In recent years, much research interest has been drawn to BiFeO3 (shortened as BFO), which is known to be the only multiferric compound that exhibits simultaneous G-type antiferromagnetic and ferroelectric orders over a broad range above room temperature (Curie temperature >800°C, Neel temperature = 370°C) [1–7]. As a partially covalent oxide, BFO has a rhombohedrally distorted perovskite structure belonging to a space group of R3c . Nevertheless, it exhibits weak magnetism at room temperature due to a spiral magnetic spin cycloid with a periodicity of ∼62 nm . Although rhombohedral BFO has been studied since first discovery in 1960s, electrical properties of the pure BFO rhombohedral have been rarely reported due to its high conductivity, which may originate from uncertain oxygen stoichiometry, high defect density and poor sample quality [9–11]. In order to understand the properties of multiferroic BFO, the fabrication procedure of pure BFO R-phase is very important and should be studied. If oxygen partial pressure and temperature are not controlled accurately during crystallization of the BFO R-phase, the kinetics of phase formation always lead to other impurity phases in Bi–Fe–O system, such as Bi2Fe4O9, Bi2 O2.75 and Bi46Fe2O72 [10, 12, 13].
Potential applications of BFO in the memory devices, satellite communications, sensors, smart devices and optical filters are greatly limited due to its leakage current, which is usually caused by defects and non-stoichiometry. In order to overcome these disadvantages, various wet chemical methods were applied to prepare pure-phase BFO powders, such as hydrothermal synthesis , co-precipitation , microemulsion techniques  and ferrioxalate precursor method . Sol–gel process is also widely used for preparation of pure-phase powders. In the sol–gel synthesis of BFO, the sol is usually prepared based on citric acid route [18–20].
In this paper, we describe the synthesis procedures to obtain high purity as well as homogeneous nanoscale BFO rhombohedral powders by a microwave, a Pechini method and an assisted method by different polymer. The final BFO R-phase powders were characterized by X-ray, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and vibrating sample magnetometer (VSM) measurements.
Materials and physical measurements
All chemical reagents used in our experiments were of analytical grade and used as received without further purification. X-ray diffraction (XRD) patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Kα radiation. Elemental analyses were obtained from Carlo ERBA Model EA 1108 analyzer. SEM images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. FT-IR spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets.
Synthesis of BFO nanoparticles
Synthesis of BFO nanoparticles by Pechini method: at first 1.668 g Bi(NO3)3.5H2O was dissolved in 20 ml distilled water and then acetic acid (65%) was added until clear solution obtained, and finally 5 ml diethylene glycol monoethyl ether (DGME) was added. At last, 4 g anhydrous citric acid, C6H8O7, was added to this solution and dissolved at 50°C for 1 h. After complete dissolution, 1.389 g Fe(NO3)3.9H2O in 10 mL DGME was added. This solution is further heated at 70°C for 1 h to remove excess water. The heating continued at 120°C for 1 h, the solution became more and more viscous and finally became a xerogel. To complete drying, xerogel was placed in electric oven at 250°C for 1 h. The resulted brownish powder was then used as precursor. In the electric furnace, the precursor was heat-treated at 400–600°C in air, in an Al2O3 boat, and then cooled to room temperature (Figure 1).
Synthesis of BFO nanostructures assisted by different polymers: in this part, at first 2.5 g polyvinylpyrrolidone (PVP) was dissolved in 20 ml distilled water (solution A). Then 1 g Bi(NO3)3.5H2O was dissolved assisted by acetic acid (solution B) and 0.89 g Fe(NO3)3.9H2O was dissolved in 20 ml distilled water (solution C). Then, solutions B and C were added into solution A under vigorous stirring. The solution was then heated at 120°C under constantly stirring until all liquid evaporated out from the solution. Finally, brownish powder was calcined at 600°C for 1 h. This test was repeated by chitosan polymers and polyethylene glycol (PEG) instead of PVP.
Synthesis of BFO nanoparticles assisted by microwave
At first 0.24 g Bi(NO3)3.5H2O was dissolved in 20 ml distilled water, 2 ml DMGE and 3 ml acetic acid (solution A). About 0.2 g Fe(NO3)3.9H2O was dissolved in 10 ml distilled water and 10 ml propylene glycol (solution B). Then these solutions were mixed together and stirred for 20 min. Finally, the solution contents were exposed to microwave irradiation in a domestic microwave oven, operating at 2,450 MHz, for different power and 6 microwave cycles (each cycle includes 30 s on and 60 s off).
Results and discussion
The effect of different methods for preparation of BFO and treatment temperatures on a solid phase and purity of the products are shown in Figure 2(a–e). Figure 2(a) is related to the XRD pattern of those samples prepared by the Pechini method. Diffraction peaks of BFO correspond well with the reported data (JCPDS: 72–2112). The diffraction patterns of bismuth ferrite have been indexed as a rhombohedral distorted perovskite structure with space group R3m. However, the formation of impurity phase, Fe2O3 (JCPDS: 73–0603), was observed. Thus we applied other methods to synthesize pure BFO. Microwave irradiation was applied and synthesis process was performed assisted by different polymers. BFO obtained assisted by microwave were composed of Fe2O3 and Bi2O3 as impurity. BFO obtained from this method has rhombohedral phase (JCPDS: 72–2112) (Figure 2(b)). Impurity phase observed again which made us to try other methods. In other methods, we synthesized bismuth ferrite with the use of chitosan polymers. Figure 2(d), (e) shows XRD pattern of the products that are obtained in the presence of chitosan polymers before and after calcinations, respectively. It can be seen that before calcinations, amorphous BFO produced and after calcinations, BFO obtained with high purity. BFO obtained by this method has rhombohedral phase (JCPDS: 72–2112) and is pure. In the fourth method BFO was synthesized assisted by PVP. In Figure 2(c) BFO was synthesized (JCPDS: 72–2112) with few Bi2O3 as impurity. We choose the fourth method for more investigation because BFO that obtained by this method was small sized and contained less impurity, also PVP is much cheaper than chitosan. Crystal structure of bismuth ferrite is shown in Figure 10.
We investigated the effect of different parameters on the morphology of the products such as the amount of PVP and treatment temperatures. FT-IR spectra of crystalline bismuth ferrite nanoparticles derived from the different methods are shown in Figure 3. The broad band at 3,000–3,600 cm−1 is due to the antisymmetric and symmetric bond stretching of H2O and OH groups while a band at 1,630 cm−1 corresponds to bending vibrations of H2O [21, 22]. Specifically, strong absorptive peaks at 400–600 cm−1 are imputation to the Fe–O bending and stretching vibrations which are characteristics of octahedral FeO6 groups in perovskite compounds. The formation of a perovskite structure can be confirmed by the presence of a metal–oxygen bond [21, 23]. Other peaks can be attributed to the remained polymers of precursor.
Scanning electron microscope was used for investigation of morphology of the products. SEM images of BFO powders prepared by different methods are shown in Figures 4(a–d) and 5(b). In all cases, the particles are sufficiently fine. In the third method (Figure 4(a)), the powders are smaller than those of powders prepared by other methods but agglomerated products are more than the fifth method (Figure 5(b)). However, in the third method pure BFO was obtained but we chose fifth method for more investigation because this method led to formation of sheet-like BFO. Chitosan polymers are also much expensive. It was observed that there are no changes in the shape and morphology of the products by changing methods and the shape and size of the particles are uniform in all cases (Figure 4(b–d)). The typical SEM micrographs of the BFO powders heat treated at 400°C and 600°C are presented in Figure 5. According to Figure 5(a), at 400°C sheet-like bismuth ferrite formed and by increasing calcination temperature, BFO nanoparticle was obtained (Figure 5(b)).
Also BFO powders were synthesized by different amounts of PVP (0.7, 2 and 2.5 g) and their SEM photographs are shown in Figure 6. When 0.7 g of PVP was used sheet-like bismuth ferrite was obtained (Figure 6(a)). Figure 6(b) is related to SEM image of BFO which is prepared in with 2 g PVP. In this case cubic-like BFO was synthesized. By increasing PVP amount to 2.5 g, cubic-like BFO changed to particle (Figure 5(b)). Further morphological characterization of the BFO nanoparticles was carried out by TEM. The TEM and HRTEM images of BFO nanoparticles are shown in Figure 7. According to Figure 7(a), the morphology of BFO is sphere-like and well isolated. Figure 7(b) shows the HRTEM images of BFO nanoparticles.
The magnetic properties of the materials are created from the quantum couplings at the atomic level, including the coupling between electron spin and the angular momentum of the electron orbital (L–S coupling) and the coupling between the electron spins (S–S coupling) . Each of the magnetic nanoparticles tends to take a single magnetic domain. Thus, the nanoparticles provide excellent opportunity for the fundamental studies on the relationship between magnetic behavior and the magnetic couplings at the atomic level . The magnetization curves of the bismuth ferrite nanoparticles measured at 27°C are shown in Figures 8 and 9. It is clear that the synthesis method can influence the magnetization curves as a result of a change in the particle size and distribution of cations. When the nanoparticle diameter is smaller than the critical single-domain diameter, avoiding the configuration of magnetic domain walls decreases the magnetization. Even though the diameter of particle becomes even smaller, the thermal stability of the magnetization orientation decreases. The limited super paramagnetic, as it is known, refers to the particle size because the thermal energy at room temperature causes fluctuations in the magnetization orientation of a bit (Figure 8) .
Figure 9 shows the typical magnetic hysteresis loops of bismuth ferrite nanoparticle synthesized by the second method. As can be seen in Figure 9, the bismuth ferrite nanoparticles show a weak ferromagnetic order at room temperature, which is quite different from the linear M–H relationship in bulk bismuth ferrite. Park et al.  reported the size-dependent magnetic properties of single crystalline bismuth ferrite nanoparticles. They explained that the weak magnetic property of bismuth ferrite nanoparticles can be attributed to the size-confinement effects of the bismuth ferrite nanostructures, which correlate with (a) the increased repression of the known spiral spin structure (period length of 62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Their results showed that the bismuth ferrite nanoparticles with the typical size of nearly 90 nm presented the weak magnetic property quite different from bulk bismuth ferrite. It should be noted that the particle size of our products is nearly 90 nm, which is close to that of their result. Thus, the origin of the weak magnetic property in our samples may be attributed to the size-confinement effects of the bismuth ferrite nanostructures. Figure 10 shows polymer structures that were used in this work.
In current investigation, bismuth ferrite powders were synthesized by five methods. We successfully prepared the pure bismuth ferrite nanoparticles by a simple method. SEM, XRD, FT-IR and TEM were used to confirm the phase and size distribution of the nanoparticles and VSM was utilized to measure the size-dependent magnetic behaviors of the as-prepared nanoparticles. The data obtained from SEM, FTIR, XRD and VSM confirmed that:
The method has strong effect on the purity of the products. BFO synthesized by third method was pure.
The crystallite size has a great effect on the magnetic properties.
BFO was synthesized with new morphology.
Authors are grateful to the Council of Iran National Science Foundation and University of Kashan for supporting this work by grant no. 159271/26.
 W. Eerenstein, N.D. Mathur and J.F. Scott, Nature, 442 (2006) 756–760. Google Scholar
 P. Fischer, M. Polomska, I. Sosnowska and M. Szymanski, J. Phys. C, 13 (1980) 1931–1940.Google Scholar
 C. Tabares-Munoz, J.P. Rivera, A. Monnier and H. Schmid, Jpn. J. Appl. Phys., 24 (1985) 1051–1053.Google Scholar
 Y.P. Wang, L. Zhou, M.F. Zhang, X.Y. Chen, J.M. Liu and Z.G. Liu, Appl. Phys. Lett., 84 (2004) 1731–1733. Google Scholar
 M. Fiebig, T.H. Lottermoser, D. Frohlich, A.V. Goltsev and R.V. Pisarev, Nature, 419 (2002) 818–820.Google Scholar
 N. Hur, S. Park, J.S. Sharma, A.,S. Guha and S.W. Cheong, Nature, 429 (2004) 392–395.Google Scholar
 J. Wang, J.B. Neaton and H. Zheng, Science, 299 (2003) 1719–1722. Google Scholar
 J.R. Teague, R. Gerson and W.J. James, Solid State Commun., 8 (1970) 1073–1074. Google Scholar
 K.Y. Yun, M. Noda and M. Okuyama, J. Korean Phys. Soc., 42 (2003) 1153–1156. Google Scholar
 I. Sosnowska, T.P. Neumaier and E. Steichele, J. Phys. C, 15 (1982) 4835–4846. Google Scholar
 M.M. Kumar, V.R. Palkar, K. Srinivas and S.V. Suryanarayana, Appl. Phys. Lett, 76 (2000) 2764–2766. Google Scholar
 C. Chen, J.R. Cheng, S.W. Yu, L.J. Che and Z.Y. Meng, J. Cryst. Growth, 291 (2006) 135–138. Google Scholar
 S. Shetty, V.R. Palkar and R. Pinto, J. Phys., 58 (2002) 1027–1030. Google Scholar
 N. Das, R. Majumdar, A. Sen and H.S. Maiti, Mater. Lett., 61 (2007) 2100–2104. Google Scholar
 S. Ghosh, S. Dasgupta, A. Sen and H.S. Maiti, Mater. Res. Bull., 40 (2005) 2073–2079. Google Scholar
 F. Chen, Q.F. Zhang, J.H. Li, Y.J. Qi and C.J. Lu, Appl. Phys. Lett., 89 (2006) 92910–92910–3. Google Scholar
 Y. Wang, Q.H. Jiang, H.C. He and G.W. Nan, Appl. Phys. Lett., 88 (2006) 259902. Google Scholar
 A.H.M. Gonzalez, A.Z. Sim˜oes, L.S. Cavalcante, E. Longo, J.A. Varela and C.S. Riccardi, Appl. Phys. Lett., 90 (2007) 52906–52906–3.Google Scholar
 E.C. Aguiar, M. Ramirez, F. Mour, J.A. Varela, E. Longo and A.Z. Sim, Ceram. Int., 39 (2013) 13–20. Google Scholar
 A.Z. Sim, B.D. Stojanovic, M.A. Ramirez, A.A. Cavalheiro, E. Longo and J.A. Varela, Ceram. Int., 34 (2008) 257–261. Google Scholar
 D. Lee, M.G. Kim, S. Ryu, H.M. Jang and S.G. Lee, Appl. Phys. Lett., 86 (2005) 222903–222903–3. Google Scholar
 L.D. Tung, V. Kolesnichenko, D. Caruntu, N.H. Chou and C.J. O’Connor, J. Appl. Phys., 93 (2003) 7486–7488. Google Scholar
 I. Sharifi, H. Shokrollahi, M.M. Doroodm and R. Safi, J. Magn. Magn. Mater, 324 (2012) 1854–1861. Google Scholar
 T.J. Park, G.C. Papaefthymiou, A.J. Viescas, A.R. Moodenbaugh and S.S. Wong, Nano Lett., 7 (2007) 766–772. Google Scholar
About the article
Published Online: 2015-07-07
Published in Print: 2016-06-01