furnace with a heating rate of 40 °C/min. The Setaram labsys TG/DSC was applied to detect the phase transformation of precursor with a heating rate 20 °C/min. The crystal structure of calcined samples was detected by D/max-2000 X-ray diffraction with Cu Ka radiation. The phase volume percent was calculated on the basis of the highest diffraction peaks intensity of the different phases from the XRD patterns. The magnetic properties were measured via the vibrating sample magnetometer (VSM) with a maximum applied magnetic field of 1.8 T at room temperature.

3 Results and Discussion

Figure 1 illustrates the TG/DSC curves of the Sr_1−x La _x Fe₁₂O₁₉ (x = 0.15, 0.25, 0.5) precursors. The weight loss appears at 100 °C deriving from the evaporation of free water and bound water in the precursors. A precipitous exothermic peak is observed from 250 to 425 °C, which can be attributed to the citrates decomposition and the formation of metal oxides. Weight loss also occurs from 580 to 800 °C, accompanied with a minor exothermic peak at about 780 °C, which suggests the formation of M-strontium ferrite [16, 17]. Those curves show that the substitution of La³⁺ almost has no effect on the phase transition temperature of the precursors. According to our previous study [16, 18], the precursors are calcined at 800 °C for 80 min in microwave furnace.

Fig. 1

TG/DSC curves of Sr_1−x La _x Fe₁₂O₁₉ (x = 0.15, 0.25, 0.5) precursors

Figure 2 shows the XRD patterns of Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) powders with microwave-assisted calcination at 800 °C for 80 min. A strong diffraction peak of SrFe₁₂O₁₉ phase is detected in all the samples. The single SrFe₁₂O₁₉ hexaferrite has been identified with the La³⁺ substitution at x = 0.15. The diffraction peaks of α-Fe₂O₃ phases are observed in the samples with x = 0.25 and 0.5, respectively. Comparing the ionic radius of Sr²⁺ (1.27 Å) with La³⁺ (1.22 Å), the Sr²⁺ substitution by smaller ionic radius La³⁺ could reduce the cell volume. Table 1 lists the lattice constants of Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) and the volume percentage of α-Fe₂O₃. The lattice constant a and c are calculated by Formula (1), where d _{h k l} represents the crystal distance and (h k l) is the Miller indices.

$$ d_{(hkl)} =\left( \frac{4}{3}\frac{h^{2}+hk+k^{2}}{a^{2}}+\frac{l^{2}}{c^{2}}\right)^{-\frac{1}{2}} $$

(1)

It indicates that the lattice constant c of strontium ferrites unsubstituted with La³⁺ is bigger than that of substituted samples, because the smaller ionic radius of La³⁺ would reduce the distance along c-axis of the stacking fault, while the lattice constant a has a different change. The mössbauer spectra analysis has figured out that the substitution of Sr²⁺ by La³⁺ is accompanied with the change of Fe³⁺(0.64 Å) to Fe²⁺(0.75 Å) at 2a or 4f₂ crystallographic sites to keep the electrical neutrality of strontium hexaferrite [19]. The Fe²⁺ ions have a large ionic radius than Fe³⁺ ions, which results in the enlargement of the lattice constant a. The volume percentage of α-Fe₂O₃ decrease to 10.5 % after reaching to 21.1 % at x = 0.25, which is different from the substitution by Pr³⁺ and Dy³⁺ [16].

Fig. 2

X-ray diffraction patterns of the Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) powders

Table 1 Lattice constants of Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) and the volume percentage of α-Fe₂O₃

Figure 3 shows the hysteresis loops of Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) powders at room temperature. The saturation magnetization (σ _s) increases slightly with the La³⁺ doping content improvement, accompanied with the slight decrease in coercivity. As reported, some reasons have been discussed for the effects of the La³ substitution. When a divalent Sr²⁺ is replaced by a trivalent La³⁺, Fe³ would be converted to Fe²⁺ to maintain the electrical neutrality. The emergence of Fe²⁺ could cause the decrease of net magnetic moment in Sr_1−x La _x Fe₁₂O₁₉ for the valence change of Fe³⁺ to Fe²⁺, the spin canting deviating from the collinear to non-collinear arrangement, and the weaker super-exchange of Fe³⁺–O²⁻–Fe²⁺ [6, 20]. Those combined effects could cause the degradation in σ _s. The substitution of Sr²⁺ by La³⁺ with smaller ionic radius leads to the reduction of lattice constant c [20, 21], as shown in Table 1, which causes the decrease of the Fe–O distance that is parallel to the c-axis. The superexchange interaction of Fe³⁺–O²⁻–Fe³⁺ increases at the 12k and 2b site, corresponding with the improvement of σ _s. For the sample with La³⁺ substitution at x = 0.15, the stronger superexchange interaction of Fe³⁺–O²⁻–Fe³ could be dominant factor for the slight increase of σ _s. As listed in Table 2, the ratio of σ _r/σ _s is 0.447 with La³⁺ substitution at x = 0.15, showing its magnetic multi-domain structure [22]. The magnetization of samples with multi-domain structure is mainly determined by the domain wall displacement. The movement of multi-domain walls is much easier than flipping the magnetization of an entire particle coherently which is the magnetization mechanism of single domain, so the multi-domain grain has lower coervicity. For the sample at x = 0.25, the σ _s reduces to 50.449 emu/g. The decrease of σ _s and σ _r could be attributed to the presence of the α-Fe₂O₃ phase. The content of α-Fe₂O₃ is about 21.1 % and it dilutes the magnetization values. The ratio of σ _r/ σ _s is 0.532 and a remanence enhancement effect is observed. The exchange coupling effects take place between the hard magnetic SrFe₁₂O₁₉ phase and soft magnetic α-Fe₂O₃ phase and the smooth hysteresis loop also verify the effect. This coupling could hide the domain rotation and block the displacement of domain walls. The grain size calculated by scherrer formula at x = 0.25 for La³⁺ substitution has only 29.5 nm, resulting in higher coercivity. The ratio of σ _r/σ _s is 0.493 at x = 0.5 and the grain has no interaction when the value ratio of σ _r/σ _s below the 0.5. The σ _s and σ _r increased at x = 0.5 because of the lower phase composition of α-Fe₂O₃ (Table 1).

Fig. 3

The hysteresis loops of Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) powders at room temperature

Table 2 The grain size and magnetic properties of Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5)

4 Conclusion

Sr_1−x La _x Fe₁₂O₁₉ (x = 0, 0.15, 0.25, 0.5) hexaferrites were prepared by microwave-assisted sol-gel method. The grain size of strontium ferrite reduces when the La³⁺ doping content below x = 0.25. The substitution of La³⁺ can remarkably improve the coercivity to 5960.2 Oe at x = 0.25. This derived from the fine grain and exchange coupling between the higher volume percentage of α-Fe₂O₃ and the strontium ferrite with signal domain.