Synthesis and Characterization of Ba 0.6 Sr 0.4 Fe 12 O 19 /LaMnO 3 Composites as Microwave Absorbers

The synthesis and characterization of Ba 0.6 Sr 0.4 Fe 12 O 19 /LaMnO 3 composite material has been successfully carried out by mechanical alloying method using high energy milling. Crystal structure and surface morphology were characterized using x-ray diffraction and scanning electron microscopy. While the value of magnetization and the ability to absorb microwaves, vibrating sample magnetization and vector network analyzing were used, respectively. With variations in weight, does not change the crystal structure. The Ba 0.6 Sr 0.4 Fe 12 O 19 phase has a hexagonal structure and the LaMnO 3 phase has an orthorhombic structure. Surface morphology has a heterogeneous size in the range of 200-450 nm with the form of platelets. The best composite material Ba 0.6 Sr 0.4 Fe 12 O 19 /LaMnO 3 (75/25 wt%) is with composition of 75/25 wt%, and has magnetic properties with a magnetic saturation of Ms ~ 46.83 emu/g, Mr ~ 28.8 emu/g, and a coercive field of Hc ~ 3.88 kOe. The minimum reflection loss value is – 13.0 dB at 11.2 GHz frequency and broadband absorber with RL of -8dB at 8-12 GHz.


INTRODUCTION
Hexaferrite-based materials, especially M-type hexaferrite (MFe12O19, M = Ba or Sr) are still a concern of researchers [1][2][3]. At room temperature type-M hexaferrite material is hard magnetic, has high magnetic saturation Ms, wide Hc coercivity field, strong uniaxial magnetic anisotropy, high curie temperature, and good chemical stability. Therefore, this material is very applicable in technology, for example as a recording medium [1], ceramic color pigments [2], photocatalysts [3], and microwave absorbers at a frequency of 8-12 GHz [4].
From the previous study, the substitution of Mn 2+ ions on Fe 3+ ions (Ba0.6Sr0.4Fe12-zMnzO19) with z = 0, 1, 2, and 3. They reported that for z = 0 (Ba0.6Sr0.4Fe12O19) the magnetic properties were Ms = 92.9 emu/g and Hc = 0.424 T, while a maximum reflection loss (RL) of about -4.9 dB in a frequency range of 12.5 GHz and RL of -2 dB in a frequency range of 8-10 GHz for a sample thickness of 1.5 mm [4]. In other research for La1-xBaxMnO3 perovskite sample with a sample thickness of 1.5 mm has a value of RL ~ -2.6 dB at a frequency of 11 GHz [11] with a magnetic saturation value of Ms ~ 9 emu/g and Hc ~ 90 Oe [12]. Meanwhile, the La1-xCaxMnO3 perovskite sample for x = 0.1 has Rl ~ -42 dB (f = 10.5 GHz) for a sample thickness of 2.0 mm [13]. Based on the results of previous studies, it was stated that Ba/SrFe12O19 material has low reflection loss but the absorption frequency area is relatively wide, while LaMnO3 material has high reflection loss but relatively narrow absorption. Thus, in this study, a combination of the two materials was carried out to form a composite in the hope of obtaining a material that has a high reflection loss with a relatively wide absorption frequency area. So the purpose of this research is to synthesize and characterize the superior material in the form of Ba0.6Sr0.4Fe12O19/LaMnO3 composite as a microwave absorbing material at a frequency of 8-12 GHz using mechanical milling method. This research is an initial study on the development of composite materials which can then be used as a reference for the modification of the two materials so that a more optimal microwave absorbing material can be obtained.

RESEARCH METHOD 2.1. Precursor Preparation
Raw materials consisting of BaCO3, SrCO3, and Fe2O3, each Merck product with purity > 99% were mixed by stoichiometric calculations to form the sample composition Ba0.6Sr0.4Fe12O19. The equations used are as follows: 0.6 BaCO3 + 0.4 SrCO3 + 6 Fe2O3 → Ba0.6Sr0.4Fe12O19 + CO2 (1) For samples, LaMnO3 was made from materials La2O3 (Aldrich, 99%) and MnCO3 (Merck, 99%) with equation (2): (2) The Ba0.6Sr0.4Fe12O19/LaMnO3-∂ composite was made with three different compositions, each with a weight % ratio of 85/15, 75/25, and 65/35. Each composite composition was mixed in a stainless steel vial, given iron balls with a ratio of iron:sample weight = 1:1 and added ethanol up to two-thirds. Each sample was milled for 30 hours using high energy milling at 700 rpm with a mechanism of every hour milling, interspersed with 0.5 hours of rest. This is to avoid the heat that occurs due to the impact of the iron balls with the vial. Then the sample was sintered at 1000 o C for 5 hours under atmospheric pressure.

Characterization
X-ray diffractometer (XRD) type PAN Analytical Empyrean ( Cu-Kα, = 1.5406 Å) was used to characterize the phase formation and the composition formed. Data analysis of XRD results both qualitatively and quantitatively using Rietvelt software. The surface morphology was characterized using a scanning electron microscopy (SEM) type-JEOL JED 2300. Meanwhile, the characterization of magnetic magnitude at room temperature and the ability to absorb microwaves were characterized by vibrating-sample magnetometer (VSM) OXFORD VSM 1.2H and Vector Network, respectively. Analysis (VNA) type-Anritsu MS46322A in the 8-12 GHz frequency range.

RESULTS AND DISCUSSION
The Ba0.6Sr0.4Fe12O19/LaMnO3 composite with a weight ratio of 85/15 wt%, 75/25 wt%, and 65/35 wt% will be referred to as YEG-1, YEG-2, and YEG-3, respectively. Figure 1 shows the results of X-ray diffraction characterization for samples YEG-1, YEG-2, and YEG-3. All samples showed typical peaks of their constituent materials, namely the phases of Ba0.6Sr0.4Fe12O19 (JCPDS card no. 00-051-1879) and LaMnO3 (JCPDS Card No. 54-1275). See Figure 1 show the results of refining the diffraction pattern using the Rietvelt program. The complete refinement results using the Rietvelt program can be seen in Table 1. In the three samples, the fase Ba0.6Sr0.4Fe12O19 phase has a hexagonal crystal structure, the P63/mmc space group and the LaMnO3 phase has an orthorhombic crystal structure, the Pbnm space group. The same result was also obtained in our previous study, where Ba0.6Sr0.4Fe12-3xZn2xTixO19 for x = 0 had a hexagonal crystal structure [14]. Different results were obtained for the LaMnO3 sample, where the crystal structure was hexagonal [15]. According to A. Gholizadeh, LaMnO3 can have an orthorhombic, rhombohedral or cubic structure, depending on the concentration of Mn 4+ ions [12]. If the Mn 4+ ion content is around 0-12%, it has an orthorhombic structure. In this paper, the Mn ion is in the Mn 2+ (MnCO3) state which then becomes Mn 3+ (LaMnO3). Thus the state of the Mn 4+ ion does not exist. From Table 1. it can be seen that as the percentage by weight of LaMnO3 decreases (the percentage by weight of Ba0.6Sr0.4Fe12O19 decreases), the crystal density of the Ba0.6Sr0.4Fe12O19 phase also decreases, although it is relatively not large. The composition ratio by weight of Ba0.6Sr0.4Fe12O19-LaMnO3 formed was 86%-14%, 74%-26%, and 63%-37%, respectively. The results of the analysis of the three samples have met the criteria of the Rietveld method with a value of χ 2 close to 1. This means that the sintering process of the mixture does not affect the phase change of the two composite-forming materials. However, it is hoped that with this sintering process, there will be intergrain interactions between Ba0.6Sr0.4Fe12O19 and LaMnO3 materials. This condition is very necessary for microwave absorbing materials because the microwave absorption mechanism is strongly influenced by the magnetic and electrical properties of the material [5]. Table 1 shows that the volume of the YEG-1, YEG-2, and YEG-3 composite V unit cells for the BaFe12O19 phase ranges from 693.4 to 693.6 Å 3 with atomic density ranging from 5.874-5.876 g/cm3, while for the LaMnO3 phase it ranges from 240.7 to 240.9 Å 3 . with atomic densities ranging from 6.666 to 6.673 g/cm 3 . This means that these two materials do not undergo structural changes after being composited through the sintering process. The results of the XRD analysis are also supported based on observations of the microstructure of this composite which is expected to produce a homogeneous mixture and have a relatively uniform particle size as shown in Figure 2. Figure 2 (a)-(c) shows a scanning electron microscopy (SEM) image of the microstructure of the Ba0.6Sr0.4Fe12O19 composite with perovskite LaMnO3 with various weight percentage fractions of 85/15, 75/25, and 65/35. The granules form like platelets, as seen in the insert in Figure 2 (c). The grain size ranges from 200 to 450 nm. This measure is obtained by comparing the grain length to the existing scale. Ba0.6Sr0.4Fe12O19 phase and LaMnO3 phase were mixed evenly. However, we suspect that based on the crystal size results, the small grains represent the LaMnO3 phase, while the larger grains represent the Ba0.6Sr0.4Fe12O19 phase. Similar results were also obtained by J. N. Dahal et al., where the hexaferrite particle size was larger than the perovskite particle size [7]. The size of the particles formed is also influenced by the sintering temperature. The higher the temperature, the larger the particle size that's formed [16]. From the observation of the microstructure, it is shown that these three composites are homogeneous mixtures with relatively uniform particle sizes. This means that the distribution of particles in the mixture between BaFe12O19 and LaMnO3 is evenly distributed over the entire sample surface in these three composites. Besides that, it also appears that the particles are not dispersed but there is good interconnection between the particles so that intergrain interactions are expected between the BaFe12O19 and LaMnO3 materials. The presence of this intergrain interaction will greatly affect the magnetic properties and microwave absorption characteristics of the composite, especially in composites consisting of a mixture of materials that have much different characteristics [17]. BaFe12O19 material is a ferrous magnetic material that has hard magnetic characteristics with relatively large magnetic properties so that it tends to have a relatively large permeability value, while LaMnO3 material is an antiferrous magnetic material but has a relatively high permittivity value. Magnetic hysteresis loop curves at room temperature for YEG-1, YEG-2 and YEG-3 composites are shown in Figure 3. The three samples have remanent magnetic magnitude Mr and coercivity field Hc are relatively the same, but differ in magnetic saturation values Ms. Compared to the other two samples, YEG-2 sample has the highest Ms saturation magnetic value, around 46.83 emu/g. The complete results of VSM can be seen in Table 2. At room temperature Ba0.6Sr0.4Fe12O19 is hard magnetic, has a magnetic saturation of Ms of 92.9 emu/g and a coercive field of Hc = 4.24 kOe [4,14], and LaMnO3 has a value of Ms ~ 3.42 emu/g and Hc ~ 0.755 kOe are soft magnetic [7,18,19]. The Ba0.6Sr0.4Fe12O19-LaMnO3 composite is a hard-soft magnetic. The magnitude of the magnetization can be seen in Table 2. Based on the measurement results in Table 2, it appears that the magnetic properties of this material are relatively unchanged, this indicates that the level of particle homogeneity and the effect of the presence of intergrain interactions in the particles plays an important role in maintaining the magnetic properties of this composite. It is also suspected that this LaMnO3 system when interacting with Ba 2+ ions will be ferromagneticly so that it is able to maintain the Ms of this composite even though the BaFe12O19 fraction is decreasing [20]. It is different if this composite is purely mixed without any sintering process, in general the saturation magnetization of Ms tends to decrease due to the reduced mass fraction of the BaFe12O19 phase in the composite. These results are in agreement with the results of the previous analysis using XRD and microstructure observations using SEM. This study can also be seen from the measurement results of microwave absorption, as the main goal of this research.
The ability of a material to absorb electromagnetic waves is usually referred to as the reflection loss of electromagnetic radiation, RL (dB), calculated by the following equation: [21][22] RL(dB)= 20 log|(Zin -Z0 )/(Zin+Z0 | (3) where Zin and Z0 represent the characteristic impedance of the material and the characteristic impedance of vacuum, respectively. The Zin value is obtained from the equation: Z_in= (μr/εr ) ( where µr and ℇr are the relative permeability and permittivity of the material, d is the thickness of the sample, c is the speed of electromagnetic waves in a vacuum, and f is the frequency with which the waves arrive. According to equation (3), when RL = -10 dB, the wave absorption reaches 90%. The reflection loss for the YEG-1, YEG-2, and YEG-3 samples can be seen in Figure 4. The best minimum reflection loss is obtained for the YEG-2 sample, which is -13.0 dB, f = 11.12 GHz, and 1.52 GHz bandwidth. The increase in the weight percentage of LaMnO3 in the Ba0.6Sr0.4Fe12O19-LaMnO3 composite, although only slightly, the bandwidth increased from 1.42 GHz to 1.52 GHz, there was also a shift in the peak frequency from 10.96 GHz to 11.22 GHz. The amount of reflection loss (RL) by the absorbing material is influenced by electromagnetic parameters, such as permittivity and permeability of the material. Therefore, the soft magnetic properties of LaMnO3 strengthen the microwave attenuation performance in the high frequency region [20]. While Ba0.6Sr0.4Fe12O19 is a hard magnetic having a high Ms, so it will widen the absorption band [23]. In Table 3, it can be seen that the performance improvement of this composite-based microwave absorbing material is obtained, namely reflection loss and the width of the microwave absorption area. It is known that before being modified these two materials have low reflection loss, but when they become composites and it is strongly suspected that interconnection occurs between grains, this results in the composite having higher permeability and permittivity so as to produce higher microwave absorption. The most interesting thing is that the absorption frequency area becomes wider. The YEG-2 composite found microwave absorption with RL = -8 dB at a wide frequency from 8-12 GHz. This composition becomes a reference and initial study for the development of further composite materials, so that it is expected that composite materials with more optimum absorption capabilities can be obtained.

CONCLUSION
The Ba0.6Sr0.4Fe12O19/LaMnO3 composite material which has been successfully synthesized by mechanical alloying method has a hexagonal/orthorhombic crystal structure with a homogeneous mixture and relatively uniform particle size of about 200 to 450 nm. This sintering process produces interconnections between particles in the composite so that it is suspected. Intergrain interactions occur which are able to maintain their magnetic properties even though the BaFe12O19 fraction is reduced. Besides that, it produces a significant increase in microwave absorption capability with a relatively wide absorption frequency area of 8-12 GHz with a reflection loss of -8 dB. This research can be used as a reference and initial study for the development of further composite materials in order to obtain composite materials with more optimum absorption capabilities.