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Structural, magnetic and dielectric properties of Sr and V doped BiFeO3 multiferroics
Journal of Magnetism and Magnetic Materials 385 (2015) 175–181
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Structural, magnetic and dielectric properties of Sr and V doped BiFeO3 multiferroics Reetu Dahiya, Ashish Agarwal n, Sujata Sanghi, Ashima Hooda, Priyanka Godara Department of Applied Physics, Guru Jambheshwar University of Science & Technology, Hisar 125001, Haryana, India
art ic l e i nf o
a b s t r a c t
Article history: Received 8 August 2013 Received in revised form 15 May 2014 Accepted 2 March 2015 Available online 3 March 2015
Bi0.85Sr0.15FeO3 (BSFO), Bi0.85Sr0.15Fe0.97V0.03O3 (BSFVO1) and Bi0.85Sr0.15Fe0.95V0.05O3 (BSFVO2) ceramics were synthesized by solid state reaction method. X-ray diffraction studies and Rietveld refinement results indicate that all the samples crystallized in rhombohedrally distorted perovskite structure. The remnant magnetization and coercive field of BSFVO2 were greatly enhanced in comparison with BSFO. The enhancement of remnant magnetization was attributed to collapse of the spiral spin structure caused by change in bond length and bond angles of BSFO on V substitution. The enhanced value of coercive field might be attributed to decreased grain size with V substitution. BSFO sample shows dispersion in dielectric constant (έ) and dielectric loss (tan δ) values in lower frequency region. With V doping this dispersion is reduced resulting in frequency independent region. Dielectric anomaly peak due to charge defects in BSFO sample is also suppressed significantly on V substitution. BSFVO2 sample shows almost temperature stable behavior in έ and tan δ in the studied temperature range. Temperature dependence of index ‘s’ of power law suggests that overlapping large polaron tunneling model is applicable for describing the conduction mechanism in BSFO sample while small polaron tunneling model is appropriate for BSFVO1 and BSFVO2 samples in the studied temperature range. & 2015 Elsevier B.V. All rights reserved.
Keywords: BiFeO3 Rietveld refinement Dielectric properties
1. Introduction Existence of spontaneous magnetic ordering and ferroelectric polarization in a single phase material has been a subject of growing interest for both the dielectric and magnetic scientific communities. Coupling between magnetic and electric ordering leads to magnetoelectric (ME) effect. The ME effect provides an additional degree of freedom in designing of functional sensors, current devices, transducers and multistate memory devices. Besides application potentials, the fundamental physics of multiferroic materials is rich and fascinating [1–4]. However, there are very few naturally occurring materials that can exhibit both spontaneous magnetization and electric polarization. This is possibly due to the fact that the transition metal ‘d’ electron essential in the presence of magnetic moment, also reduces lattice distortion. Lattice distortion is essential in the presence of ferroelectric behavior. Also, known single-phase magnetic ferroelectrics usually have low magnetic ordering temperatures, thus constricting the possibilities for their applications [4]. From this point of view, the most interesting results are expected for the BiFeO3-based perovskite materials. Stoichiometric BiFeO3 (BFO) crystallizes in a n
Corresponding author. Fax: þ 91 1662 276240. E-mail address: [email protected] (A. Agarwal).
http://dx.doi.org/10.1016/j.jmmm.2015.03.013 0304-8853/& 2015 Elsevier B.V. All rights reserved.
rhombohedrally distorted perovskite structure. It has been considered as one of the fascinating multiferroics because of its ferroelectric transition (at about 1100 K) and antiferromagnetic Neel temperatures (at about 640 K) are well above room temperature. The ferroelectric mechanism in BFO is caused by the steriochemically active 6s2 lone pair of Bi3 þ while the weak ferromagnetism is due to residual moment from the canted Fe3 þ spin structure. The magnetic moments of Fe3 þ cations in BFO couple ferromagnetically within the pseudocubic (111) planes and antiferromagnetically between the adjacent planes showing the G-type antiferromagnetic order [5–7]. However, it has been shown that a long-range incommensurate cycloidal spiral magnetic structure with a large period of 62 nm is present in BFO. This cycloidal structure results in the disappearance of weak ferromagnetism and the linear ME effect due to averaging over the period [5,7]. In addition, the bulk BFO is characterized by serious current leakage problems due to the existence of large number of charge centers caused by oxygen ion vacancies and Bi2O3 evaporation during sintering process which makes it difficult to achieve high resistivity. These problems limit the use of BFO for fabrication of multifunctional devices [8–10]. Several research groups have reported that the multiferroic properties of BFO can be improved with various ion substitution at A or/and B site [11–18]. Recently, divalent cations (e. g., Ca, Sr, Pb, Ba) substitution at A-site of BFO ceramics have been reported to enhance the
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magnetization of BFO ceramic [14,19,20]. Interestingly, the magnetic moment with divalent alkaline earth metal substitution is comparable to that with rare earth substituted BFO. Also partial substitution of Fe3 þ at the B-site with higher valence cations like Nb5 þ and Ti4 þ are reported to decrease the leakage current density significantly [21,22]. V5 þ is also a higher valence cation than Fe3 þ . Therefore, B site doping with V5 þ is expected to increase the electrical resistivity of BFO by compensating the charge defects that cause high conduction. This would allow the ferroelectricity and magnetoelectric coupling to be determined at room temperature. A significant improvement of ferroelectric properties in La and V co-doped BFO ceramic have been observed by Yu et al. [23,24]. But investigations on co-substitution of alkaline earth metal Sr and V in bulk BFO have not been reported so far. Therefore, aim of the present work is to investigate the structural, magnetic and dielectric properties of Bi0.85Sr0.15Fe1–xVxO3 multiferroic ceramics.
2. Experimental details Polycrystalline Bi0.85Sr0.15FeO3 (BSFO), Bi0.85Sr0.15Fe0.97V0.03O3 (BSFVO1) and Bi0.85Sr0.15Fe0.95V0.05O3 (BSFVO2) ceramics were synthesized by the conventional solid state reaction method using high purity analytical grade Bi2O3, SrCO3, Fe2O3 and V2O5 (purity Z99.0%) reagents. These materials were carefully weighed in stoichiometric proportion, mixed thoroughly and ground in an agate mortar till a homogeneous mixture was formed. The mixtures were first calcined at 953 K for 2 h and after cooling, grinding was again done for 1 h to get better homogeneity. Calcined powder sample was pressed to form pellets (of 13 mm diameter and 1mm thickness) by applying a pressure of 10 tons using pellet press. Final sintering of the pellet was carried out at 1153 K for 30 min at heating rate 10 K/min. All the pellets were quenched by removing it from the furnace immediately after sintering. The crystal structure of the powder samples was determined from X-ray diffraction (XRD) data. XRD patterns were collected at room temperature using a Rigaku Miniflex-II diffractometer with Cu Kα radiation in the 2θ range (20–80°) at slow scanning rate of 1°/min. The data was further analyzed by Rietveld refinement using General Structure Analysis System (GSAS). Microstructure of the surface was observed using scanning electron microscope (SEMTRAC-mini of NIKKISO). Magnetic evaluation was carried out at room temperature up to a field of 20 kOe using Vibrating Sample Magnetometer (Lakeshore VSM 7410). Dielectric characterization was made with an impendence/gain phase analyzer (Newton's 4th Ltd.) in the frequency range 10 Hz–5 MHz and in the temperature range 323–573 K. For dielectric measurements, powdered sample was pressed to form pellets (of 13 mm diameter and 1 mm thickness) by applying a pressure of 10 tons using pellet press. The sintered pellets were then polished and coated uniformly with silver paste on both sides for making electrodes.
3. Results and discussion 3.1. Structural analysis Fig. 1(a) shows the XRD patterns of BSFO, BSFVO1 and BSFVO2 ceramics. As shown in the figure, the diffraction peaks in each XRD pattern characterized a polycrystalline rhombohedrally distorted perovskite structure. A weak diffraction peak appeared near 2θ E 27.8° along with the main phase and was associated with the unconsumed excess Bi2O3 due to excessive Bi used for compensating volatilization during synthesis. This diffraction peak is very common and has also been observed in La and V codoped BFO
Fig. 1. (a) XRD patterns of BSFO, BSFVO1 and BSFVO2 samples at room temperature. The diffraction peaks corresponding to impurity phase Bi2O3 are marked by n. (b) The magnified patterns in the vicinity of 22.7°.
ceramics by Yu et al. [24]. To investigate the effect of V5 þ doping on the structure of BSFO, subtle XRD patterns of 2θ near 22.5° is shown in Fig. 1(b). It can be clearly seen in the patterns that with V5 þ substitution the (012) diffraction peak shifts gradually to a low diffraction angle, indicating a gradually increased lattice distance of the (012) planes, which might originate from the lattice distortion of the samples due to varying V5 þ contents and difference in ionic radii of V5 þ and Fe3 þ . For detailed structure analysis XRD data was further analyzed by Rietveld refinement using GSAS þ EXPGUI program [25,26]. The Rietveld method uses a least square approach to refine a theoretical line profile until it matches the measured profile. In Rietveld refinement a structural model is required that has an approximation for the actual structure. In this method, the entire spectrum, including the background intensity, is considered to be a single discrete function against which a multi parameter model must be fitted. In Rietveld refinement we need to minimize the function Δ given by [27]
Δ=
∑ wi {IiO i
− IiC }2
(1)
where wi is the weight parameters given by (1/wi)¼ si 2, si being the standard deviation associated with the intensity at each 2θi value, IiO and IiC are observed and calculated intensities for diffraction angle 2θi. In order to achieve convergence and to make the refinement more quantitative, agreement indices or residuals are defined. The profile residual [27] Rp
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Table 1 Refined structural parameters of BSFO, BSFVO1 and BSFVO2 samples. Atomic positions and volume
R-factors (%)
Bi/Sr (0, 0, 0.2782) Fe (0, 0, 0) O (0.8215, 0.6228, 0.4511) Volume¼ 371.653 Å3
Rp ¼3.11 Rwp ¼ 4.35 χ2 ¼2.85
BSFVO1 Rhombohedral a ¼ 5.5848 b¼ 5.5848 c ¼ 13.7304
Bi/Sr (0, 0, 0.2636) Fe (0, 0, 0) O(1.8417, 0.6845, 0.3846) Volume¼ 370.889 Å3
Rp ¼ 5.17 Rwp ¼ 3.97 χ2 ¼3.52
BSFVO2 Rhombohedral a ¼ 5.5737 b¼ 5.5737 c ¼ 13.7662
Bi/Sr (0, 0, 0.2304) Fe (0, 0, 0) O (1.7274, 1.0646, 0.4158) Volume¼ 370.377 Å3
Rp ¼3.00 Rwp ¼ 3.99 RF2 ¼1.96
Sample
Crystal structure
Lattice parameters (Å)
BSFO
Rhombohedral a ¼ 5.5832 b¼ 5.5832 c ¼ 13.7672
The fitting between the experimental spectra and the calculated values is relatively good based on the consideration of relatively lower Rp and Rwp (o10%) values. Profile R-factors measure goodness of fit (GOF) for structure, peak shapes and background. Refined structural parameters along with profile R-factors are listed in Table 1 and different bond lengths and bond angles are given in Table 2 for all the samples. Value of volume for the sample Bi0.85Sr0.15FeO3 is smaller as compared to BiFeO3 reported in literature [28]. The effective ionic radius of Sr2 þ ion under the anion co-ordination number 6 is 1.18 Å which is almost same as the ionic radius of Bi3 þ (1.17 Å). This small difference in ionic radii would hardly influence the lattice dimensions. Therefore low value of volume for Sr doped BiFeO3 sample may be attributed to oxygen vacancies increased due to charge compensation mechanism. It is clear from Table 1 that cell volume decreases with V5 þ substitution. This is expected as the smaller in size (0.54 Å) and higher in valence (V5 þ ) vanadium ions are accommodated in the rhombohedral structure in place of the larger, predominant iron species (0.64 Å, Fe3 þ ). Further SEM investigation performed on surface of the Bi0.85Sr0.15FeO3 sample pointed out a microstructure consisting of non-uniform (as shape and size) grains (Fig. 3). It is clear from the micrographs that average grain size decrease with V5 þ doping. V5 þ plays the role of donor because it possesses a higher valence than Fe3 þ . Addition of V5 þ requires charge compensation by
Table 2 Different bond lengths and bond angles of BSFO, BSFVO1 and BSFVO2 samples obtained from refinement. Fig. 2. The observed, calculated and difference Rietveld refined XRD patterns of (a) BSFO, (b) BSFVO1 and (c) BSFVO2 multiferroics.
Rp =
∑i IiO − IiC ∑i IiO
(2)
and the weighted profile [27] Rwp
⎡ ∑ w (I − I )2 ⎤1/2 i iO iC ⎥ R wp = ⎢ i 2 ⎢⎣ ⎥⎦ ∑i wi IiO
(3)
are used to obtain convergence and this leads to the value of goodness of fit parameter, χ2 approaching one [27]. Fig. 2 shows the experimental and calculated XRD patterns for all the samples.
Sample
BSFO
Bond distances (Å) Bi–O 2.857(5), 2.745 (5) 2.615(10), 2.999 (11) Bi–Fe 3.054(17), 3.312 (6) 3 Fe–O 1.945(3), 2.143 (7) Bond angles (deg) Fe–O–Fe O–Fe–O
150.9(1) 87.7(2), 87.2(2) 108.7(1)
BSFVO1
BSFVO2
2.256(8), 2.86(22) 2.870(22), 3.374(8) 3.246(5), 3.364 (14) 3 2.200(7), 1.838(6)
2.222(4) 2, 2.648 (9) 3.082(12) 3.172(18), 3.516 (6) 3 2.038(4), 2.080(5)
156.4(4) 87.4(5), 73.9(28) 106.3(25)
157.3(1) 88.3(2), 87.8(2) 91.9(2)
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0.2 BSFO BSFVO1
Moment (emu/g)
BSFVO2
0.15 0.1 0.05 0
-20
-10
-0.05
0
10
20
-0.1 -0.15 -0.2 Applied field (kOe) Fig. 4. Room temperature M–H hysteresis loops of BSFO, BSFVO1 and BSFVO2 samples.
D = Kλ /βCosθ
(4)
where D is the crystallite size, K (constant) ¼0.89 and β is the full width at half maxima (in rad). Effects of strain, instruments and other defects were ignored in the calculations. Average crystallite size for all the samples obtained using Eq. (4) are 38, 35 and 31 nm. 3.2. Magnetic properties The room temperature magnetic hysteresis loops of Bi0.85Sr (x¼ 0.0, 0.03, 0.05) ceramics are shown in Fig. 4. All the samples show nonlinear magnetic hysteresis loops. BFO is known to have G-type antiferromagnetic spin structure. The crystal structure of BFO permits the appearance of weak ferromagnetism arising from the canting of antiferromagnetic sublattices [7]. However, the possible net magnetization is canceled by the spiral spin structure. The appearance of remnant magnetization (Mr) of BSFO may be related to canting of antiferromagnetic sublattice and/or contribution of the Fe4 þ ions. Because of the different valence states of Sr2 þ and Bi3 þ ions, addition of Sr2 þ to BFO requires charge compensation which can be achieved by the formation of Fe4 þ or oxygen vacancies. This can be understood by the following defect chemistry model [29]: 0.15Fe1–xVxO3
Fig. 3. SEM micrograph of the surface of the multiferroics: (a) BSFO; (b) BSFVO1 and (c) BSFVO2 samples.
filling of oxygen vacancies. Therefore, reduction of grain size with vanadium ion substitution can be interpreted by the suppression of oxygen vacancies due to charge compensation mechanism, which results in slower oxygen ion motion and consequently lower grain growth rate [13]. The average crystallite size for all the samples was calculated from XRD peak broadening using the Debye Scherrer formula:
+ + − 0.4Bi2 O3 + 0 .5 SrO+ 0.5Fe2 O3 → Bi30.8 Sr 20.2 Fe3 +O22.9
(5)
+ + + + 2− 0 .4 Bi2 O3 + 0.2 SrO+ 0.5Fe2 O3 → Bi30.8 Sr 20.2 Fe 30.8 Fe 40.2 O3
(6)
If Fe4 þ exists, statistical distribution of Fe3 þ and Fe4 þ ions in the octahedral results in net magnetization. But the spiral spin structure of BFO is not completely destroyed by 15% Sr doping. Mr is found to increase with increase in V5 þ ion concentration and the corresponding values for BSFO, BSFVO1 and BSFVO2 samples are 0.0038, 0.009 and 0.016 emu/g, respectively. The enhanced remnant magnetization in BSFVO2 sample is comparable to that for La (30%) doped BFO ceramic [12]. The enhanced Mr value in 30% La doped BFO is attributed to destruction of spiral spin structure. But it is clear from structural analysis that there is no structural transformation in BSFO with V substitution. It is also known that there is no magnetic interaction between V5 þ and Fe3 þ ions as V5 þ ion is nonmagnetic. But a small amount of V5 þ ion doping at Fe-site of BSFO perturbs the superexchange
R. Dahiya et al. / Journal of Magnetism and Magnetic Materials 385 (2015) 175–181
Fig. 5. Frequency dependence of dielectric constant (έ) and dielectric loss (tan δ) (inset) of all the samples at 573 K.
interaction. It can be seen from Table 2 that Fe–O–Fe angle and Fe– O distances are changed by V5 þ doping. As the superexchange interaction is sensitive to bond angles and bond lengths, the spiral spin structure might be collapsed by V5 þ doping resulting in enhanced remnant magnetization. The values of coercive fields for BSFO, BSFVO1 and BSFVO2 samples are 350, 750 and 2200 Oe, respectively. Very large value of coercive field for BSFVO2 sample than for BSFO (see Fig. 3) sample may be due to the decreased grain size in BSFVO2 sample. The grain size of BSFVO2 sample might be close to the single magnetic domain size resulting in an increase in coercivity [30]. 3.3. Dielectric properties Fig. 5 depicts the frequency dependence of dielectric constant and dielectric loss (see inset) at 300 K for all the samples. Dielectric constant exhibits larger dispersion at lower frequencies while it becomes constant at higher frequencies for all the samples. This behavior is well explained by the Maxwell–Wagner type relaxation, often occurring in the heterogeneous systems [31,32]. When an electric current passes through interfaces between two different dielectric media, because of their different conductivities, surface charges pile up at the interfaces giving rise to interfacial polarization at the boundaries. These space charges align with the applied electric field at lower frequencies but as frequency increases the dipoles cannot synchronize with the frequency of the applied field so their contribution is reduced, giving rise to low dielectric constant. According to this model, the sample consists of perfectly conducting grains separated by insulating grain boundaries. The Koop's phenomenological theory postulates that grain boundaries are effective at low frequencies and grains are effective at high frequencies [33]. Thus low polarization at higher frequencies, leads to decrease in dielectric constant. Due to higher resistance posed by the grain boundaries at low frequencies, more energy is required for the motion of charge carriers; hence energy loss (tan δ) is high in this region. On the other hand, at higher frequencies, as low resistance is offered by grains, less energy is required by the charge carriers for motion, so the dielectric loss is also low. BSFO sample shows dispersion in έ and tanδ over larger range of frequency as compared to vanadium doped samples. This is due to the reason that Sr2 þ is expected to introduce more oxygen vacancies (Eq. (5)) thereby increasing the probability of hopping conduction mechanism resulting in large dielectric dispersion. Fig. 6 depicts the temperature dependence of έ and tan δ (see inset) for the prepared samples at 100 kHz. For BSFO sample, both dielectric constant and dielectric loss increase as the temperature
179
Fig. 6. Temperature dependence of dielectric constant (έ) and dielectric loss (tan δ) (inset) for all samples at 100 kHz.
rises, but an anomalous peak is observed around 453 K. This type of anomaly has also been reported by Catalan et al. and attributed to the interaction of oxygen ion vacancies with Fe3 þ /Fe2 þ redox couple [5]. Oxygen vacancies (V 2+ O ) come mainly from Bi volatility and the transition from Fe3 þ to Fe2 þ which can be described by Eqs. (7) and (8):
2Fe 3 + + O2 − → 2Fe2 + + V 2O+ + 0 .5 O2
(7)
2Bi3 + + 3O2 − → 2V 3Bi− + 3V 2O+ + Bi2 O3
(8)
For BSFVO1 sample, the anomaly peak is suppressed and a steplike increase in dielectric constant is observed. Dielectric constant shows almost temperature independent behavior for BSFVO2 sample in the studied temperature range. As the anomaly peak in BSFO is removed by V5 þ ion doping, therefore the anomaly peak observed is not intrinsic type but related to charge defects (oxygen vacancies) which are suppressed by substitution of V5 þ ions due to charge compensation mechanism. Fig. 7 shows the frequency dependence of conductivity for all the samples at 323 K. The phenomenon of conductivity dispersion in solids is generally analyzed using Jonsher’s power law [34]:
σ (ω) = σo + σac = σo + Aωs
(9)
where ω is the angular frequency of applied field. s(ω) is the total conductivity of the system. so is the frequency independent (electronic or dc) part of conductivity, ‘s’ (0 r s r1) is
Fig. 7. Frequency dependence of conductivity s(ω) at different temperatures of all the samples at 323 K. (Inset) s(ω) variation with frequency at different temperatures for BSFVO1 sample.
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mechanism in this sample can be explained on the basis of overlapping large polaron tunneling model. For such polarons, overlap of the potential wells of neighboring sites is possible because of the long range nature of the dominant coulomb interaction. The increase in ‘s’ parameter with increase in temperature for V substituted BSFO samples may be explained in the light of small polaron tunneling mechanism.
4. Conclusions
Fig. 8. Variation of exponent ‘s’-parameter with temperature for BSFO, BSFVO1 and. BSFVO2 samples.
the index, A [ ¼ πN2e2/6kBT (2p)] is a constant, e is the electronic charge, T is the temperature, p is the polarizability of a pair of sites and N is the number of sites per unit volume among which hopping takes place. It is observed that the conductivity remains almost constant at lower frequencies but exhibits dispersion for higher frequencies which is well in accordance with Eq. (9). Fig. 7 shows that conductivity for vanadium substituted samples is lower than BSFO sample. This might be attributed to the suppression of oxygen vacancies with vanadium substitution. Inset shows frequency dependence of s(ω) at different temperatures for BSFVO1 sample. It is clear from inset that conductivity variation is small with temperature for this sample. This reveals increased temperature stability of conductivity of BSFO sample with vanadium doping. From the slopes of log sac vs. log ω curves (not shown here) the values of index ‘s’ are estimated and plotted in Fig. 8 as a function of temperature for all the samples. For random hopping of carriers ‘s’ is equal to zero (frequency independent conductivity) and tends to approach unity as correlation increases. It is well known that ac conductivity of a system is governed by the relaxation mechanism. In a dielectric material, the dipoles formed between two different valence states act as relaxing species, which have a distribution of relaxation time as its length changes with the distribution of sites. The models developed for different microscopic relaxation processes assume that the relaxation is due to transfer of charge carriers in pairs. The term Aωs can often be explained on the basis of different mechanisms for carrier conduction; correlated barrier hopping (CBH) over the same barrier, quantum mechanical tunneling (QMT) through the barrier separating the localized sites and overlapping large polaron tunneling (OLPT) model. Correlated barrier hopping shows a decrease in ‘s’ with increasing temperature, which indicates the thermally activated behavior of electron transfer over the barrier between two sites having their own Coulombic potential wells. Quantum mechanical tunneling is usually associated with temperature independent behavior of ‘s’. In the present study, the parameter ‘s’ varies with temperature in all the samples (Fig. 8). This can be explained by considering polaron tunneling model. If small polarons are formed, the tunneling model predicts an increase in ‘s’ with increase in temperature indicating the activated behavior of polarons, which is independent of intersite separation. On the other hand, when overlapping large polarons are formed, the tunneling model predicts a decrease in ‘s’ with increase in temperature upto a certain range and then an increase in ‘s’ with further increase in temperature. For BSFO sample, ‘s’ parameter decreases with increase in temperature up to 463 K and then increases with increase in temperature. Therefore, conduction
The phase purity, structure, surface morphologies, magnetic and dielectric properties of Bi0.85Sr0.15Fe1–xVxO3 (x ¼0.0, 0.03, 0.05) ceramics were investigated. All the samples were found to crystallize in rhombohedrally distorted perovskite structure with space group R3c. The grain size of BSFO (x ¼0.0) ceramic was greatly reduced and homogeneity was increased on vanadium substitution. Significant enhancement in magnetization in BSFVO2 (x ¼0.05) might be attributed to collapse of spiral spin structure caused by change in bond angles and bond distances of BSFO on V substitution. Values of έ and tan δ for vanadium substituted samples are lower as compared to BSFO (x¼0.0) sample indicating high resistivity of these samples. Temperature stability of dielectric constant and conductivity are improved with V substitution. Therefore improved magnetic and dielectric properties make Sr and V doped BFO multiferroic suitable for device applications.
Acknowledgements Authors are thankful to DST, New Delhi (FIST Scheme) for providing XRD facilities. One of the authors (A.A.) is also thankful to UGC, New Delhi for providing financial (F.42-829/2013(SR)).
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Structural, magnetic and dielectric properties of Sr and V doped BiFeO3 multiferroics Reetu Dahiya, Ashish Agarwal n, Sujata Sanghi, Ashima Hooda, Priyanka Godara Department of Applied Physics, Guru Jambheshwar University of Science & Technology, Hisar 125001, Haryana, India
art ic l e i nf o
a b s t r a c t
Article history: Received 8 August 2013 Received in revised form 15 May 2014 Accepted 2 March 2015 Available online 3 March 2015
Bi0.85Sr0.15FeO3 (BSFO), Bi0.85Sr0.15Fe0.97V0.03O3 (BSFVO1) and Bi0.85Sr0.15Fe0.95V0.05O3 (BSFVO2) ceramics were synthesized by solid state reaction method. X-ray diffraction studies and Rietveld refinement results indicate that all the samples crystallized in rhombohedrally distorted perovskite structure. The remnant magnetization and coercive field of BSFVO2 were greatly enhanced in comparison with BSFO. The enhancement of remnant magnetization was attributed to collapse of the spiral spin structure caused by change in bond length and bond angles of BSFO on V substitution. The enhanced value of coercive field might be attributed to decreased grain size with V substitution. BSFO sample shows dispersion in dielectric constant (έ) and dielectric loss (tan δ) values in lower frequency region. With V doping this dispersion is reduced resulting in frequency independent region. Dielectric anomaly peak due to charge defects in BSFO sample is also suppressed significantly on V substitution. BSFVO2 sample shows almost temperature stable behavior in έ and tan δ in the studied temperature range. Temperature dependence of index ‘s’ of power law suggests that overlapping large polaron tunneling model is applicable for describing the conduction mechanism in BSFO sample while small polaron tunneling model is appropriate for BSFVO1 and BSFVO2 samples in the studied temperature range. & 2015 Elsevier B.V. All rights reserved.
Keywords: BiFeO3 Rietveld refinement Dielectric properties
1. Introduction Existence of spontaneous magnetic ordering and ferroelectric polarization in a single phase material has been a subject of growing interest for both the dielectric and magnetic scientific communities. Coupling between magnetic and electric ordering leads to magnetoelectric (ME) effect. The ME effect provides an additional degree of freedom in designing of functional sensors, current devices, transducers and multistate memory devices. Besides application potentials, the fundamental physics of multiferroic materials is rich and fascinating [1–4]. However, there are very few naturally occurring materials that can exhibit both spontaneous magnetization and electric polarization. This is possibly due to the fact that the transition metal ‘d’ electron essential in the presence of magnetic moment, also reduces lattice distortion. Lattice distortion is essential in the presence of ferroelectric behavior. Also, known single-phase magnetic ferroelectrics usually have low magnetic ordering temperatures, thus constricting the possibilities for their applications [4]. From this point of view, the most interesting results are expected for the BiFeO3-based perovskite materials. Stoichiometric BiFeO3 (BFO) crystallizes in a n
Corresponding author. Fax: þ 91 1662 276240. E-mail address: [email protected] (A. Agarwal).
http://dx.doi.org/10.1016/j.jmmm.2015.03.013 0304-8853/& 2015 Elsevier B.V. All rights reserved.
rhombohedrally distorted perovskite structure. It has been considered as one of the fascinating multiferroics because of its ferroelectric transition (at about 1100 K) and antiferromagnetic Neel temperatures (at about 640 K) are well above room temperature. The ferroelectric mechanism in BFO is caused by the steriochemically active 6s2 lone pair of Bi3 þ while the weak ferromagnetism is due to residual moment from the canted Fe3 þ spin structure. The magnetic moments of Fe3 þ cations in BFO couple ferromagnetically within the pseudocubic (111) planes and antiferromagnetically between the adjacent planes showing the G-type antiferromagnetic order [5–7]. However, it has been shown that a long-range incommensurate cycloidal spiral magnetic structure with a large period of 62 nm is present in BFO. This cycloidal structure results in the disappearance of weak ferromagnetism and the linear ME effect due to averaging over the period [5,7]. In addition, the bulk BFO is characterized by serious current leakage problems due to the existence of large number of charge centers caused by oxygen ion vacancies and Bi2O3 evaporation during sintering process which makes it difficult to achieve high resistivity. These problems limit the use of BFO for fabrication of multifunctional devices [8–10]. Several research groups have reported that the multiferroic properties of BFO can be improved with various ion substitution at A or/and B site [11–18]. Recently, divalent cations (e. g., Ca, Sr, Pb, Ba) substitution at A-site of BFO ceramics have been reported to enhance the
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magnetization of BFO ceramic [14,19,20]. Interestingly, the magnetic moment with divalent alkaline earth metal substitution is comparable to that with rare earth substituted BFO. Also partial substitution of Fe3 þ at the B-site with higher valence cations like Nb5 þ and Ti4 þ are reported to decrease the leakage current density significantly [21,22]. V5 þ is also a higher valence cation than Fe3 þ . Therefore, B site doping with V5 þ is expected to increase the electrical resistivity of BFO by compensating the charge defects that cause high conduction. This would allow the ferroelectricity and magnetoelectric coupling to be determined at room temperature. A significant improvement of ferroelectric properties in La and V co-doped BFO ceramic have been observed by Yu et al. [23,24]. But investigations on co-substitution of alkaline earth metal Sr and V in bulk BFO have not been reported so far. Therefore, aim of the present work is to investigate the structural, magnetic and dielectric properties of Bi0.85Sr0.15Fe1–xVxO3 multiferroic ceramics.
2. Experimental details Polycrystalline Bi0.85Sr0.15FeO3 (BSFO), Bi0.85Sr0.15Fe0.97V0.03O3 (BSFVO1) and Bi0.85Sr0.15Fe0.95V0.05O3 (BSFVO2) ceramics were synthesized by the conventional solid state reaction method using high purity analytical grade Bi2O3, SrCO3, Fe2O3 and V2O5 (purity Z99.0%) reagents. These materials were carefully weighed in stoichiometric proportion, mixed thoroughly and ground in an agate mortar till a homogeneous mixture was formed. The mixtures were first calcined at 953 K for 2 h and after cooling, grinding was again done for 1 h to get better homogeneity. Calcined powder sample was pressed to form pellets (of 13 mm diameter and 1mm thickness) by applying a pressure of 10 tons using pellet press. Final sintering of the pellet was carried out at 1153 K for 30 min at heating rate 10 K/min. All the pellets were quenched by removing it from the furnace immediately after sintering. The crystal structure of the powder samples was determined from X-ray diffraction (XRD) data. XRD patterns were collected at room temperature using a Rigaku Miniflex-II diffractometer with Cu Kα radiation in the 2θ range (20–80°) at slow scanning rate of 1°/min. The data was further analyzed by Rietveld refinement using General Structure Analysis System (GSAS). Microstructure of the surface was observed using scanning electron microscope (SEMTRAC-mini of NIKKISO). Magnetic evaluation was carried out at room temperature up to a field of 20 kOe using Vibrating Sample Magnetometer (Lakeshore VSM 7410). Dielectric characterization was made with an impendence/gain phase analyzer (Newton's 4th Ltd.) in the frequency range 10 Hz–5 MHz and in the temperature range 323–573 K. For dielectric measurements, powdered sample was pressed to form pellets (of 13 mm diameter and 1 mm thickness) by applying a pressure of 10 tons using pellet press. The sintered pellets were then polished and coated uniformly with silver paste on both sides for making electrodes.
3. Results and discussion 3.1. Structural analysis Fig. 1(a) shows the XRD patterns of BSFO, BSFVO1 and BSFVO2 ceramics. As shown in the figure, the diffraction peaks in each XRD pattern characterized a polycrystalline rhombohedrally distorted perovskite structure. A weak diffraction peak appeared near 2θ E 27.8° along with the main phase and was associated with the unconsumed excess Bi2O3 due to excessive Bi used for compensating volatilization during synthesis. This diffraction peak is very common and has also been observed in La and V codoped BFO
Fig. 1. (a) XRD patterns of BSFO, BSFVO1 and BSFVO2 samples at room temperature. The diffraction peaks corresponding to impurity phase Bi2O3 are marked by n. (b) The magnified patterns in the vicinity of 22.7°.
ceramics by Yu et al. [24]. To investigate the effect of V5 þ doping on the structure of BSFO, subtle XRD patterns of 2θ near 22.5° is shown in Fig. 1(b). It can be clearly seen in the patterns that with V5 þ substitution the (012) diffraction peak shifts gradually to a low diffraction angle, indicating a gradually increased lattice distance of the (012) planes, which might originate from the lattice distortion of the samples due to varying V5 þ contents and difference in ionic radii of V5 þ and Fe3 þ . For detailed structure analysis XRD data was further analyzed by Rietveld refinement using GSAS þ EXPGUI program [25,26]. The Rietveld method uses a least square approach to refine a theoretical line profile until it matches the measured profile. In Rietveld refinement a structural model is required that has an approximation for the actual structure. In this method, the entire spectrum, including the background intensity, is considered to be a single discrete function against which a multi parameter model must be fitted. In Rietveld refinement we need to minimize the function Δ given by [27]
Δ=
∑ wi {IiO i
− IiC }2
(1)
where wi is the weight parameters given by (1/wi)¼ si 2, si being the standard deviation associated with the intensity at each 2θi value, IiO and IiC are observed and calculated intensities for diffraction angle 2θi. In order to achieve convergence and to make the refinement more quantitative, agreement indices or residuals are defined. The profile residual [27] Rp
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Table 1 Refined structural parameters of BSFO, BSFVO1 and BSFVO2 samples. Atomic positions and volume
R-factors (%)
Bi/Sr (0, 0, 0.2782) Fe (0, 0, 0) O (0.8215, 0.6228, 0.4511) Volume¼ 371.653 Å3
Rp ¼3.11 Rwp ¼ 4.35 χ2 ¼2.85
BSFVO1 Rhombohedral a ¼ 5.5848 b¼ 5.5848 c ¼ 13.7304
Bi/Sr (0, 0, 0.2636) Fe (0, 0, 0) O(1.8417, 0.6845, 0.3846) Volume¼ 370.889 Å3
Rp ¼ 5.17 Rwp ¼ 3.97 χ2 ¼3.52
BSFVO2 Rhombohedral a ¼ 5.5737 b¼ 5.5737 c ¼ 13.7662
Bi/Sr (0, 0, 0.2304) Fe (0, 0, 0) O (1.7274, 1.0646, 0.4158) Volume¼ 370.377 Å3
Rp ¼3.00 Rwp ¼ 3.99 RF2 ¼1.96
Sample
Crystal structure
Lattice parameters (Å)
BSFO
Rhombohedral a ¼ 5.5832 b¼ 5.5832 c ¼ 13.7672
The fitting between the experimental spectra and the calculated values is relatively good based on the consideration of relatively lower Rp and Rwp (o10%) values. Profile R-factors measure goodness of fit (GOF) for structure, peak shapes and background. Refined structural parameters along with profile R-factors are listed in Table 1 and different bond lengths and bond angles are given in Table 2 for all the samples. Value of volume for the sample Bi0.85Sr0.15FeO3 is smaller as compared to BiFeO3 reported in literature [28]. The effective ionic radius of Sr2 þ ion under the anion co-ordination number 6 is 1.18 Å which is almost same as the ionic radius of Bi3 þ (1.17 Å). This small difference in ionic radii would hardly influence the lattice dimensions. Therefore low value of volume for Sr doped BiFeO3 sample may be attributed to oxygen vacancies increased due to charge compensation mechanism. It is clear from Table 1 that cell volume decreases with V5 þ substitution. This is expected as the smaller in size (0.54 Å) and higher in valence (V5 þ ) vanadium ions are accommodated in the rhombohedral structure in place of the larger, predominant iron species (0.64 Å, Fe3 þ ). Further SEM investigation performed on surface of the Bi0.85Sr0.15FeO3 sample pointed out a microstructure consisting of non-uniform (as shape and size) grains (Fig. 3). It is clear from the micrographs that average grain size decrease with V5 þ doping. V5 þ plays the role of donor because it possesses a higher valence than Fe3 þ . Addition of V5 þ requires charge compensation by
Table 2 Different bond lengths and bond angles of BSFO, BSFVO1 and BSFVO2 samples obtained from refinement. Fig. 2. The observed, calculated and difference Rietveld refined XRD patterns of (a) BSFO, (b) BSFVO1 and (c) BSFVO2 multiferroics.
Rp =
∑i IiO − IiC ∑i IiO
(2)
and the weighted profile [27] Rwp
⎡ ∑ w (I − I )2 ⎤1/2 i iO iC ⎥ R wp = ⎢ i 2 ⎢⎣ ⎥⎦ ∑i wi IiO
(3)
are used to obtain convergence and this leads to the value of goodness of fit parameter, χ2 approaching one [27]. Fig. 2 shows the experimental and calculated XRD patterns for all the samples.
Sample
BSFO
Bond distances (Å) Bi–O 2.857(5), 2.745 (5) 2.615(10), 2.999 (11) Bi–Fe 3.054(17), 3.312 (6) 3 Fe–O 1.945(3), 2.143 (7) Bond angles (deg) Fe–O–Fe O–Fe–O
150.9(1) 87.7(2), 87.2(2) 108.7(1)
BSFVO1
BSFVO2
2.256(8), 2.86(22) 2.870(22), 3.374(8) 3.246(5), 3.364 (14) 3 2.200(7), 1.838(6)
2.222(4) 2, 2.648 (9) 3.082(12) 3.172(18), 3.516 (6) 3 2.038(4), 2.080(5)
156.4(4) 87.4(5), 73.9(28) 106.3(25)
157.3(1) 88.3(2), 87.8(2) 91.9(2)
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0.2 BSFO BSFVO1
Moment (emu/g)
BSFVO2
0.15 0.1 0.05 0
-20
-10
-0.05
0
10
20
-0.1 -0.15 -0.2 Applied field (kOe) Fig. 4. Room temperature M–H hysteresis loops of BSFO, BSFVO1 and BSFVO2 samples.
D = Kλ /βCosθ
(4)
where D is the crystallite size, K (constant) ¼0.89 and β is the full width at half maxima (in rad). Effects of strain, instruments and other defects were ignored in the calculations. Average crystallite size for all the samples obtained using Eq. (4) are 38, 35 and 31 nm. 3.2. Magnetic properties The room temperature magnetic hysteresis loops of Bi0.85Sr (x¼ 0.0, 0.03, 0.05) ceramics are shown in Fig. 4. All the samples show nonlinear magnetic hysteresis loops. BFO is known to have G-type antiferromagnetic spin structure. The crystal structure of BFO permits the appearance of weak ferromagnetism arising from the canting of antiferromagnetic sublattices [7]. However, the possible net magnetization is canceled by the spiral spin structure. The appearance of remnant magnetization (Mr) of BSFO may be related to canting of antiferromagnetic sublattice and/or contribution of the Fe4 þ ions. Because of the different valence states of Sr2 þ and Bi3 þ ions, addition of Sr2 þ to BFO requires charge compensation which can be achieved by the formation of Fe4 þ or oxygen vacancies. This can be understood by the following defect chemistry model [29]: 0.15Fe1–xVxO3
Fig. 3. SEM micrograph of the surface of the multiferroics: (a) BSFO; (b) BSFVO1 and (c) BSFVO2 samples.
filling of oxygen vacancies. Therefore, reduction of grain size with vanadium ion substitution can be interpreted by the suppression of oxygen vacancies due to charge compensation mechanism, which results in slower oxygen ion motion and consequently lower grain growth rate [13]. The average crystallite size for all the samples was calculated from XRD peak broadening using the Debye Scherrer formula:
+ + − 0.4Bi2 O3 + 0 .5 SrO+ 0.5Fe2 O3 → Bi30.8 Sr 20.2 Fe3 +O22.9
(5)
+ + + + 2− 0 .4 Bi2 O3 + 0.2 SrO+ 0.5Fe2 O3 → Bi30.8 Sr 20.2 Fe 30.8 Fe 40.2 O3
(6)
If Fe4 þ exists, statistical distribution of Fe3 þ and Fe4 þ ions in the octahedral results in net magnetization. But the spiral spin structure of BFO is not completely destroyed by 15% Sr doping. Mr is found to increase with increase in V5 þ ion concentration and the corresponding values for BSFO, BSFVO1 and BSFVO2 samples are 0.0038, 0.009 and 0.016 emu/g, respectively. The enhanced remnant magnetization in BSFVO2 sample is comparable to that for La (30%) doped BFO ceramic [12]. The enhanced Mr value in 30% La doped BFO is attributed to destruction of spiral spin structure. But it is clear from structural analysis that there is no structural transformation in BSFO with V substitution. It is also known that there is no magnetic interaction between V5 þ and Fe3 þ ions as V5 þ ion is nonmagnetic. But a small amount of V5 þ ion doping at Fe-site of BSFO perturbs the superexchange
R. Dahiya et al. / Journal of Magnetism and Magnetic Materials 385 (2015) 175–181
Fig. 5. Frequency dependence of dielectric constant (έ) and dielectric loss (tan δ) (inset) of all the samples at 573 K.
interaction. It can be seen from Table 2 that Fe–O–Fe angle and Fe– O distances are changed by V5 þ doping. As the superexchange interaction is sensitive to bond angles and bond lengths, the spiral spin structure might be collapsed by V5 þ doping resulting in enhanced remnant magnetization. The values of coercive fields for BSFO, BSFVO1 and BSFVO2 samples are 350, 750 and 2200 Oe, respectively. Very large value of coercive field for BSFVO2 sample than for BSFO (see Fig. 3) sample may be due to the decreased grain size in BSFVO2 sample. The grain size of BSFVO2 sample might be close to the single magnetic domain size resulting in an increase in coercivity [30]. 3.3. Dielectric properties Fig. 5 depicts the frequency dependence of dielectric constant and dielectric loss (see inset) at 300 K for all the samples. Dielectric constant exhibits larger dispersion at lower frequencies while it becomes constant at higher frequencies for all the samples. This behavior is well explained by the Maxwell–Wagner type relaxation, often occurring in the heterogeneous systems [31,32]. When an electric current passes through interfaces between two different dielectric media, because of their different conductivities, surface charges pile up at the interfaces giving rise to interfacial polarization at the boundaries. These space charges align with the applied electric field at lower frequencies but as frequency increases the dipoles cannot synchronize with the frequency of the applied field so their contribution is reduced, giving rise to low dielectric constant. According to this model, the sample consists of perfectly conducting grains separated by insulating grain boundaries. The Koop's phenomenological theory postulates that grain boundaries are effective at low frequencies and grains are effective at high frequencies [33]. Thus low polarization at higher frequencies, leads to decrease in dielectric constant. Due to higher resistance posed by the grain boundaries at low frequencies, more energy is required for the motion of charge carriers; hence energy loss (tan δ) is high in this region. On the other hand, at higher frequencies, as low resistance is offered by grains, less energy is required by the charge carriers for motion, so the dielectric loss is also low. BSFO sample shows dispersion in έ and tanδ over larger range of frequency as compared to vanadium doped samples. This is due to the reason that Sr2 þ is expected to introduce more oxygen vacancies (Eq. (5)) thereby increasing the probability of hopping conduction mechanism resulting in large dielectric dispersion. Fig. 6 depicts the temperature dependence of έ and tan δ (see inset) for the prepared samples at 100 kHz. For BSFO sample, both dielectric constant and dielectric loss increase as the temperature
179
Fig. 6. Temperature dependence of dielectric constant (έ) and dielectric loss (tan δ) (inset) for all samples at 100 kHz.
rises, but an anomalous peak is observed around 453 K. This type of anomaly has also been reported by Catalan et al. and attributed to the interaction of oxygen ion vacancies with Fe3 þ /Fe2 þ redox couple [5]. Oxygen vacancies (V 2+ O ) come mainly from Bi volatility and the transition from Fe3 þ to Fe2 þ which can be described by Eqs. (7) and (8):
2Fe 3 + + O2 − → 2Fe2 + + V 2O+ + 0 .5 O2
(7)
2Bi3 + + 3O2 − → 2V 3Bi− + 3V 2O+ + Bi2 O3
(8)
For BSFVO1 sample, the anomaly peak is suppressed and a steplike increase in dielectric constant is observed. Dielectric constant shows almost temperature independent behavior for BSFVO2 sample in the studied temperature range. As the anomaly peak in BSFO is removed by V5 þ ion doping, therefore the anomaly peak observed is not intrinsic type but related to charge defects (oxygen vacancies) which are suppressed by substitution of V5 þ ions due to charge compensation mechanism. Fig. 7 shows the frequency dependence of conductivity for all the samples at 323 K. The phenomenon of conductivity dispersion in solids is generally analyzed using Jonsher’s power law [34]:
σ (ω) = σo + σac = σo + Aωs
(9)
where ω is the angular frequency of applied field. s(ω) is the total conductivity of the system. so is the frequency independent (electronic or dc) part of conductivity, ‘s’ (0 r s r1) is
Fig. 7. Frequency dependence of conductivity s(ω) at different temperatures of all the samples at 323 K. (Inset) s(ω) variation with frequency at different temperatures for BSFVO1 sample.
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mechanism in this sample can be explained on the basis of overlapping large polaron tunneling model. For such polarons, overlap of the potential wells of neighboring sites is possible because of the long range nature of the dominant coulomb interaction. The increase in ‘s’ parameter with increase in temperature for V substituted BSFO samples may be explained in the light of small polaron tunneling mechanism.
4. Conclusions
Fig. 8. Variation of exponent ‘s’-parameter with temperature for BSFO, BSFVO1 and. BSFVO2 samples.
the index, A [ ¼ πN2e2/6kBT (2p)] is a constant, e is the electronic charge, T is the temperature, p is the polarizability of a pair of sites and N is the number of sites per unit volume among which hopping takes place. It is observed that the conductivity remains almost constant at lower frequencies but exhibits dispersion for higher frequencies which is well in accordance with Eq. (9). Fig. 7 shows that conductivity for vanadium substituted samples is lower than BSFO sample. This might be attributed to the suppression of oxygen vacancies with vanadium substitution. Inset shows frequency dependence of s(ω) at different temperatures for BSFVO1 sample. It is clear from inset that conductivity variation is small with temperature for this sample. This reveals increased temperature stability of conductivity of BSFO sample with vanadium doping. From the slopes of log sac vs. log ω curves (not shown here) the values of index ‘s’ are estimated and plotted in Fig. 8 as a function of temperature for all the samples. For random hopping of carriers ‘s’ is equal to zero (frequency independent conductivity) and tends to approach unity as correlation increases. It is well known that ac conductivity of a system is governed by the relaxation mechanism. In a dielectric material, the dipoles formed between two different valence states act as relaxing species, which have a distribution of relaxation time as its length changes with the distribution of sites. The models developed for different microscopic relaxation processes assume that the relaxation is due to transfer of charge carriers in pairs. The term Aωs can often be explained on the basis of different mechanisms for carrier conduction; correlated barrier hopping (CBH) over the same barrier, quantum mechanical tunneling (QMT) through the barrier separating the localized sites and overlapping large polaron tunneling (OLPT) model. Correlated barrier hopping shows a decrease in ‘s’ with increasing temperature, which indicates the thermally activated behavior of electron transfer over the barrier between two sites having their own Coulombic potential wells. Quantum mechanical tunneling is usually associated with temperature independent behavior of ‘s’. In the present study, the parameter ‘s’ varies with temperature in all the samples (Fig. 8). This can be explained by considering polaron tunneling model. If small polarons are formed, the tunneling model predicts an increase in ‘s’ with increase in temperature indicating the activated behavior of polarons, which is independent of intersite separation. On the other hand, when overlapping large polarons are formed, the tunneling model predicts a decrease in ‘s’ with increase in temperature upto a certain range and then an increase in ‘s’ with further increase in temperature. For BSFO sample, ‘s’ parameter decreases with increase in temperature up to 463 K and then increases with increase in temperature. Therefore, conduction
The phase purity, structure, surface morphologies, magnetic and dielectric properties of Bi0.85Sr0.15Fe1–xVxO3 (x ¼0.0, 0.03, 0.05) ceramics were investigated. All the samples were found to crystallize in rhombohedrally distorted perovskite structure with space group R3c. The grain size of BSFO (x ¼0.0) ceramic was greatly reduced and homogeneity was increased on vanadium substitution. Significant enhancement in magnetization in BSFVO2 (x ¼0.05) might be attributed to collapse of spiral spin structure caused by change in bond angles and bond distances of BSFO on V substitution. Values of έ and tan δ for vanadium substituted samples are lower as compared to BSFO (x¼0.0) sample indicating high resistivity of these samples. Temperature stability of dielectric constant and conductivity are improved with V substitution. Therefore improved magnetic and dielectric properties make Sr and V doped BFO multiferroic suitable for device applications.
Acknowledgements Authors are thankful to DST, New Delhi (FIST Scheme) for providing XRD facilities. One of the authors (A.A.) is also thankful to UGC, New Delhi for providing financial (F.42-829/2013(SR)).
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