Preparation of Superparamagnetic Zn0.5Mn0.5Fe2O4 Particle by Coprecipitation-Sonochemical Method for Radar Absorbing Material

One of many applications of spinel ferrite nanoparticles is related to their performance as radar absorbing materials. In this work, we report developing synthesis method through combined coprecipitation-sonochemical routes in preparing Zn0.5Mn0.5Fe2O4 nanoparticle from iron sand in Indonesia as a vital raw material. The structure, size, morphology, and elements of the Zn0.5Mn0.5Fe2O4 nanoparticle were investigated via X-Ray diffractometry and Transmission/Scanning Electron Microscopy (TEM/SEM) combining Energy Dispersive Spectroscopy (EDS). The magnetic properties of the Zn0.5Mn0.5Fe2O4 nanoparticle were characterized by using Vibrating Sample Magnetometer (VSM). Furthermore, the reflection loss character of the Zn0.5Mn0.5Fe2O4 nanoparticle was determined via Vector Network Analyzer (VNA). From the qualitative and quantitative analysis of the XRD data, it can be identified that the Zn0.5Mn0.5Fe2O4 particle formed a spinel cubic structure in a single phase with the lattice parameter of approximately 8.401 Å. It is known from the TEM image that the Zn0.5Mn0.5Fe2O4 particle had a size of about 9.7 nm and tended to agglomerate. Furthermore, the data analysis of the M(H) curve presented that the Zn0.5Mn0.5Fe2O4 nanoparticle has a superparamagnetic behavior with the saturation magnetization of approximately 43 emu/g. Finally, the data analysis of the reflection loss as a function of frequency showed that the Zn0.5Mn0.5Fe2O4 nanoparticle performs as a radar absorbing material with the absorption performance of approximately -11.0 dB at the frequency of 10.8 GHz


Introduction
Spinel ferrite nanoparticles have general formula AB 2 O 4 and form spinel crystal structure, where A represents the metallic cations located at the tetrahedral site, and B represents the metallic cations positioned at the octahedral site [1]. Spinel ferrite nanoparticles are a group of functional magnetic materials that have attracted significant attention due to their high performance in various applications. Nowadays, several researchers have extensively reported that the spinel ferrite nanoparticles have excellent application performances in many fields. The applications include for hyperthermia treatment [2], energy storage [3], core, switching and MLCI's [4], as a catalyst to eliminate organic pollutants [5], electromagnetic shielding [6], microwave absorption and high-frequency [7], and so forth.
Numerous efforts have been conducted intensively and continuously by many researchers to obtain the high-performance spinel ferrite nanoparticles as radar absorbing materials. It is imperative because radar absorbing materials have an essential role in fulfillment in civil and military purposes [8]. The efforts have been conducted through developing synthesis method, expanding characterization technique, and also developing a theoretical explanation to achieve the thin, flexible and light absorbers with high reflection loss performance.
For mass production, it is essential to develop the synthesis method efficiently and inexpensively in preparing spinel ferrite nanoparticles as radar absorbing materials. However, many papers reported that the ferrite nanoparticles as radar absorbing materials are prepared from commercial precursors which are relatively expensive. Therefore, to reduce production cost, in this experiment, we propose the utilization of iron sand from natural resources in Indonesia in preparing Zn 0.5 Mn 0.5 O 4 nanoparticle. Among various synthesis methods, the coprecipitation approach has advantages due to its simplicity, effectivity, low price, and ability in producing spinel ferrite nanoparticles [9]. Furthermore, the sonochemical approach also is also superior regarding its high performance in preparing the nanoparticles in various shapes and sizes with high homogeneity. Therefore, we also propose a combination of the two approaches in synthesizing the Zn 0.5 Mn 0.5 O 4 nanoparticle. Finally, we complete this works by discussing in detail the structural, magnetic, and microwave absorption characteristics of the prepared Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle.

Sample Preparation
The Zn 0.5 Mn 0.5 Fe 2 O 4 particle was prepared by using a combined coprecipitation-sonochemical method using natural iron sand as a primary raw material. The iron sand, was taken from Sine Beach Tulungagung -East Java Indonesia, was extracted and purified by using a magnetic separator to obtain the Fe 3 O 4 powder following our previous works [10,11]. The process of the powder was reacted with HCl (Merck, PA) to form a FeCl 2 and FeCl 3 solution tracking our previous experiments [12][13][14]. The solution was then mixed with ZnCl 2 (Merck, PA) and MnCl 2 .4H 2 O (Merck, PA) by using a magnetic stirrer. The process was continued by wisely dropping NH 4 OH (Merck, PA) and combined with the sonochemical process by using an Ultrasonic Batch also at room temperature to obtain the black precipitated sample. The precipitated sample was washed several times by using distilled water until it reached a normal pH condition. Finally, the precipitated sample was filtered before drained at 100 °C to obtain the Zn 0.5 Mn 0.5 Fe 2 O 4 particle.

Characterizations
The crystal structure was investigated using X-ray Diffractometry (XRD) with Cu-Kα radiation in the range of 10° -80°. The XRD data were analyzed by using Rietica program [15] [16] to obtain the phase purity, crystal structure, lattice parameters, and crystallite size of the sample. The particles size, morphology and elemental composition of the sample were investigated by means of Transmission/Scanning Electron Microscopy (TEM/SEM) combined with energy-dispersive spectroscopy (EDS). The magnetic properties of the sample, as well as saturation magnetization, coercive field, and remanent magnetization, were characterized by using Vibrating Sample Magnetometer (VSM) at room temperature. Finally, the performance of the sample as a radar absorbing material was investigated by using vector network analyzer (VNA) at a frequency in the range of 8 -12 GHz.

Results and Discussion
The chemical reaction of the Fe 3 O 4 from iron sand dissolved in HCl tracks the following equation of reaction [17].
From reaction (1), the metal salt solution containing Fe 3+ and Fe 2+ can result only through one step of reacting natural Fe 3 O 4 with HCl. The prepared solution from the first reaction was then mixed with ZnCl 2 and MnCl 2 .4H 2 O, and then reacted with NH 4 OH by dropping process to form the Zn 0.5 Mn 0.5 Fe 2 O 4 particle. The sonochemical route was also applied to reducing the agglomerated particles of the sample during the precipitating process. The detail of the chemical reaction is presented in equation of reaction (2). The remained or unwanted product in this reaction was removed by a washing process using distilled water for several times to achieve a normal pH condition and then followed by a heating process at 100 o C.
The obtained Zn 0.5 Mn 0.5 Fe 2 O 4 particle was characterized using XRD to investigate the phase purity, crystal structure, as well as lattice parameter and crystal volume, and also crystallite size. The XRD pattern of the Zn 0.5 Mn 0.5 Fe 2 O 4 particle is presented in Figure 1. The XRD data as illustrated in Figure 1 were analyzed qualitatively and quantitatively. From the qualitative analysis, the Zn 0.5 Mn 0.5 Fe 2 O 4 particle formed a single phase without impurity. Meanwhile, from the quantitative analysis, the Zn 0.5 Mn 0.5 Fe 2 O 4 particle built a spinel crystal structure in the cubic system. The measured XRD data and the Rietveld refinement plot are, respectively, shown by the circles and solid line. The lattice parameter of the crystal can be calculated using the following equation [18].
Where a,  , (hkl), and  represent the lattice parameter, wavelength of X-ray, Miller indices, and Bragg angle, respectively. For this case, the lattice parameter is a = b = c, and the crystal volume is V = a × b × c as the cubic structure with the space group of F d -3 m Z. From the quantitative analysis, the Zn 0.5 Mn 0.5 Fe 2 O 4 particle has the lattice parameters and crystal volume of approximately 8.401 Å and 592.9 Å 3 , respectively. These are in excellent agreement with the results reported by other researchers [19,20].  Figure 2, the crystallite size of the Zn 0.5 Mn 0.5 Fe 2 O 4 particle was calculated by using Debye-Scherrer's formula to ensure that the particle formed in nanometric size. The fitting result of the FWHM for the highest peak is presented in Figure 2. The equation (4) represents the Debye-Scherrer's formula [21].
Where D is the crystallite size (nm), K is the constant of 0.9,  is the Bragg angle (degree),  is the excessive line broadening (rad), and  is the wavelength of X-ray (0.15406 nm). From the data analysis in this experiment, the crystallite size of the Zn 0.5 Mn 0.5 Fe 2 O 4 particle prepared by the coprecipitation-sonochemical method is 10.7 nm. This result is quite similar to the crystallite size of the Zn 0.5 Mn 0.5 Fe 2 O 4 prepared by precursor-based combustion method [22]. It means that the combined method is effective and efficient to prepare Zn 0.5 Mn 0.5 Fe 2 O 4 particle in nanometric size from local natural iron sand. Furthermore, the particle size is also compared to the TEM characterization of the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle as shown in Figure 3.  Figure 3, it is clear that the prepared Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle in this experiment has nanometric size. Quantitative analysis of the TEM image presented that the average particle size of the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle is 8.7 nm with a standard deviation of 2.4 nm. This result is close to the size calculated from the XRD data. In this work, the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle from iron sand has smaller particle size than that of Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle from commercial precursors prepared by microwave induced urea nitrate and coprecipitation processes [23,24]. The Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle had a tendency to form a spherical shape with aggregation. The small particle size can contribute the phenomenon of agglomeration in the magnetic nanoparticles. The result of Rietveld analysis of the XRD data for the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle became physical evidence that the Mn and Zn ions successfully substituted Fe ion in the crystal structure. In order to enhance our knowledge about the composition and distribution of the metallic elements and oxygen, further investigation was also carried out by using SEM combined with the EDS. The investigation resulted in EDS spectra, elemental quantification as shown in Figure 4, and also elemental mapping for all elements and individual element as presented in Figure 5. From the data, the selected elements such as iron, zinc, manganese, and oxygen tended to distribute randomly in all space of the material. Furthermore, the distribution of iron was dominant because it has the highest composition. Interestingly, the atomic composition of zinc and manganese was quite similar representing that the theoretical calculation (Mn : Zn = 0.5 : 0.5) was appropriate for the experimental data.  [25]. Moreover, to study the magnetic properties comprehensively, the quantitative analysis of the M(H) curve was conducted via fitting analysis. The data analysis was done by employing Langevin function according to the following equation [26].
Where M is the magnetization, M s is the saturation magnetization,  is the magnetic moment, H is the magnetic field, k B is the Boltzmann constant, and T is the temperature. The solid line and star represent the fitting model and experimental data, respectively. The theoretical calculation fits the experimental data well with the R-square of 99.8%. From the result of data analysis, the M s and μH/k B T values were approximately 43 emu/g and 40, respectively. The saturation magnetization value of the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle prepared in this work is higher than that of Mn-ZnFe 2 O 4 nanoparticles reported by other researchers [23,27]. The saturation magnetization of the Zn 0.5 Mn 0.5 Fe 2 O 4 in the nanoparticle is lower than that of the bulk as ascribed to the particle size. The saturation magnetization of the spinel-structured materials such as Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle can be determined from the magnetic moments of individual ion, the ions distribution in the spinel crystal structure, and the exchange interaction between tetrahedral and octahedral positions.
Where RL, Z 0, and Z in are the reflection loss (dB), the characteristic impedance of free space, and input impedance, respectively. The characteristic impedance and the input impedance are presented in equation (7) and (8).
Where r  and r  represent the complex permeability and permittivity, f represents the frequency of operation, t represents the thickness of the sample, and c is the light velocity. Based on Figure 7, there were two reflection losses of the absorption peaks at frequencies of approximately -7.0 dB and -11.0 dB at 8 GHz 10.8 GHz, respectively. The electromagnetic wave absorption after calculating reaches about 80 % at the highest peak frequency of 10.8 GHz with a thickness of 1.0 mm. Low reflectivity of the material makes excellent absorption performance. It means that the strongest performance of the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle in term of absorbing electromagnetic wave is -11.0 dB at the frequency of 10.8 GHz. Therefore, the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle prepared by a coprecipitation-sonochemical method in this experiment exhibits its sustainability as a radar absorbing material, especially for military purposes. Usually, the radar absorbing materials for military applications are maintained at a frequency in the range of 8 to 12 GHz [8]. The materials should have magnetic and dielectric characters with proper impedance and interaction with the electromagnetic energy to give a high performance in absorbing electromagnetic wave.

Conclusion
The Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle was successfully prepared by employing a mixed coprecipitationsonochemical method using iron sand as the core precursor. From the XRD data analysis, the Zn 0.5 Mn 0.5 Fe 2 O 4 particle formed a spinel cubic structure in a single phase with the particle size in nanometric scale and a lattice parameter of approximately 8.401 Å. Furthermore, the data analysis of the M(H) curve presented that the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle has a superparamagnetic behavior with the saturation magnetization of approximately 43 emu/g. Finally, the data analysis of the reflection loss as a function of frequency showed that the Zn 0.5 Mn 0.5 Fe 2 O 4 nanoparticle performs a radar absorbing material with the absorption performance of -11.0 dB at the frequency of 10.8 GHz.