Abstract
Nanocrystalline materials were synthesized using sol-gel technique. The structural and magnetic properties were investigated in detail. Rietveld analysis from XRD revealed the structural formation of BiFeO3. As the Mn doping concentration was increased, the structure of BiFeO3 changed from rhombohedral to tetragonal. All the M-H loops showed the ferromagnetic behavior in the prepared samples. Magnetization was observed to enhance as the Mn doping concentration was increased. The enhanced magnetization may be due to the collapse of the space modulated spin structure as observed from the structural changes.
1. Introduction
BiFeO3 (BFO) is an interesting material due to the coexistence of G-type antiferromagnetic (AFM) structure having Néel temperature around 640 K and ferroelectric polarization with Curie temperature around 1000 K [1, 2]. BFO is considered to be superior candidate for the next generation devices such as spintronics, piezoelectric sensors, optoelectronic devices, nonvolatile memories, and high density microactuators [3, 4]. The spontaneous magnetization in BFO could be induced by changing Fe-O-Fe bond angle or by the statistical octahedral distribution of Fe [5, 6]. Room temperature ferromagnetic properties of rare earth and transition metal ions doped BFO materials were observed by several researchers. The enhanced magnetization was observed in BFO due to the divalent, trivalent, and rare earth ions on Fe site [7, 8]. Literature suggests that the superexchange interaction within Fe3+-O-Fe2+ gives rise to a finite value of magnetization in BFO [9]. The enhanced magnetization in Mn and Ho doped BFO thin films prepared by chemical solution deposition was due to the collapse of space modulated spin structure by the structural transition [10]. The magnetic ordering in BFO is G-type antiferromagnetic; the contribution of the antiferromagnetic sublattices could lead to weak ferromagnetism [11]. Doping of trivalent, divalent, and rare earth elements in BFO was observed to enhance the magnetization by suppressing the cycloid spin structure and reducing the leakage current density and enhancing the multiferroic properties [12–16]. When Mn is substituted for Fe site, ferroelectric properties were observed to improve [17]. It was observed that BFO shows weak magnetization and poor polarization due to the spiral spin structure and leakage problem [18, 19]. The secondary phases and oxygen vacancies in BFO may also affect the magnetic and electrical properties [1, 20]. The magnetic properties of BFO are also related to the local spin structure of FeO6 octahedron [18]. Among all the elements Mn doped BFO was observed to contribute to enhanced magnetization [21, 22]. Bulk BFO crystallizes in a rhombohedral distorted perovskite structure R3c space group. Partially orbital of Fe ions leads to G-type antiferromagnetic structure [23–25]. BFO multiferroic materials in nanofilm, bulk film, and thin film forms have been extensively investigated during the last decade. Since BFO materials are having interesting properties, therefore, in the present work, we aimed to present their structural and magnetic properties in detail.
2. Experimental
Mn doped BFO was synthesized using sol-gel method [7, 8, 26]. The A.R Grade citric acid (C6H8O7·H2O), bismuth nitrate (Bi(NO3)3·5H2O), manganese nitrate (Mn(NO3)3·4H2O), and ferric nitrate (Fe(NO3)3·9H2O) (≥99%) were used as starting materials. During synthesis, an excess of 5 wt.% Bi was added to compensate bismuth oxide loss during the thermal treatment. Without this additional bismuth addition, the pure phase of BFO could not be obtained. The entire synthesis process is described elsewhere [26]. The as-prepared powder samples were heat treated at temperature of 600°C. The X-ray diffraction of the ferrite powders was carried using a Philips PW 1820 powder diffractometer with CuK graphite monochromated radiation, operating at 40 kV and 30 mA, with solar slits and divergent and receiving slits of 0.2 mm. The diffracted intensity was registered in the angular range 8° ≤ 2θ ≤ 80°, with a step size of 0.02° (). The micrographs of all samples were taken on FEI Quanta FEG 200 High Resolution Scanning Electron Microscope. Room temperature magnetic properties were investigated using Lakeshore’s (USA) vibrating sample magnetometer (VSM 7410).
3. Results and Discussions
Figure 1(a) shows Rietveld analysis for BiFeO3 sample. All the XRD peaks correspond to BiFeO3 structure. Apart from the main BiFeO3 peaks few minor peaks belonging to Bi2Fe4O9/Bi25FeO29/Bi2O3 are visible in the 2θ° range between 25 and 30. It is reported most often that BiFeO3 is accompanied by secondary phases such as Bi2Fe4O9 and Bi25FeO29 [27–29]. The impurity phases in the XRD patterns could be attributed to the volatilization of Bi3+ ions which resulted due to excess addition of Bi. The high vapour pressure of Bi atoms tends easy evaporation of Bi3+ ions during synthesis conditions, such as drying and annealing conditions. Therefore, for Mn and BFO synthesis, we have used excess Bi3+ ions. Without this additional bismuth addition, the pure phase of BFO could not be obtained. Berbenni et al. [29] showed that if BFO is annealed at temperatures ≥ 750°C, the secondary phases such as BiO3, Fe2O3, and Bi2Fe4O9 will disappear. It seems that higher annealing temperature increases the intensity of characteristic BFO peaks and reduces the intensity of secondary phases but it may not be possible to avoid the secondary phases even though the samples were annealed at 800°C [29]. When we annealed BFO powders at temperatures ≥ 750°C, the BFO powder melted and stuck to the crucible. For this reason, we have chosen 600°C as the final annealing temperature for our samples.
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From Figure 1(b), it is clearly observed that, for , the diffraction can be indexed to a rhombohedral (R3c) structure, whereas for it is tetragonal (P4mm) phase. Similar kind of results was observed for () synthesized by sol-gel method [30]. The XRD diffraction peaks for all the samples were similar to those published in the literature. The diffraction peak at , 22.5° (012) and diffraction peak around 32° (104) and (110) are indexed for rhombohedral structure. With the increase in Mn concentration to the diffraction at 46° splits into two peaks (002) and (200) as a characteristic peak for tetragonal (P4mm) space group. The rhombohedral (R3c) structure was observed for pure BFO and was considered as the distortion from cubic perovskite pmsm structure [31]. When the pmsm and R3c crystal structures were present in samples simultaneously, they were expected to show both weak ferromagnetic and ferroelectric properties [31].
Figure 2 shows the SEM micrographs for all the samples. SEM analysis showed the average grain size around 200 nm. There was no significant change in the grain size with the increase in Mn doping concentration. The magnetic hysteresis loops of samples are shown in Figure 3. All the samples show unsaturated magnetization. The highest magnetization was observed for the composition . Similar kinds of results were observed for for () BFOBCZT ceramics [23]. Pure BFO is observed to be antiferromagnetic with G-type spin ordering below Néel’s temperature. As there was no significant change in the BMFO grain size with doping concentration (see Figure 2), it can be said that the enhancement of magnetization may not be attributed to the dependency of size on the doping concentration. Similar kinds of results were observed for Mn doped BFO powder samples [18]. The weak ferromagnetism in BFO powder samples was observed to depend not only on the DM interactions but also on the single-ion anisotropy [31, 32]. It is well known that BFO is G-type antiferromagnet and each Fe3+ spin is surrounded by six canted antiparallel spins on the nearest Fe neighbors which allow net magnetic moment [8, 9]. Except for sample all other samples showed very small remanent magnetization; the exact reason for this kind of behavior is not known. The observed hysteresis of all the samples without saturation point was explained due to the persistence of the uncompensated antiferromagnetism in the samples [23]. As the Mn doping concentration was increased, the magnetization was observed to increase. Mn substitution induced the structural phase change from R3c to P4mm and this results in the destruction of spin cycloid structure and thus the homogeneous structure was formed. Wu et al. [30] reported the enhanced magnetization due to Mn and La codoping. However, the observed magnetization value for our samples was small compared to those in literature. This might be due to several factors such as synthesis technique, synthesis conditions, annealing conditions, and temperature. The enhanced magnetization with further substitution of Fe3+ ions with Mn3+ (d4) ions could break the balance within antiparallel sublattice magnetization due to the Jahn-Teller distortion of MnO6 octahedron [30, 33]. For small doping concentration of Mn, Jahn-Teller distortion would enhance and may affect the DM interactions which in turn enhance the weak ferromagnetism [20, 23, 30–32, 34]. It was observed that ferromagnetism in Mn doped BFO might be due to induced defects created by excess Bi in the interstitial region of the BFO lattice [32].
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The magnetic interactions of BFO can be modified by the substitution of different elements in BFO [30]. Several researchers have tried to improve the structural and magnetic properties of BFO by substituting transition metals [1, 35–42]. Structural defects, control of grain size, or thickness of thin films can regulate the magnetic properties of thin films [43]. Introducing defects like oxygen vacancies during synthesis in BFO powders or thin films also induces ferromagnetism [44]. For multiferroic materials, they are required to have both ferroelectric and ferromagnetic properties. As observed from our results, these samples showed both rhombohedral (R3c) and tetragonal (P4mm) structures; therefore, these materials could be suitable for ferroelectric materials.
4. Conclusions
Mn doped BFO nanopowders were successfully synthesized using sol-gel method. Pure BFO showed rhombohedral structure. As the Mn doping content was increased, the structure of the sample changed from rhombohedral to tetragonal structure. Grain size did not show any considerable change with increasing doping concentration. The magnetization was observed to enhance as the doping content was increased due to Jahn-Teller distortion. With the appearance of rhombohedral (R3c) and tetragonal (P4mm) structures, these materials could be suitable candidate for ferroelectric materials.
Competing Interests
The authors declare that there are no competing interests regarding the publication of this paper.
Acknowledgments
The authors are grateful to sophisticated analytical instrument facility (SAIF), Indian Institute of Technology (IIT), Madras, India, for providing the experimental facilities. K. Vijaya Kumar is thankful to Professor N. V. Ramana, Principal, JNTUH CEJ, Nachupally (Kondagattu), Karimnagar District.