Rietveld analysis results for the XRD patterns of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders.
Ferrite-based nanoparticles, namely, bismuth ferrite (BiFeO3) and calcium ferrite (CaFe4O7), have been synthesized via sol-gel and chemically dissolved method, respectively, employing hematite (α-Fe2O3) as the Fe3+ ion source. Firstly, α-Fe2O3 nanoparticles were prepared from natural iron sand containing mostly magnetite (Fe3O4) phase through coprecipitation technique continued by sintering process at 800°C for 2 h. Higher BiFeO3 phase content was achieved after Bi-Fe gel being annealed at 650°C for 1 h in air atmosphere. Furthermore, major phase of CaFe4O7 was formed with molar ratio of Fe3+/Ca2+ = 6 and sintering temperature of 800°C for 3 h. Interestingly, the powders with dominant CaFe4O7 phase, known as calcium biferrite, exhibit higher ferromagnetism at room temperature. The magnetic properties of the calcium biferrite are comparable to those of barium hexaferrite which can be applied for radar-absorbing material. Meanwhile, BiFeO3 powders also show weak room temperature ferromagnetism. It has also demonstrated that Ni doping in the bismuth ferrite (BiFe1−xNixO3 with x = 0.1) nanoparticles results in enhancement of the magnetic properties. Moreover, a ferroelectric hysteresis loop and a trend of frequency dependence of the dielectric constant have been observed, which were enhanced by Pb doping (Bi1−yPbyFeO3 with y = 0.1). These results suggest a multiferroic behavior in the BiFeO3 nanoparticles.
- bismuth ferrite
- calcium ferrite
- iron sand
Development of functional nanomaterials for scientific and industrial applications is very crucial for advanced technologies. The use of natural resources as the starting compounds for producing nanomaterials is currently developing. Many researchers are exploring natural materials and even waste biomass applied as a functional material that has a high selling value for various specific applications. For example, the use of silica sand from Tanah Laut, Kalimantan, Indonesia, as a raw material for manufacturing pure SiO2, zircon, and zirconia with high phase purity and crystallite size in nanometer range was reported . Moreover, natural iron sand exploration as a starting material has been shown to produce magnetite (Fe3O4) nanoparticles as magnetic coating, magnetic fluid (ferrofluid), and magnetic gel (ferrogel) for radar-absorbing materials, biomedical applications, and tissue engineering, respectively [2, 3, 4, 5].
Fe3O4 is one of the magnetic particles that can be obtained from natural iron sand after conducting the separation technique from its impurities by mechanical and chemical processes. In nature, iron sand consists of more than 90 wt% of Fe3O4 particles. Generally, Fe3O4 has been synthesized using commercial raw materials, such as FeCl2.4H2O and FeCl3.6H2O . The commonly used synthesis methods are sol-gel, hydrothermal, and coprecipitation techniques [7, 8, 9]. Because Fe3O4 nanoparticles tend to agglomerate among particles, the addition of surfactants or templates has been widely applied to produce homogeneous nanoparticles with certain sizes and morphologies [10, 11, 12, 13, 14]. Research on preparing Fe3O4 nanoparticles from iron sand has been the main topic for the past few years. The use of doping, for example, doping Mn and Zn, on Fe3O4 makes it superparamagnetic so that it can be applied in biomedicine applications [15, 16, 17, 18].
Hematite (α-Fe2O3) is the most stable iron oxides at high temperatures. α-Fe2O3 is commonly obtained from iron rust which is one of the dominant corrosion products of iron metal or iron alloys. In general, α-Fe2O3 nanoparticles have been successfully prepared by several methods, namely, hydrothermal  and coprecipitation technique , using commercial raw materials, such as Fe(NO3)3·9H2O and FeCl3.6H2O, respectively. It is found that the concentration of Fe3+ ions used in the preparation of α-Fe2O3 nanoparticles may influence the particle size and morphology, as well as the optical bandgap . α-Fe2O3 nanoparticles with particle size of 8 nm possess superparamagnetic properties with relatively high magnetization at room temperature . Therefore, it is possible to be applied for biomedical and spintronic applications. Moreover, Liu et al. have successfully prepared porous Fe2O3 nanorods with particle size of ~10 nm and pore sizes in the range of 5–50 nm. These porous Fe2O3 nanorods exhibit excellent photocatalytic properties .
In the field of environmental engineering, α-Fe2O3 nanoparticles can be synthesized from hydrated ferric chloride and ferrous sulfate salt solution through chemical coprecipitation method and calcination process at relatively high temperature of 500°C . In addition, a simple chemically coprecipitation method has been employed to obtain Fe3O4 nanoparticles using HCl and NH4OH as dissolving and precipitating agent, respectively . Some researchers have investigated the transformation from Fe3O4 to α-Fe2O3 phase through oxidation process of Fe2+ to Fe3+ ions . It is noted that Fe3O4 nanoparticles could be transformed into maghemite (γ-Fe2O3) and hematite (α-Fe2O3) via dry oxidation process at temperature range between 350 and 400°C and 600 and 800°C, respectively . Focusing on the use of natural resources as raw materials for synthesizing functional materials, in this chapter, α-Fe2O3 nanoparticles were synthesized from natural iron sand through chemical coprecipitation method followed by sintering process at temperature of 800°C. Then, the obtained α-Fe2O3 nanoparticles were utilized as one of the raw materials for preparing calcium ferrite (Ca-ferrites) and bismuth ferrite (BiFeO3) nanoparticles as potential materials for radar-absorbing and data storage materials, respectively. The physical characterizations for all obtained ferrite-based nanoparticles include elemental and phase identification, particle morphology, and magnetic and electrical properties.
Based on the phase diagram of CaO-Fe2O3 system [26, 27], it is known that there are three main phases of calcium ferrite compounds and those are 2CaO.Fe2O3 (Ca2Fe2O5), CaO.Fe2O3 (CaFe2O4), and CaO.2Fe2O3 (CaFe4O7). It is possible that the reaction between CaO and Fe2O3 results in other unstable calcium ferrite phases, such as CaFe12O19. In addition, Boyanov  has pointed out that the mixture of CaCO3-Fe2O3 after thermal treatment has produced various types of calcium ferrite compounds consisting of ~50% CaO.2Fe2O3, ~20% CaO.Fe2O3, ~8% 2CaO.Fe2O3, and other ferrite products. The formation of calcium ferrite compounds depends on the kinetics of chemical reaction at the boundary between the phases and oxide diffusion during the reaction affected by the concentration ratio of the existing Ca2+ and Fe3+ ions as the precursors and also the atmospheric condition .
Calcium ferrite compounds exhibit soft ferromagnetism, and, therefore, it can be used for radar-absorbing materials in the calcium ferrite/graphite nanocomposites . In this case, calcium ferrite nanoparticles have magnetic properties that are comparable to barium ferrite (BaO.6Fe2O3) and strontium ferrite (SrO.6Fe2O3) known as M-type hexaferrite for microwave-absorbing applications. In order to be used for this application and also for biomedical applications as targeted drug delivery, calcium ferrite should exhibit superparamagnetic behavior . Compared with the other ferrites, such as MFe2O4 (M = Zn, Mn, Ni, and Cu), CaFe2O4 is one of the biocompatible materials and environmentally friendly due to the use of calcium rather than heavy metals. Moreover, Ca2Fe2O5 with the brownmillerite structure has a specific application as p-type thermoelectric device . This is due to the fact that this compound has interesting electrical properties [33, 34]. Oxygen deficiencies in the Ca2Fe2O5 crystals may enhance the electrochemical activity . On the other hand, CaFe4O7 has not been explored yet regarding its magnetic properties. In contrast to the other calcium ferrites, in this chapter, CaFe4O7 nanoparticles were prepared by mixing Fe2O3 from natural iron sand and CaCO3 from natural limestone.
Bismuth ferrite (BiFeO3) is one of multiferroic system showing a magnetic-electric coupling at room temperature. Multiferroic material has perovskite structure with chemical formula ABO3. The type of A and B sites, the cation nonstoichiometry, and the presence of oxygen vacancies may have an impact on the structural, electronic, and magnetic properties . BiFeO3 crystallizes in a distorted rhombohedral perovskite with space group R3c . It has high Curie temperature and Néel temperature of 1100 and 640 K, respectively . It is difficult to obtain a pure phase of BiFeO3 because the kinetics of phase formation leads to the formation of secondary phases, such as Bi25FeO40 (sillenite) and Bi2Fe4O9 (mullite). Various techniques have been reported to prepare single phase of BiFeO3, and those are chemical coprecipitation , hydrothermal , and sol-gel methods [41, 42, 43]. The ideas of those techniques are to achieve a single phase of BiFeO3 with a simple route, low temperature, and cost-effectiveness. Wang et al. have found that the formation of BiFeO3 phase starts at 425°C with impurity phases about 30% by the low-heating temperature solid-state precursor method [44, 45]. Further calcination from 450 to 550°C results in a pure BiFeO3 phase without any impurity phases. However, impurity phase of Bi2Fe4O9 has been detected in the powder calcined at above 650°C. Moreover, BiFeO3 nanoparticles synthesized by microwave-assisted sol-gel method at calcination temperature of 450°C exhibit a pure phase of BiFeO3 structure with particle size of 40 nm and no detected secondary phase .
Magnetic and dielectric properties of BiFeO3 nanoparticles are determined by the introduction of doping and particle size influenced by the synthesis method, temperature, and duration of calcination. It has been found that all magnetic parameters, such as saturation magnetization, enhance with decreasing particle size . BiFeO3 nanoparticles with the size below 100 nm have weak ferromagnetism at room temperature. This ferromagnetic behavior in the nanoparticles is due to the presence of oxygen vacancies in BiFeO3 system [41, 47]. Enhancement of magnetic as well as dielectric properties in BiFeO3 can be achieved by adding doping of Mn, Ni, Pb, Ti, Sr, and Zn [48, 49, 50, 51, 52, 53, 54, 55, 56]. Up to the present, there have been various studies examining the doping effects of BiFeO3 nanoparticles with numerous advanced techniques to improve their performance. In the case of the enhancing magnetization induced by doping, it has been suggested that this is probably due to increasing distortion of local structure, increasing the effect of Dzyaloshinskii-Moriya (DM) interaction, distortion of Fe and O bonding, destruction of spin cycloid structure, and the presence of impurity phase in the BiFeO3 systems [53, 57]. Besides affecting the magnetic properties, introduction of doping in BiFeO3 leads to the improvement of dielectric and ferroelectric properties [50, 58, 59]. Yuan et al.  have found that a sufficient amount of Sr/Pb doping can improve the magnetic properties as well as high-frequency dielectric properties.
In addition, the dielectric properties of pure BiFeO3 phase strongly depend on the atmospheric condition during the powder synthesis. Liu et al.  have found a higher spontaneous polarization and lower breakdown field based on polarization-electrical field (P-E) hysteresis loops in the samples annealed in H2 and N2 atmospheres. In this chapter, BiFeO3 nanoparticles were synthesized by sol-gel method using natural iron sand as one of the raw materials and calcined in air atmosphere. Then, the ferroelectric and the dielectric properties were intensively investigated in the Pb- and Ni-doped BiFeO3 nanoparticles.
2. Preparation of hematite (α-Fe2O3) nanoparticles
Prior to the preparation of α-Fe2O3 nanoparticles, at first, Fe3O4 nanoparticles were synthesized from natural iron sand as the raw material by coprecipitation technique using HCl as dissolving agent and NH4OH as precipitating agent. The detail of experimental procedure to synthesize Fe3O4 nanoparticles was also described in elsewhere . First of all, the extracted iron sand was collected and dissolved in 12 M HCl at ~70°C under continuous and constant stirring of 600 rpm. The obtained solution from the reaction process was filtered and added slowly with 6.49 M NH4OH under the same temperature and stirring speed for 30 minutes. Then, the black precipitates were formed. The precipitate (Fe3O4 phase) was initially washed with distilled water until pH 7 and then dried at 70°C for 5 h. In order to get α-Fe2O3 phase, the dried nanopowder (Fe3O4 phase) was calcined at 800°C for 2 h, as shown in Figure 1. Finally, the Fe2O3 powders from this calcination were continued by performing coprecipitation process again with the same experimental procedure as before until the precipitation process. A reddish precipitate (Fe2O3.H2O) was formed. The resulted precipitate was then washed and collected for further synthesis of CaFe4O7 and BiFeO3 (without and with doping of Pb and Ni) nanoparticles.
3. Preparation of calcium ferrite nanoparticles
Calcium biferrite (CaFe4O7) nanoparticles were synthesized by the so-called chemically dissolved method using precipitated CaCO3 and Fe2O3 as Ca2+ and Fe3+ ion sources, respectively. Fe2O3 powders were obtained as described previously from natural iron sand, whereas the precipitated CaCO3 particles were synthesized from natural limestone through carbonation process. First, the natural limestone was extracted from the existing impurities, such as silica, and then it was calcined at 900°C for 6 h to produce CaO. The CaO powder was dissolved into distilled water to produce Ca(OH)2 solution. The carbonation process using CO2 gas flow was performed until it formed a precipitation at pH around 7. The precipitated CaCO3 was filtered and dried for further synthesis. The detail procedure was also explained in the former paper by Arifin et al. .
In the synthesis of the calcium ferrite nanoparticles using the chemically dissolved method, the obtained Fe2O3 and precipitated CaCO3 were dissolved in HNO3 to get Fe(NO)3 and Ca(NO)2 solutions, respectively, with a molar ratio of 1:6. Both solutions were mixed homogeneously and heated at constant temperature (80°C) and stirring rate (600 rpm) until it formed slurry precipitates. The precipitates were washed using distilled water and dried at 80°C for 10 h. The resulted powders were collected and then sintered at 800°C for 3 h.
4. Preparation of bismuth ferrite (BiFeO3) nanoparticles without and with Pb and Ni doping
Nanoparticles of undoped, Pb- and Ni-doped BiFeO3 (BiFeO3, Bi0.9Pb0.1FeO3, and BiFe0.9Ni0.1O3, respectively) were prepared by sol-gel method. The starting materials were Fe2O3 synthesized previously from iron sand (94%) as the Fe3+ ion source and Bi2O3 (Aldrich, 99.9%) as the Bi3+ ion source. Pb(NO3)2 (powder, 99%) and Ni(NO3)2.6H2O (powder, 99%) were used as the Pb and Ni doping, respectively. Fe2O3, Bi2O3, Pb(NO3)2, and Ni(NO3)2.6H2O powders were dissolved separately by HNO3 (Merck, 65%) to form solutions of ferrite nitrate, bismuth nitrate, lead nitrate, and nickel nitrate, respectively, with the stoichiometric molar ratio of (Bi, Pb):(Fe, Ni) = 1:1. Acetic acid was added into each solution under constant stirring and temperature for 30 minutes. Then, it was followed by addition of ethylene glycol under the same condition. Next, the obtained solutions were mixed together under the same temperature and stirring rate for 1 h. The resulted solution was dried at 80°C for 6 days to obtain the undoped and doped BiFeO3 xerogels. The dried gels were ground and collected. Finally, the powders were calcined in air at 650 and 700°C for 1 h to form undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3), respectively, for further characterizations.
A thermogravimetric/differential thermal analysis (TG/DTA) was performed to determine the thermal behaviors of the dried gel of bismuth ferrite. The phase formation and crystal structure of all samples were characterized by X-ray diffraction (XRD) with Cu-Kα radiation and λ = 1.54056 Å for scanning 2θ range of 20–70°. The lattice parameters and average crystallite sizes were determined by XRD patterns which were analyzed by the Rietveld method using the Rietica and MAUD programs [62, 63]. Transmission electron microscopy (TEM) with selected area electron diffraction (SAED) pattern was conducted to investigate the particles’ morphology and crystal structure confirmation of all ferrite-based samples. The magnetic properties of the nanoparticles were measured using vibrating sample magnetometry (VSM, Oxford VSM1.2H) and superconducting quantum interference device (SQUID) magnetometer in external magnetic field range of ±1 T at room temperature. The ferroelectric properties of the bismuth ferrites were studied from the polarization-electric field (P-E) hysteresis loops using a polarization meter (Radiant Technologies 66A). Frequency dependence of the dielectric constant of all bismuth ferrites was estimated by two-probe electrical resistance using Automatic RCL Meter (type PM6303A).
6. Structural and magnetic properties of calcium ferrites from natural iron sand and limestone
Figure 2 shows the XRD pattern of calcium ferrite compound synthesized by the chemically dissolved method from natural iron sand and limestone as the raw materials and then sintered at 800°C for 3 h. Based on the analysis of phase identification, it can be seen that the resulted powder contains several phases of calcium ferrites, CaFe4O7, Ca4Fe14O25, and Ca2Fe9O13, with weight percentages of 28.8, 46.6, and 24.6 wt%, respectively. The formation of those phases is possible to occur due to the atmospheric condition during calcination. Generally, at relatively high calcination temperatures, the most stable phases are those that have higher coordination numbers, in this case with surrounding oxygen. Hughes et al.  have also identified these distinct calcium ferrite phases in the mixture of CaO and Fe2O3 calcined in air at high temperatures between 1180 and 1240°C. In addition, the phase formation of Ca2Fe9O13 can be present in the compound at the lower temperatures . With the increase of temperature, the phase formation becomes more complex. Related to the phase transformation, it strongly depends on the crystallization kinetics of the reaction, the ratio concentration between Ca and Fe ions, and the atmospheric condition .
Focusing on the high intensities of the diffraction peaks, the sample exhibits XRD lines of both CaFe4O7 and Ca4Fe14O25 phases as the dominant phases. CaFe4O7 has monoclinic structure and Ca4Fe14O25 has hexagonal structure. Both phases have similar crystalline structure related to hexagonal ferrite structures . The XRD pattern in Figure 2 shows that CaFe4O7 and Ca4Fe14O25 phases have broad diffraction peaks. This indicates that the average crystallite sizes are in a nanometer scale. Based on the Rietveld analysis, CaFe4O7 phase in the calcium ferrite compound has average crystallite size of about 46 nm. In order to clarify the nano-sized particles, TEM image is important to be investigated in detail.
Figure 3 displays TEM image of the calcium ferrite sample together with the selected area electron diffraction (SAED). The TEM image proves that the particle size of the sample is in the range of 40–60 nm. This is in a good agreement with the Rietveld analysis of the XRD pattern in Figure 2. The analysis of electron diffraction from SAED pattern reveals that CaFe4O7 and Ca4Fe14O25 phases are dominantly present and Ca2Fe9O13 is the minor phase in the sample. This result is also consistent with the XRD pattern analysis.
Magnetic properties of the calcium ferrite compound were studied by the magnetic hysteresis curve (M-H curve) at room temperature as shown in Figure 4. It is clear that the sample exhibits ferromagnetic behavior. A detailed observation on the M-H curve of the sample shows that the values of remanent magnetization and magnetization at 1 T are 2.11 and 10.94 emu/g, respectively. This indicates that a soft magnetism is realized in the calcium ferrite compound. It has been found that the dominant phase existing in the sample has a contribution to the ferromagnetic behavior . The value of magnetism in the sample is comparable with that of the barium-calcium hexaferrite prepared by sol-gel and microemulsion techniques, in which the saturation magnetization value is approximately 24 emu/g . Moreover, Samariya et al.  have studied the magnetic properties of calcium ferrite, in the form of CaFe2O4, nanoparticles. They have found similar value of magnetization compared with the present result in this work. Concerning the multiphase compound, the magnetic parameters in the sample are influenced by the presence of nonmagnetic phase, magnetic domain and its orientation, and defect formation. Therefore, it is important to investigate more detail on how to prepare a pure certain phase of calcium ferrite from natural resources as the starting materials. Accordingly, this result demonstrates that the present calcium ferrite nanoparticles could be used as one of the potential materials for microwave absorption application.
7. Magnetoelectric properties of bismuth ferrite nanoparticles
TG/DTA curve of the uncalcined powder of the undoped BiFeO3, shown in Figure 5, exhibits about 29% weight loss from room temperature to 550°C due to the evaporation of water, organics, and nitrate decomposition [71, 72]. Based on this thermal behavior, the powder could be thermally treated at temperatures from 500 up to 700°C for 1 h. Carvalho et al.  have reported that the increasing time of the heat treatment increases the formation of secondary phases and, therefore, they have suggested to avoid a long heat treatment to synthesize BiFeO3 nanoparticles.
Figure 6 shows the XRD patterns of the undoped and doped BiFeO3 samples calcined at 650 and 700°C, respectively, for an hour in air atmosphere. This heat treatment was conducted to form BiFeO3 phase. The influence of the atmosphere in the phase formation has been investigated by Xu et al. . They have reported that crystallization in the atmosphere is important to obtain a pure BiFeO3 phase prepared by sol-gel method. It can be seen from the phase identification of the XRD patterns that multiphases of bismuth ferrite compounds such as BiFeO3, Bi25FeO40, and Bi2Fe4O9 were observed in the synthesized powders. Moreover, Bi2O3 was still observed in the XRD patterns in minor composition. BiFeO3 is a metastable phase which easily decomposes to secondary phases, Bi25FeO40 and Bi2Fe4O9, at high temperatures . In this present work, it is found that higher BiFeO3 phase is achieved with heat treatment at 650°C for 1 h. This result is consistent with the TG/DTA and XRD data analyzed by Sakar et al.  which corresponds to sharp diffraction peaks of the BiFeO3 phase. The formation of secondary phases increases at higher temperature than 650°C. BiFeO3 began to decompose because of its unstable thermodynamic character when the calcination temperature was further increased. The relative weight percent and average crystallite size of the BiFeO3 phase were determined from the diffraction patterns by Rietveld method using Rietica and MAUD program, respectively. Overall, the analysis results show that the bismuth ferrite powders contain about 75 wt% of BiFeO3 phase. The average crystallite size of the BiFeO3 sample prepared at 650°C is about 84 nm.
The addition of doping substituting the A and B sites in the ABO3 perovskite structure of BiFeO3 greatly affects the crystal distortion and changes in the composition of the secondary bismuth ferrite phases. Pb ion substitutes A site, namely, the Bi3+ ion, in the structure of BiFeO3. As a result, Pb doping has an effect on the diffraction peak shift of the BiFeO3 phase to the lower diffraction angle. This is because the ionic radius of Pb2+ ion (0.119 nm) is greater than that of Bi3+ ion (0.103 nm). Moreover, it can also be seen that there is a combination of the diffraction peaks for the crystal plane (006) and (202) into the diffraction peak (111) at 2θ of 31–32o. This indicates a small change in the distortion of the crystal from distorted rhombohedral to pseudocubic system. XRD analysis confirms that Bi0.9Pb0.1FeO3 has cubic structure with space group of Pm-3 m, compared with the undoped BiFeO3 having rhombohedral structure with space group of R3c. It is important to mention that the secondary phase in the Pb-doped BiFeO3 (Bi0.9Pb0.1FeO3) sample, which is PbFe12O19, has been reported to be one of the hexaferrite materials exhibiting good superparamagnetic behavior . Further Rietveld analysis from the XRD patterns gives the values of lattice parameters of BiFeO3, Bi0.9Pb0.1FeO3, and BiFe0.9Ni0.1O3 as shown in Table 1.
|Sample||Structure||Lattice parameters (Å)|
|BiFeO3||Rhombohedral||a = b = 5.578 (1)|
|c = 13.862 (3)|
|Bi0.9Pb0.1FeO3||Cubic||a = b = c = 3.958 (1)|
|BiFe0.9Ni0.1O3||Rhombohedral||a = b = 5.574 (1)|
|c = 13.840 (4)|
On the XRD pattern of the Ni-doped BiFeO3 (BiFe0.9Ni0.1O3) sample, shown in Figure 6, it is clear that there is no change of the crystal structure due to Ni doping at the B site (Fe3+ ion) of BiFeO3 crystal. This is displayed by the rhombohedral peak which can still be observed at 2θ of 31–32o. The result of the phase composition analysis gives that there is an increase of secondary phases (Bi25FeO40) and the presence of NiFe2O4 in the sample. Interestingly, both secondary phases have also unique magnetoelectric properties. It has been reported by Zhu et al.  that Bi25FeO40 has good dielectric and electrical properties which can be used as one of integrated circuit components. NiFe2O4 is one of magnetic spinel structures with good magnetic and dielectric properties . In addition, Ni doping in the BiFeO3 system has an effect on diffraction peak shift to the lower diffraction angle because ionic radius of Ni3+ ion (0.069 nm) is slightly larger than that of Fe3+ ion (0.065 nm). The change of lattice parameter due to Pb and Ni doping in BiFeO3 system is summarized in Table 1.
Figure 7 shows the TEM image and selected area electron diffraction (SAED) patterns of BiFeO3 powders annealed at 650°C for 1 h in air. Sharp diffraction spots seen from SAED pattern confirm the formation of well crystalline bismuth ferrites. Phases identified from SAED pattern are relatively matching with the XRD patterns in Figure 6 consisting of BiFeO3, Bi25FeO40, Bi2Fe4O9, and Bi2O3. The TEM image shows typical morphology of particle agglomeration. The particle size is greater than the average crystallite size estimated by Rietveld analysis due to agglomeration of the nanoparticles.
The nonlinear magnetic hysteresis curve of the bismuth ferrite powders, as shown in Figure 8, illustrates weak ferromagnetism. The remanent magnetization of 0.044 emu/g and coercive field of 68.5 Oe in the undoped BiFeO3 confirm the weak ferromagnetism behavior at room temperature. The complete saturation of magnetization of powders was not achieved up to applied magnetic field of 1 T. The hysteresis loop of bulk BiFeO3 is generally linear indicating antiferromagnetic order at the ground state (5 K) . The weak ferromagnetic order of these powders can be understood as a result of residual magnetic moment caused by its canted spin structure . The canting of the spins can be caused by reduction of particle size. When the particle size decreases, the number of surface asymmetry atoms increases, then it changes the angle of the helical ordered spin arrangement, and finally the net magnetic moment appears . Moreover, the existence of defects, for instance, oxygen vacancies , and the secondary phases  may contribute to the weak ferromagnetic behavior.
Based on the magnetic hysteresis loops of the doped BiFeO3 nanoparticles, the Pb doping in the BiFeO3 structure seems to have a small effect on the magnetic properties. Substitution of Pb2+ ions at the Bi3+ sites induces oxygen vacancies which may lead to the enhancement of magnetic moments in the sample . However, Verma and Kotnala  have confirmed through the SQUID measurements that BiFeO3 with Pb doping exhibits a strong antiferromagnetism suggesting that the reduction of oxygen vacancies is realized in the system. Moreover, Ederer and Spaldin  have proposed that the magnetization value can be affected by the presence of oxygen vacancies but with a small change due to the formation of Fe2+ at the BiFeO3 sites adjacent to the vacancy. Therefore, there is almost no increase in the magnetic parameters after Pb doping. Moreover, the weak ferromagnetism is commonly observed in the Bi1−xAxFeO3 (A = Ca, Sr., Pb, Ba) system providing a canting of the antiferromagnetic sublattice , which is in line with this present work. On the other hand, Ni-doped BiFeO3 nanoparticles show a significant increase on the magnetic parameters, namely, remanent and saturation magnetization. This result is consistent with the previous paper by Hwang et al. , in which the Ni-doped BiFeO3 sample exhibits similar rhombohedral perovskite structure compared to that of the undoped one and the magnetic properties show enhancement with respect to the undoped one. The increase in magnetic properties can occur due to the effect of nanoparticle surface area and ferromagnetic interaction exchange between neighboring Fe3+ and Ni3+ ions in the BiFeO3 system .
The room temperature P-E loop of the prepared undoped bismuth ferrite, presented in Figure 9, exhibits unsaturated hysteresis loop. The curve was not fully saturated because of the low applied electric field. The remanent polarization (Rs) and the coercive field (Ec) of the undoped BiFeO3 nanoparticles are about 20.5 μC/cm2 and 5.5 V/cm, respectively. These values are lower than the values reported in the single crystal which has a large polarization of ~100 μC/cm2 along (111) for bulk bismuth ferrite . The existence of secondary phases, such as Bi25FeO40, Bi2Fe4O9, and Bi2O3, affects the lower values of Rs and Ec in the sample. Pradhan et al.  have reported that leakage current is one of the major reasons for obtaining lower values of saturation polarization (Ps), Rs, and Ec in BiFeO3 system.
In the Pb-doped BiFeO3 nanoparticles, the Pb substitution improves the dielectric and ferroelectric properties . It can be seen from Table 2 that the electric properties, including dielectric constant, electrical conductivity, and electrical permittivity, increase with Pb doping in the BiFeO3 crystal. It has been found that Pb substitution on the Bi site in the BiFeO3 may destroy ferroelectricity ordering induced by Bi lone pair in the rhombic structure until it reaches a stable pseudocubic structure of BiFeO3 . In this work, addition of Pb doping in BiFeO3 with x = 0.1 has already resulted in a pseudocubic structure, and, hence, the enhancement of the electrical properties is realized in the present sample. The value of dielectric constant with Pb doping, x = 0.1, at 1 kHz is in a good agreement with the work done by Zhang et al. . The defect of oxygen vacancy due to Pb doping can increase the polarity of the sample and finally increase its dielectric constant. In addition, oxygen vacancy created as the consequence of Pb substitution on Bi site in the BiFeO3 system plays an important role related to the ferroelectricity for Pb-doped BiFeO3 sample. Moreover, the presence of Pb doping causes the existence of Fe2+ ion at Fe3+ sites which can produce holes around the Fe3+ site . This effect is shown by the increasing value of electrical conductivity. It has been suggested that the relatively low number of oxygen vacancies in this sample may result in an improvement of the ferroelectric properties , as shown in Table 2.
|Sample||Dielectric constant (εr)|
f = 1 kHz,T = 300 K
|Conductivity [×10−4 (Ω m)−1]|
T = 300 K
|Permittivity (×10−10 F/m)|
T = 300 K
As mentioned earlier, the Ni doping in BiFeO3 nanoparticles enhances the magnetic properties as reported in the former paper . However, the dielectric and other electrical properties of the Ni-doped BiFeO3 have lower values than those of the undoped one, as displayed in Table 2. This means that the sample has inappropriate Ni doping concentration to improve the ferroelectricity. Moreover, the reduction in the dielectric constant is attributed to the decrease in the total polarization occurring in the sample. It is well known that the total polarization of a dielectric material is a combination of electronic, ionic, dipolar, and interfacial/space charge polarizations. The lower value of dielectric constant is probably caused by the effect of Ni doping on the ionic transformation from Fe2+ to be Fe3+ again. As the consequence of the charge stability, it may consume holes. Hence, the holes as charge carrier decrease. This is one reason of the decrease of sample’s conductivity . Another possible reason on decreasing value of electrical properties in Ni-doped BiFeO3 sample is the impurity effect. It should be noticed that the impurity phases such as Bi2Fe4O9 and Bi25FeO40 may also contribute to the electrical properties in BiFeO3 . The existence of multiphase in the sample leads to the increase of insulating grain boundaries affecting the electrical conductivity as well as the total polarization in the sample. The increase in the amount of grain boundaries, acting as the barrier for charge carrier mobility, results in the decrease of conductivity in the system.
Exploration related to the use of natural materials for functional materials has been applied in this study. Natural iron sand with the dominant magnetite (Fe3O4) content has been successfully synthesized through the chemical coprecipitation method as a starting material for producing hematite (α-Fe2O3). α-Fe2O3 has been successfully used as the source of Fe3+ ions to synthesize calcium ferrite and bismuth ferrite nanoparticles. The calcium ferrite powders synthesized by the chemical dissolved technique produce nano-sized crystals with the dominant phases of CaFe4O7 and Ca4Fe14O25. The calcium ferrite powder has soft magnetic properties at room temperature which is attributed to the presence of dominant ferromagnetic phase and also oxygen vacancy in the nanoparticles. Magnetic parameters, such as saturation magnetic, are comparable to the barium-calcium hexaferrites, so that these nanoparticles have the potential application as microwave-absorbing materials. The bismuth ferrite powder, synthesized by the sol-gel method, exhibits multiferroic properties. The undoped BiFeO3 possesses a weak ferromagnetism at room temperature. The magnetic parameters can be enhanced by Ni doping in the form of BiFe0.9Ni0.1O3 nanoparticles. On the other hand, the electrical properties, i.e., dielectric constant, permittivity, and electrical conductivity, can be improved by Pb doping in the nanoparticles of Bi0.9Pb0.1FeO3. The multiferroic behaviors are strongly determined by the nano-sized effects, the presence of oxygen vacancies and impurities, and also the doping type affecting the phase stability in the perovskite structure of BiFeO3 crystals. Considering the importance of applying these ferrite-based nanoparticles, investigations for obtaining pure phases of the nanoparticles from natural resources are very important and need further study.
This work was partially supported by the grant of the International Research Collaboration, provided by DRPM, Ministry of Research, Technology and Higher Education, 2017–2019. We are thankful to Prof. Y. Kohori and Dr. H. Fukazawa, Chiba University, Japan, for the use of magnetic hysteresis measuring apparatus.
Conflict of interest
We state that the article is original and all authors are aware of its content and approve its submission. This article has not been published previously, and it is not under consideration for publication elsewhere. I confirm that there is no conflict of interest exists.