Enhanced Microwave Absorbing Properties of La³⁺ Substituting Barium Hexaferrite

Abstract

Barium hexagonal ferrites doped with La³⁺, Ba_1−x La _x CoTiFe₁₀O₁₉ (x = 0.10, 0.15, 0.20), were prepared by sol-gel combustion method. The structure, magnetic properties, and microwave absorption of the samples were investigated. The X-ray diffraction patterns of the doped ferrites confirmed the formation of the M-type barium ferrite, and no other impurities could be found. The electromagnetic parameter spectra showed that The La³⁺ ions doped in the ferrite improved complex permittivity and permeability so that microwave absorption was enhanced. The minimum reflection loss of −39.15 dB was obtained for sample x = 0.20 with thickness of 2.4 mm, and the maximum bandwidth of RL <−10 dB can reach to 11.5 GHz when the thickness is 2.0 mm.

1 Introduction

The rapid developments of electrical and electronic devices cause a growing request in microwave absorbing materials, which could be used extensively in commerce, industry, and defense applications [1–6]. Due to their strong absorption of electromagnetic (EM) wave, the microwave absorbers can minimize various EM radiations and interferences. In this way, an appropriate microwave absorber can eliminate the excessive electromagnetic waves to ensure the people’s health and the undisturbed functioning of equipment. Moreover, the appropriate absorbers can help military aircrafts and vehicles to avoid detection by the radar in defense and aerospace industries, as the application of microwave absorbing coating on the exterior surfaces of military equipments. However, owing to the advancing technology, the EM wave absorbing materials are expected to have broad bandwidth, minimum reflection loss (RL), and light weight (or small thickness) in recent years [7]. Therefore, broadening bandwidth and minimizing reflection loss have been the vital problem for microwave absorbers.

Among the microwave absorbing materials, M-type barium hexagonal ferrite (BaFe₁₂O₁₉, BaM) has generated considerable recent research interests because of its low cost, good stability, large magnetocrystalline anisotropy, high natural resonance frequency (about 47.6 GHz), and excellent magnetic loss [7–10]. At the microwave band, the microwave attenuation properties of BaM are closely related to the magnetic losses which result mainly from the resonance absorption of moving magnetic domains in lower frequency and the spin relaxation of magnetization in higher frequency. It has been known that the microwave absorption performance can be improved by choosing suitable cations to substitute the Fe³⁺ in BaM. M. K. Tehrani et al. [11] have confirmed that the bandwidth of EM wave absorption for BaM can be broadened when the Fe³⁺ was partially replaced by metal cations (Mg²⁺, Mn²⁺, Co²⁺) and Ti⁴⁺. C. S. Dong et al. [12] have demonstrated that the minimum reflection loss of BaCo_0.3Ti_0.3 Fe_11.4O₁₉ can reach −47 dB at the thickness of 0.8 mm. X. Tang and his co-workers [13] have reported that the non-stoichiometric Ni²⁺-Ti⁴⁺ can improve the absorption performance of BaM and the natural resonance frequency of the doped ferrite shift to X-band (8–12 GHz). Thus, using the right cations to substitute BaM is significant for the excellent microwave absorption properties of the absorbing materials. However, influence of substituting Ba²⁺ on properties of barium hexaferrite has been rarely reported.

In the current work, the La³⁺-doped ferrites Ba_1−x La _x CoTiFe₁₀O₁₉ were synthesized and the phase composition was confirmed. Moreover, the magnetic and microwave absorption properties were investigated in the frequency range of 2–18 GHz.

2 Experimental

The powders of Ba_1−xLa_xCoTiFe₁₀O₁₉ (x=0.10, 0.15, 0.20) were synthesized via the sol-gel combustion method. As starting materials, stoichiometric amount of Fe(NO₃)₃⋅9H₂O, Ba(NO₃)₂, Co(NO₃)₂⋅6H₂O, Ti(OC₄ H ₉)₄, and La(NO₃)₃⋅6H₂O were dissolved in deionized water by stirring it constantly using a magnetic stirrer. After dissolved completely, appropriate citric acid solution was added into the solution, and then ammonia solution was added dropwise with vigorous stirring to maintain the pH value of the solution at 7. Subsequently, the neutralized solution was heated at 100^∘C with continuous magnetic stirring to obtain the dried gel. With further heating, the dried gel would burn up in a self-propagating combustion manner, and some brown precursors could be obtained. Finally, the precursors were pre-heated at 450^∘C for 2 h and then calcined at 1100^∘C for 4 h to obtain the ferrite powders.

The phase composition of ferrites was identified by the X-ray diffractometer (XRD; PANalytical X’Pert PRO), with Cu K α radiation (λ = 1.540598 Å) in the range of 20^∘–75^∘. The morphology was observed by using the field emission scanning electron microscopy (FE-SEM; Zeiss Ultra 55). The vibrating sample magnetometer (VSM) was used to measure magnetic properties of the ferrites at room temperature. The resulting ferrite powders were mixed with paraffin wax at ratio of 6:1 and then compacted into a circular cylinder with the inner diameter of 3.04 mm, the outer diameter of 7.0 mm. The electromagnetic parameters of composite samples in the frequency range of 1–18 GHz were measured by network analyzer (Agilent Technologies, E8363A) using a coaxial method.

3 Results and Discussion

The X-ray diffraction patterns of Ba_1−x La _x CoTiFe₁₀O₁₉ (x=0.10, 0.15, 0.20) are presented in Fig. 1. The results show that all the samples have similar diffraction patterns representing a single phase with hexagonal crystal structure and no second phase can be found, indicating that the La³⁺ can enter the lattice and do not affect the crystal structure of hexagonal barium ferrite. However, the substitution of Ba²⁺ with La³⁺ results in a slight change of diffraction peak position due to the slight change in lattice parameters of the respective compounds. The lattice parameters (a and c) are tabulated in Table 1 and which were calculated by the following formula:

$$ d=\frac{1}{\sqrt {4(h^{2}+hk+k^{2})/3a^{2}+l^{2}/c^{2}} } $$

(1)

where (h k l) is the Miller indices, and d is the crystal face distance. It can be seen that the value of a decreases little, the value of c decreases from 23.423 to 23.136 Å and the ratio of c/a decreases from 3.954 to 3.913 with increasing x. The variations of lattice parameters can be attributed to partial replacement of Ba²⁺ (ionic radius 149 pm) with the smaller La³⁺ (ionic radius 117 pm) ions.

Fig. 1

The X-ray patterns for Ba_1−xLa_xCoTiFe₁₀O₁₉ with x = 0.10, 0.15, and 0.20

Table 1 Lattice parameters for all compositions

A representative scanning electron micrograph (SEM) for Ba_0.8La_0.2CoTiFe₁₀O₁₉ is shown in Fig. 2. The micrograph clearly illustrates hexagonal structure. From the image, it can be seen that the particles appear fine grain growth with the grains size varying from 100 to 300 nm. The micrographs of other samples also show similar features and no obvious microstructure change is seen with La³⁺ substitution.

Fig. 2

Scanning electron micrograph (SEM) for sample with x= 0.20

The magnetic hysteresis loops for Ba_1−x La _x CoTiFe₁₀O₁₉ with different values of x are shown in Fig. 3. The saturation magnetization M _sand coercivity H _c are plotted in Fig. 4 as a function of La³⁺ substitution. The hysteresis loops show that all the samples exhibit typical characteristics of soft magnetic materials. The value of M _s increases from 57 to 66 emu/g with increasing La³⁺ amount as revealed in Fig. 4. On the contrary, the value of H _cdecreases with increasing x. These can be explained by the ferromagnetic theory of ferrite. The increase of M _s is ascribed to the occupation of La³⁺ in the lattice site of Ba²⁺ enhancing molecular magnetic moment, and the decease of H _c is due to the weakened uniaxial magnetocrystalline anisotropy of the barium ferrite for La³⁺ substitution being consistent with variation of lattice parameter c/a.

Fig. 3

Magnetic hysteresis loops for the Ba_1−xLa_xCoTiFe₁₀O₁₉ with x= 0.10, 0.15, and 0.20

Fig. 4

Saturation magnetization M _s and coercivity H _c of Ba_1−x La_xCoTiFe₁₀O₁₉

In order to understand the possible microwave absorption mechanism, the complex permittivity (ε _r = ε ^′−j ε ^″) and complex permeability (μ _r = μ ^′−j μ ^″) were investigated. The complex permittivity and complex permeability can be used to represent the dielectric and dynamic magnetic properties of materials. For microwave absorber, the real part of complex permittivity and permeability is proportional to storage capability of electric and magnetic energy when the material is under an applied electric or magnetic field. The imaginary part represents the loss of electric and magnetic energy.

Figure 5 shows the frequency dependence of the real and imaginary parts of the permittivity for Ba_1−x La_xCoTiFe₁₀O₁₉ samples over the frequency range 1–18 GHz. From Fig. 5a, we can see that the real part (ε ^′) of complex permittivity exhibits indistinctive variation with frequency for all compositions; this is a normal dielectric behavior of ferrites, which was consistent with reported before [14]. However, the value of ε ^″ shows strong frequency dispersion, which can be attributed to intrinsic electric dipole polarization and interfacial polarization. Furthermore, with the amount of doped La³⁺ increasing, the average value of ε ^′ and ε ^″ rises continuously. In barium hexaferrites, Ba²⁺ are partially replaced when La³⁺ enter into the lattice, resulting in partial conversion of Fe³⁺ into Fe²⁺ in order to keeping charge equilibrium [15]. Since dielectric polarization mechanism is closely associated with electron hopping mechanism in ferrites, the greater electron hopping chance between Fe³⁺ and Fe²⁺ is the higher the permittivity would be [16]. As a result, the real part of complex permittivity and imaginary part increase with more addition of La³⁺ amount.

Fig. 5

Frequency dependence of the real (a) and imaginary part (b) of the complex permittivity for Ba_1−xLa_xCoTiFe₁₀O₁₉(x= 0.10, 0.15, 0.20)

Figure 6 shows the frequency dependence of the real (μ ^′) and imaginary (μ ^″) parts of the permeability for Ba_1−xLa_xCoTiFe₁₀O₁₉. All curves of μ ^″ vs. frequency had a broad peak corresponding to natural ferromagnetic resonance (f _r), and the peak shift to lower frequency with increasing amount of doped La³⁺. This can be ascribed to weakening uniaxial magnetocrystalline anisotropy. Above f _r, the real parts of permeability falls sharply to unity. As we mentioned above, the partial Fe³⁺ ions convert to Fe²⁺ ions with increase in La³⁺ content. It is well known that the static magnetic moment of Fe²⁺ (4 μ _B) is lower than that of Fe³⁺ (5 μ _B), resulting in overall magnetic interaction weakening in ferrites, thereby decreasing the values of μ ^′ and μ ^″ with increasing La³⁺ content. However, as shown in permeability spectra, the values of μ ^′ and μ ^″ increase slightly with increasing La³⁺ content instead of decreasing. According to Snoek’s law [17], a change in .μ due to variations in material structure is followed by an opposite change in resonance frequency (f _r), i.e., (μ _s−1)f _r = (2/3)(γ4π M _s), where 4 π M _s is the saturation magnetization, γ is gyromagnetic ratio. As shown in hysteresis loops above, M _s is enhanced with increasing La³⁺ content. Therefore, the value of μ ^′ increases with more addition of La³⁺ amount below f _r.

Fig. 6

Frequency dependence of the real (a) and imaginary part (b) of the complex permeability for Ba_1−xLa_xCoTiFe₁₀O₁₉(x= 0.10, 0.15, 0.20)

From the complex relative permittivity and the complex relative permeability, the microwave absorption performance of Ba_1−x La _x CoTiFe₁₀O₁₉ composites with paraffin wax was investigated. The reflection loss (RL) can be calculated from the relative complex permeability and permittivity with a given frequency range and a given absorber thickness (d) by the following equation [18]:

$$\begin{array}{@{}rcl@{}} \textit{RL~}=20{\text{log}}\left| (Z_{in}-1)/(Z_{in}~+~1)\right| \end{array} $$

(2)

Z _{i n} is determined as

$$\begin{array}{@{}rcl@{}} Z_{in}~=~(\mu_{\mathrm{r}}/\varepsilon_{\mathrm{r}})^{1/2}{\text{tan}} h[j(2\pi \textit{fd}/c)(\mu_{\mathrm{r}}\varepsilon_{\mathrm{r}})^{1/2}] \end{array} $$

(3)

where Z _{i n} is the input impedance of absorber, ε _rand μ _r are complex permittivity and permeability of the composite, respectively, f is the microwave frequency, and c is the velocity of light in free space.

The frequency dependences of RL for all composition with different thicknesses are exhibited in Fig. 7. It is clear that the matching frequency decreases with increase in value of x. As shown in reflectivity curves, two absorption peaks with significantly different values and positions appear for samples with x= 0.15 and x=0.20 at all different thickness. This dispersion can be attributed to domain wall motion at the lower frequency and spin resonance at the higher frequency. However, there is only one absorption peak for composition x = 0.10 due to its peak from spin resonance beyond measuring frequency. For the sample x = 0.10, the matching frequencies are equal to 14.7 GHz with a minimum RL of −28.36 dB, and the corresponding matching thickness is 2.0 mm. For the sample x = 0.15, the RL values of −29.84 and −21.85 dB were achieved in the matching frequencies of 9.16 and 15.6 GHz, respectively. The corresponding value of matching thickness is 2.0 mm. Sample x=0.20 with matching thickness of 2.4 mm exhibits a minimum RL of −39.15 dB at 7.41 GHz. It is notable that the bandwidth of RL< −10 dB covers entirely X-band (8˜12 GHz) and Ku-band (12˜18 GHz) for sample x = 0.20 when matching thickness is 2.0 mm. The obtained results indicate that La³⁺ substituting not only enhance microwave attenuation of barium hexaferrites but also broaden its bandwidth.

Fig. 7

Frequency dependence of RL for Ba_1−xLa_xCoTiFe₁₀O₁₉ (x=0.10, 0.15, 0.20) at different thicknes

4 Conclusions

In conclusion, Lanthanum ion-doped barium hexaferrites Ba_1−x La _x CoTiFe₁₀O₁₉ (x = 0.10, 0.15, 0.20) have been fabricated successfully by the sol-gel combustion method. The high-frequency magnetic and dielectric properties can be enhanced as addition of La³⁺. Increasing of La³⁺ content leads to a significant increase in complex permittivity and permeability, while the resonance frequency is shifted to low frequency. Thus, with the incorporation of La³⁺ the absorbing peak moved to the lower frequency and the microwave absorption performance is significantly improved. The sample Ba_0.8La_0.2CoTiFe₁₀O₁₉ has the best microwave absorption performance. The maximum RL of −39.15 dB can be achieved at 7.41 GHz when the matching thickness is 2.4 mm. Furthermore, the maximum bandwidth of RL< −10 dB can reach to 11.5 GHz (6.5˜18 GHz) when the matching thickness is 2.0 mm.