**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.

#### **5. 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. [64] 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 [65]. 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 [66].

#### **Figure 2.**

*XRD pattern of calcium ferrite powders synthesized by the chemically dissolved technique from natural iron sand and limestone as the Fe3+ and Ca2+ ion sources, respectively, and then continued by calcination process at 800°C for 3 h.*

**39**

**Figure 3.**

tion application.

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source*

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 [67]. 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,

**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

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 [68]. 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 [69]. Moreover, Samariya et al. [70] 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 absorp-

*TEM image with selected area electron diffraction (SAED) pattern for calcium ferrite powders synthesized by the chemically dissolved technique from natural iron sand and limestone as the Fe3+ and Ca2+ ion sources,* 

*respectively, and then continued by calcination process at 800°C for 3 h.*

*DOI: http://dx.doi.org/10.5772/intechopen.88027*

TEM image is important to be investigated in detail.

with the XRD pattern analysis.

#### *Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source DOI: http://dx.doi.org/10.5772/intechopen.88027*

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 [67]. 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 [68]. 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 [69]. Moreover, Samariya et al. [70] 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.

#### **Figure 3.**

*TEM image with selected area electron diffraction (SAED) pattern for calcium ferrite powders synthesized by the chemically dissolved technique from natural iron sand and limestone as the Fe3+ and Ca2+ ion sources, respectively, and then continued by calcination process at 800°C for 3 h.*

*Nanocrystalline Materials*

RCL Meter (type PM6303A).

**iron sand and limestone**

ions, and the atmospheric condition [66].

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

**6. Structural and magnetic properties of calcium ferrites from natural** 

**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. [64] 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 [65]. 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

*XRD pattern of calcium ferrite powders synthesized by the chemically dissolved technique from natural iron sand and limestone as the Fe3+ and Ca2+ ion sources, respectively, and then continued by calcination process at* 

**38**

**Figure 2.**

*800°C for 3 h.*

#### **Figure 4.**

*Magnetization curve at room temperature for calcium ferrite powders synthesized by the chemically dissolved technique from natural iron sand and limestone as the Fe3+ and Ca2+ ion sources, respectively, and then continued by calcination process at 800°C for 3 h.*

#### **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. [73] 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. [72]. 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 [73]. 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. [74] 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

**41**

(111) at 2θ of 31–32o

**Figure 6.**

**Figure 5.**

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source*

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

*XRD patterns of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders* 

*synthesized by sol-gel method calcined at 650 and 700°C, respectively, for 1 h in air.*

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 [75]. 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**.

. This indicates a small change in the distortion of the crystal

*DOI: http://dx.doi.org/10.5772/intechopen.88027*

*TG/DTA curves of the uncalcined BiFeO3 powder.*

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source DOI: http://dx.doi.org/10.5772/intechopen.88027*

**Figure 5.** *TG/DTA curves of the uncalcined BiFeO3 powder.*

#### **Figure 6.**

*Nanocrystalline Materials*

**7. Magnetoelectric properties of bismuth ferrite nanoparticles**

of the BiFeO3 sample prepared at 650°C is about 84 nm.

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. [73] 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

*Magnetization curve at room temperature for calcium ferrite powders synthesized by the chemically dissolved technique from natural iron sand and limestone as the Fe3+ and Ca2+ ion sources, respectively, and then* 

**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. [72]. 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 [73]. 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. [74] 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

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

**40**

nanoparticles.

**Figure 4.**

*continued by calcination process at 800°C for 3 h.*

*XRD patterns of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders synthesized by sol-gel method calcined at 650 and 700°C, respectively, for 1 h in air.*

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 [75]. 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**.

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. [76] 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 [77]. 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) [78]. The weak ferromagnetic order of these powders can be understood as a result of residual magnetic moment caused by its canted spin structure [79]. 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 [80]. Moreover, the existence of defects, for instance, oxygen vacancies [81], and the secondary phases [82] 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 [83]. However,


**Table 1.**

*Rietveld analysis results for the XRD patterns of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders.*

**43**

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source*

*TEM image with selected area electron diffraction (SAED) pattern for barium ferrite powders synthesized by* 

Verma and Kotnala [84] 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 [85] 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−*x*A*x*FeO3 (A = Ca, Sr., Pb, Ba) system providing a canting of the antiferromagnetic sublattice [86], 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. [87], 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 neigh-

*Magnetic hysteresis curves of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3)* 

boring Fe3+ and Ni3+ ions in the BiFeO3 system [88].

*DOI: http://dx.doi.org/10.5772/intechopen.88027*

*sol-gel method and then calcined at 650°C for 1 h in air.*

**Figure 7.**

**Figure 8.**

*powders synthesized by the sol-gel method.*

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source DOI: http://dx.doi.org/10.5772/intechopen.88027*

#### **Figure 7.**

*Nanocrystalline Materials*

peak which can still be observed at 2θ of 31–32o

system is summarized in **Table 1**.

to the weak ferromagnetic behavior.

of the nanoparticles.

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

tion 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. [76] 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 [77]. 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

**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

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) [78]. The weak ferromagnetic order of these powders can be understood as a result of residual magnetic moment caused by its canted spin structure [79]. 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 [80]. Moreover, the existence of defects, for instance, oxygen vacancies [81], and the secondary phases [82] may contribute

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 [83]. However,

**Sample Structure Lattice parameters (Å)** BiFeO3 Rhombohedral a = b = 5.578 (1)

Bi0.9Pb0.1FeO3 Cubic a = b = c = 3.958 (1) BiFe0.9Ni0.1O3 Rhombohedral a = b = 5.574 (1)

*Rietveld analysis results for the XRD patterns of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and* 

. The result of the phase composi-

c = 13.862 (3)

c = 13.840 (4)

**42**

**Table 1.**

*BiFe0.9Ni0.1O3) powders.*

*TEM image with selected area electron diffraction (SAED) pattern for barium ferrite powders synthesized by sol-gel method and then calcined at 650°C for 1 h in air.*

**Figure 8.**

*Magnetic hysteresis curves of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders synthesized by the sol-gel method.*

Verma and Kotnala [84] 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 [85] 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−*x*A*x*FeO3 (A = Ca, Sr., Pb, Ba) system providing a canting of the antiferromagnetic sublattice [86], 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. [87], 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 [88].

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 (*R*s) and the coercive field (*E*c) of the undoped BiFeO3 nanoparticles are about 20.5 μC/cm<sup>2</sup> 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/ cm<sup>2</sup> along (111) for bulk bismuth ferrite [89]. The existence of secondary phases, such as Bi25FeO40, Bi2Fe4O9, and Bi2O3, affects the lower values of *R*s and *E*c in the sample. Pradhan et al. [78] have reported that leakage current is one of the major reasons for obtaining lower values of saturation polarization (*P*s), *R*s, and *E*c in BiFeO3 system.

In the Pb-doped BiFeO3 nanoparticles, the Pb substitution improves the dielectric and ferroelectric properties [90]. 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 [91]. 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. [92]. 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 [93]. 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 [94], as shown in **Table 2**.

As mentioned earlier, the Ni doping in BiFeO3 nanoparticles enhances the magnetic properties as reported in the former paper [88]. However, the dielectric and

#### **Figure 9.**

*Room temperature polarization-electric field (P-E) hysteresis loop of the undoped BiFeO3 pellet sintered at 750°C.*

**45**

the system.

**Table 2.**

**8. Conclusions**

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source*

BiFeO3 19.4 0.012 1.7 Bi0.9Pb0.1FeO3 130.8 0.162 11.6 BiFe0.9Ni0.1O3 17.5 0.010 1.6

*Dielectric constant, electrical conductivity, and permittivity of the undoped BiFeO3 and doped BiFeO3*

**Conductivity [×10<sup>−</sup><sup>4</sup>**

*T* **= 300 K**

 **(Ω m)<sup>−</sup><sup>1</sup> ]** **Permittivity (×10<sup>−</sup>10 F/m)** *T* **= 300 K**

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 [95]. 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 [48]. 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

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

*DOI: http://dx.doi.org/10.5772/intechopen.88027*

**Sample Dielectric constant** 

**(εr)** *f* **= 1 kHz,** *T* **= 300 K**

*(Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders measured at room temperature.*

*Ferrite-Based Nanoparticles Synthesized from Natural Iron Sand as the Fe3+ Ion Source DOI: http://dx.doi.org/10.5772/intechopen.88027*


**Table 2.**

*Nanocrystalline Materials*

about 20.5 μC/cm<sup>2</sup>

BiFeO3 system.

shown in **Table 2**.

cm<sup>2</sup>

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 (*R*s) and the coercive field (*E*c) of the undoped BiFeO3 nanoparticles are

values reported in the single crystal which has a large polarization of ~100 μC/

In the Pb-doped BiFeO3 nanoparticles, the Pb substitution improves the dielectric and ferroelectric properties [90]. 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 [91]. 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. [92]. 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 [93]. 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 [94], as

As mentioned earlier, the Ni doping in BiFeO3 nanoparticles enhances the magnetic properties as reported in the former paper [88]. However, the dielectric and

*Room temperature polarization-electric field (P-E) hysteresis loop of the undoped BiFeO3 pellet sintered at* 

 along (111) for bulk bismuth ferrite [89]. The existence of secondary phases, such as Bi25FeO40, Bi2Fe4O9, and Bi2O3, affects the lower values of *R*s and *E*c in the sample. Pradhan et al. [78] have reported that leakage current is one of the major reasons for obtaining lower values of saturation polarization (*P*s), *R*s, and *E*c in

and 5.5 V/cm, respectively. These values are lower than the

**44**

**Figure 9.**

*750°C.*

*Dielectric constant, electrical conductivity, and permittivity of the undoped BiFeO3 and doped BiFeO3 (Bi0.9Pb0.1FeO3 and BiFe0.9Ni0.1O3) powders measured at room temperature.*

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 [95]. 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 [48]. 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.
