A systematic study on multiferroics Bi1−xCexFe1−yMnyO3: Structural, magnetic and electrical properties

A systematic study on multiferroics Bi1−xCexFe1−yMnyO3: Structural, magnetic and electrical properties

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A systematic study on multiferroics Bi1  xCexFe1  yMnyO3: Structural, magnetic and electrical properties Anu Beniwal a, Jarnail S. Bangruwa a, Rajan Walia b, Vivek Verma a,n a b

Department of Physics, Hindu College, University of Delhi, Delhi, India Department of Physics, Hansraj College, University of Delhi, Delhi, India

ar t ic l e i nf o

a b s t r a c t

Article history: Received 6 March 2016 Received in revised form 14 March 2016 Accepted 22 March 2016

Bi1 − x Cex Fe1 − y Mny O3 samples (0 rxr 0.10 and 0 ryr 0.05) were synthesized by the sol–gel technique to investigate the effect of Ce and Mn doping on the structural, magnetic and electrical properties of multiferroic BiFeO3 (BFO). It is observed that Ce–Mn doping induces structural changes in BFO and also decreases the secondary phases noticeably. Frequency dependent dielectric properties of pure and doped BFO samples were measured from 300 K to 675 K in the frequency range of 1 KHz–1 MHz. Enhanced ferromagnetism and improved ferroelectric properties were observed which may be correlated with the structural transformation and grain morphology. As Ce concentration increased in BFO, the leakage current density decreased gradually. We have observed here exciting properties in doped bismuth ferrites which provide the possibility to create new functional material. & 2016 Published by Elsevier Ltd.

Keywords: Magnetic Electrical Hysteresis XRD Rhombohedral

1. Introduction Multiferroics are the materials which exhibit more than one ferroic order parameter co-existing in the same phase. Amongst all the multiferroics available, BFO is increasingly gaining importance as it is antiferromagnetic below the Néel temperature ( TN ¼643 K) and ferroelectric below the Curie temperature ( TC =1103 K ) [1–4]. Single-phase bulk BFO exhibits a rhombohedral structure with space group R3c, where all ions along the (111)c direction are displaced relative to the ideal centrosymmetric positions and the oxygen octahedrons surrounding the transition-metal cations rotate alternately clockwise and counterclockwise about this (111)c direction. It has canted G-type antiferromagnetic order combined with space-modulated spin structure on a long wavelength of 620 Å. However, the possible nonzero remnant magnetization (Mr) permitted by the canted G-type antiferromagnetic order is canceled by the space-modulated spin structure on the 620 Å wavelength, constraining the release and measurements of potential ME effect in the bulk BFO [5–7]. It is no less than a boon for us that BFO shows magnetoelectric properties at the room temperature. These properties of BFO have made it a very popular choice for the memory storage applications [8,9]. BFO is also used in magnetoelectric sensors and in the field of spintronics [10]. But, these interesting applications of BFO are limited due to some factors. The n

Corresponding author. E-mail address: [email protected] (V. Verma).

highly volatile nature of bismuth results in the formation of secondary phases during the synthesis. Also, the valence fluctuation of Fe ion and the inherent oxygen ion vacancies pose serious problems of leakage current. One gets high dielectric loss, small remnant polarization and degraded saturation polarization value in BFO [11,12]. In order to maximize the applications of BFO, the problems stated above need to be addressed. Site-engineering concept is one of the most popular methods to solve above problems. The multiferroic properties of BFO can be enhanced by doping with rare-earth at A-site or transition metal at B-site. In codoping, both the A-site and B-site are doped with rare-earth and transition metals respectively. The doping suppresses the oxygen ion vacancies and so, reduces the leakage current. The doping improves the magnetoelectric properties and the reason has been attributed to the different ionic radii of bismuth and the doping element. In our previous work, we have used the site-engineering concept to dope the A-site of BFO with Pr using the sol–gel technique [13]. The ferromagnetic and ferroelectric properties were effectively enhanced due to the Pr doping. In this work, we have first doped the A-site of BFO with Ce and then co-doped the A–B sites of BFO with Ce and Mn respectively. The concentration of Ce is varied from 5% to 10% in both of the cases whereas; the concentration of Mn is kept 5% during co-doping. Although, there have been a lot of research aimed on improving the magnetoelectric properties of BFO by single doping as well as co-doping. But, to the best of our knowledge, the literature available on the systematic study of structural, magnetic and electrical properties

http://dx.doi.org/10.1016/j.ceramint.2016.03.169 0272-8842/& 2016 Published by Elsevier Ltd.

Please cite this article as: A. Beniwal, et al., A systematic study on multiferroics Bi1  xCexFe1  yMnyO3: Structural, magnetic and electrical properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.169i

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Fig. 1. XRD pattern of pristine and doped BFO samples.

of Ce and Mn doped bismuth ferrites is not sufficient and so, can be further explored.

3. Results and discussions 3.1. Structural analysis

2. Experimental method Sol-gel technique is a versatile technique and is used very often for the synthesis of various inorganic materials [14]. This technique uses hydrolysis and condensation of the precursors diluted in a solvent. Pure and doped (x¼0, y¼ 0; x¼0.05, y¼0; x¼0.1, y¼ 0; x¼0.05, y¼0.05; x¼0.1, y¼0.05) samples were synthesized using the sol–gel technique. The raw materials used in the process were highly pure The Bi (NO3)3⋅5H2 O, Fe (NO3)3⋅9H2 O, Ce (NO3)3⋅6H2 O, Mn (CH3 COO)2⋅4H2 O . precursor solutions were prepared using these raw materials. The stoichiometric ratio of these starting materials were dissolved in the distilled water and HNO3. Citric acid (1:1 ratio of metal ions) was used to serve the purpose of complexing agent. It was added to the aqueous solutions in appropriate amount under constant stirring. The resultant solutions were then evaporated and dried at approximately 80 °C on a hot plate under continuous stirring. The mesoporous solid structure swollen with liquid (gel) was then converted to xerogel powders. These xerogel powders were ground in the agate mortar and then annealed at 600 °C for 4 h. The annealed powders were subjected to a hydraulic press (10t) after mixing them with PVA solution. The well dense pellets (diameter ∼8 mm and thickness ∼1 mm) thus obtained were sintered at 650 °C for an hour. The pellets were silver coated for the electric characterization. The structure and microstructure analysis was carried out by Philips X'Pert X-ray diffractometer using CuKα radiation with wavelength 1.5406 Å in a wide range of Bragg angles 2θ (20–80°) at a scanning rate of 2 ° per minute. The morphology of the samples was characterized by Scanning Electron Microscopy (FEI, QUANTA 200 F). The magnetic measurements at the room temperature were carried out by Vibrating Sample Magnetometer (EV-9, Microsense). The dielectric measurements were performed with the help of Precision 6500B Impedance Analyzer. The room temperature ferroelectric measurements were performed by using the Automatic P–E Loop Tracer (Marine India).

The XRD patterns for pure, Ce-doped and, Ce and Mn co-doped BFO samples are shown in Fig. 1. BFO is observed to possess the rhombohedrally distorted perovskite structure with the R3c space group. Pure BFO is reported to have few impurity peaks of Bi2Fe4O7/Bi46Fe2O72 labeled by * in the Fig. 1. These impurities represent the secondary phases which are generally formed due to the high volatile nature of bismuth. It is interesting to note that the impurity peaks subside when BFO is doped with Ce or co-doped with Ce and Mn. The reduction in impurity peaks is more pronounced at 10% of Ce doping in 5% Mn doped BFO. This clearly indicates that Ce and Mn co-doping is highly beneficial in subsiding the secondary phases formed in BFO. In Fig. 1, it is observed that (104) and (110) peaks tend to merge when the concentration of Ce is increased from 0% to 5% and 10%. These peaks further merge when 5% Mn is used to dope 5% and 10% Ce doped BFO. The intensity of (104) peak is observed to show a decreasing pattern when BFO is doped with 5% and 10% Ce. The intensity of (012) and (024) peaks is decreased when the concentration of Ce is increased from 5% to 10% in BFO and in 5% Mn doped BFO. Twin peaks observed at 39° {(006) and (202)}, 52° {(116) and (122)} and 57° {(018) and (300)} completely merge in to a broadened single peak when BFO is co-doped with 10% Ce and 5% Mn. The split in these peaks is present even when the concentration of Ce is 10% in BFO. But, the introduction of 5% Mn in Ce-doped BFO is observed to be dominant in merging these twin peaks. This indicates a reduction in rhombohedral distortion in Ce-doped BFO but the change from rhombohedral to orthorhombic symmetry is complete only when Mn is introduced in Ce-doped BFO. The Ce and Mn co-doping in BFO at Bi site and Fe site respectively result in structural distortion due to the lattice instability driven by the substitution and possible influence of the Jahn–Teller distortions associated with the presence of the Mn3 þ ions in the host lattice should also be taken into account [15,16].

Please cite this article as: A. Beniwal, et al., A systematic study on multiferroics Bi1  xCexFe1  yMnyO3: Structural, magnetic and electrical properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.169i

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BiFeO3

3

Bi0.95Ce0.05FeO3

Bi0.95Ce0.05Fe0.95Mn0.05O3

Bi0.90Ce0.10FeO3

Bi0.90Ce0.10Fe0.95Mn0.05O3

Fig. 2. Scanning Electron Microscopy of pristine and doped BFO samples.

3.2. Scanning Electron Microscopy (SEM) analysis The scanning electron micrographs of pure and doped BFO samples are shown in Fig. 2. These images show rectangular grains of different size and shapes with highly dense microstructures. We observed a small decrease in grain sizes of all doped samples. The decreased aspect ratio of the grains improves the density of the samples. It means that Ce and Mn doping can suppress grain growth and lead to smaller grain sizes in the materials as we can observe from SEM of samples. The decrease in grain size may be attributed to the difference in the ionic radii of the elements in the compounds [14]. Kirkendall effect may be another reason for reduction in grain size due to doping which arise due to different diffusion rates of constituting elements of the compounds [17]. The grain growth in BFO is not uniform but as we increase the concentration of Ce from 0% to 5% in BFO, the grain growth becomes slight uniform. When 5% Mn is introduced in this sample, the grain growth shows further improvement in uniformity. Further, at 10% of Ce concentration, the distribution of grains becomes homogeneous. It is worth mentioning here that an increase in homogeneity as well as uniformity is obtained when BFO is co-doped with 5% Mn and 10% Ce. The structural transformation from rhombohedral to orthorhombic completes at this particular concentration. 3.3. Magnetic properties Fig. 3 shows the hysteresis curve (M–H loops)for pure and doped bismuth ferrites at room temperature. Parameters such as saturation magnetization (Ms), retentivity (Mr)and coercivity (Hc) were determined from the M–H loops and are mentioned in Table 1. The magnetic hysteresis loop for BFO shows the linear magnetic field dependence of magnetization which suggests that

Fig. 3. M–H loops of pristine and doped BFO samples.

Table 1 Magnetic parameters of pristine and doped samples at room temperature. Sample

Ms (emu/g)

Hc (Oe)

Mr (emu/g)

BiFeO3 Bi0.95 Ce0.05FeO3 Bi0.90 Ce0.10 FeO3 Bi0.95 Ce0.05Fe0.95Mn0.05O3 Bi0.90 Ce0.10 Fe0.95Mn0.05O3

0.083 0.095 0.091 0.112 0.98

148.69 126.65 142.06 168.26 225.15

0.0037 0.0044 0.0040 0.0074 0.0075

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it is antiferromagnetic material. The parent compound (BFO) is reported to be G-type antiferromagnetic due to local spin ordering of Fe3 þ at room temperature [18,19]. At the same time, there are several reports that show the FM like magnetic hysteresis in pure BFO compound [20,21]. However, the co-doping of Ce and Mn induces weak ferromagnetism. The unsaturated hysteresis loops and presence of small remant magnetization reveals the presence of antiferromagnetism with weak ferromagnetism [22]. It is, therefore, questionable whether the room temperature weak FM character is intrinsic or extrinsic magnetic property of BFO. It may arise due to impurities like γ-Fe3O4 present in Fe3O4. The observed coercivity for γ-Fe3O4 and Fe3O4 are 450 Oe and 25 Oe respectively. In the present study, the observed coercivity

of BFO is about 150 Oe, which lies between those of γ-Fe3O4 and Fe3O4 ruling out their possibility of being present as secondary phase as also confirmed through XRD. The other possibility is the presence of the Bi2Fe4O7/Bi46Fe2O72 phase which can be confirmed by the XRD pattern. With Ce and Mn co-doping in BFO sample, the impurity phase gets suppressed which leads to improved magnetic properties. The XRD results revealed that Ce doping improves the phase formation of bismuth ferrites. In BFO, Fe3 þ ion is surrounded by the six O2  ions and O2  ion is the common apex of two adjacent FeO6 octahedra. The ionic size of Bi3 þ (0.120 nm) is larger than that of substituted Ce3 þ (0.118 nm) ions [23]. Substitution of smaller ionic size elements at Bi3 þ site decreases the average A-site ionic size which in turn, decreases

Fig. 4. P–E loops of pristine and doped BFO samples at room temperature.

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the tolerance factor. This results in an increase in the octahedral tilt and change in the Fe–O–Fe bond angle and Fe–O bond distances which affects the superexchange interaction between the two antiferromagnetically aligned Fe3 þ ions with possibility of canted structure which in turn, results in an enhancement in ferromagnetic nature of the samples. It is clearly observed that the magnetic properties of the samples are strongly dependent on doping. The remnant magnetization (Mr) value increases for the co-doped samples as compared to the pristine sample which may be due to the change in the Fe–O–Fe bond angle and Fe–O bond distances which affects the superexchange interaction. The observed variation in the value of coercive field may be associated with the change in magnetic anisotropy depending on the percentage of rhombohedral and orthorhombic crystal phase symmetries present in samples. 3.4. Ferroelectric properties The polarization versus electric field (P–E) hysteresis loops of the pristine and doped BFO were measured under a maximum applied electric field of ∼35 kV/cm (50 Hz) at room temperature and are shown in Fig. 4. Breakdown field was not observed within the applied electric field for any samples. With doping, there is an increase in observed saturation polarization (Ps), remnant polarization (Pr) and coercive field (Ec). The remnant polarization value increased in the doped samples compared to the pure one, although there was an irregularity in Ce-doped samples, while Ce– Mn co-doped samples exhibit an improvement in ferroelectric properties. From P–E loops, it is observed that doping makes the loops more resistive in nature. The B-site substitutions are made by replacing the Fe3 þ ions by other transition metal ions Mn3 þ . Since, the conduction band of BiFeO3 is related to the d orbital state of the Fe3 þ ion, B-site substitutions can have a strong influence on physical properties by changing the electronic structure near the Fermi level [24]. 3.5. Dielectric properties Frequency dependent dielectric properties of pure and doped BFO samples were measured from 300 K to 675 K in the frequency range of 1000 Hz–1 MHz. Variation of dielectric constant with temperature at different frequencies (103, 104, 105 and 106 Hz) are presented in Fig. 4. It is observed from these measurements that the dielectric constant collectively increases but does not reach a peak value because of very high Curie temperature (Tc ¼1103 K) of BFO. It is further observed that real part of dielectric (ε′) is decreased with increasing frequency. The decrease in ε′ is attributed to the dielectric relaxation. There exist different mechanisms for polarization at different frequencies. For instance, at low frequency, electron displacement polarization, ion displacement polarization, turning direction polarization and space charge polarization contribute to dielectric constant. At high frequency, dielectric constant just results from electron displacement polarization. The dielectric constant shows an increasing behavior when the Ce doping is increased from 5% to 10% in BFO. A similar pattern is also observed when BFO is doped with 5% Mn and the concentration of Ce is increased from 5% to 10%. This is true for the entire frequency range (1000 Hz–1 MHz) and therefore, it clearly indicates that Ce ions get substituted in BFO lattice and thus, improves the dielectric response of BFO. The structure of BFO changes towards orthorhombic as the concentration of Ce is increased from 5% to 10% and this can be a reason for the increment in dielectric constant. The very first conclusion that can be drawn from the loss tangent pattern of the ceramics is consistent with the pattern observed in dielectric response. The pattern is the increment in dielectric loss when the concentration of Ce is increased

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from 5% to 10% in BFO. It is also observed that the dielectric loss is maximum for the 10% Ce and 5% Mn co-doped BFO for the high frequencies. The abrupt increase in dielectric constant and dielectric loss observed in the temperature range of 550–650 K can be linked to the antiferromagnetic Néel temperature (643 K). It is worth mentioning that this increment observed in the dielectric constant and dielectric loss cannot be related to the Curie temperature (1103 K) of BFO as it is very high. The dielectric dispersion can be explained by Koop's theory. According to this theory, the decrement in ε′ with increasing frequency is assigned to the fact that atoms in the dielectric material need a finite time to align their axis along the direction of applied field. As the frequency of the electric field increases, a point is reached when the charge carriers of dielectric do not follow the frequency of the applied electric field and thus, the value of ε′ is decreased. Further, if one continues to increase frequency of the applied field, again a threshold point is reached where polarization vector would hardly respond to the change in electric field and thus no contribution to polarization is made. Therefore, ε′ becomes independent at higher frequencies. Another crucial point that can explain or held responsible for decrement in dielectric constant is related to the hopping mechanism of electrons from Fe2 þ to Fe3 þ ions. At low frequency, electric field does not provide enough energy to electron for hopping but as we increase the frequency of electric field then it becomes capable of providing sufficient energy and finally, a point is reached when hopping of electrons is initiated from Fe2 þ to Fe3 þ ions. Therefore, the conductivity of the dielectric material increases with increase in the frequency as a result of which ε′ decreases [25]. Moreover, in the present study, some multi peaks are observed for doped samples in the temperature range of 300–675 K. These multi peaks may be due to combined effect of non-Debye and Maxwell–Wagner relaxation. Another possibility of occurrence of these peaks in dielectric curves of these samples is the dielectric relaxation process superimposed on electrode interface polarization. Dielectric loss tangent variation with temperature at different frequencies (103, 104, 105 and 106 Hz) for all samples are depicted in Fig. 5(b). Major peaks in dielectric loss curves are observed in temperature range of 540–650 K except for the pristine BiFeO3. The reason for occurrence of loss peaks can be again correlated to non-Debye relaxation and Maxwell–Wagner effect as explained above in dielectric constant part. 3.6. Leakage current In BFO, oxygen vacancies can be produced by the vaporization of Bi or the presence of lower-valence Fe2 þ ions, which lead to the formation of a trap level at 0.6 eV below the bottom edge of the conduction band. In the lower-electric-field region, the electric current in BFO might be transferred by a space-charge-limited mechanism (SCL) after ohmic conduction, where as it should be predominantly governed by one of the following mechanism, Pool–Frenkel (PF) conduction, Schottky conduction (SC) or fieldassisted ionic (FAI) conduction and the Fowler–Nordheim (FN) tunneling under a higher electric field [26]. The FN tunneling and Schottky conduction are interface limited process while SCL and PF are bulk limited conduction mechanism. It is interesting to study doping/co-doping effect on the leakage current of BFO because a synergetic effect might appear from the co-doping of different cations. We observed that Ce doping is favorable for stabilizing the perovskite phase, with Mn doping resulted in the enhanced magnetic properties of BFO ceramics, which might be attributed to the possible crystal lattice distortion and low charge defects (oxygen vacancies) in the sample [27]. Fig. 6 shows the curves of the leakage current density (J) as a function of the applied electric

Please cite this article as: A. Beniwal, et al., A systematic study on multiferroics Bi1  xCexFe1  yMnyO3: Structural, magnetic and electrical properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.169i

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Fig. 5. (a) Dielectric constant variation with temperature at different frequencies (103, 104, 105 and 106 Hz) for all samples. (b) Dielectric loss tangent variation with temperature at different frequencies (103, 104, 105 and 106 Hz) for all samples.

Please cite this article as: A. Beniwal, et al., A systematic study on multiferroics Bi1  xCexFe1  yMnyO3: Structural, magnetic and electrical properties, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.169i

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References

Fig. 6. Room temperature leakage current density as a function of applied electric field of pristine and doped BFO samples.

field (E). With an applied electric field of 750 kV/cm, samples were showing a decrease in J with the increase in Ce concentration. The decrease in J with the increase in Ce concentration might be explained as the reduction in charge defects or impurity phases which is accordance with the XRD results.

4. Conclusions Pure and doped multiferroic samples of bismuth ferrite were successfully synthesized by the sol–gel technique. The studies of the structural, magnetic and electrical properties of doped BFO have shown the improved properties. XRD analysis indicated that the samples are stabilized in rhombohedral structure up to the 10% Ce doping, while orthorhombic phase was observed in Ce–Mn codoped BFO. The SEM micrographs showed that the density and microstructure of samples changed significantly with doping. Furthermore, Ce and Mn co-doping effectively reduced the dielectric loss and improved the dielectric and magnetic properties. The Ce-doped BiFeO3 also decreased the electrical conductivity with improvement of its ferroelectric behavior. From the application point of view, the ability to control the multiferroic properties of BiFeO3 through doping arises many possibilities for engineering and fine-tuning its properties in order to satisfy specific device performance requirements.

Acknowledgments The authors are grateful to Department of Science and Technology (DST) for providing Fast Track Young Scientist Project SR/ FTP/PS-161/2011 and DBT Star College Project to carry out this work. The authors are grateful to Principal, Hindu College and University of Delhi (USIC) for constant encouragement and measurement facilities.

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