Realization of structural transformation for the enhancement of magnetic and magneto capacitance effect in BiFeO3–CoFe2O4 ceramics for energy storage application

In this study, (1 − x) BFO-xCFO (CFO, x = 0.00, 0.05, 0.10 and 0.30) ceramics were synthesized by a solid-state reaction method; their compositions were driven by structural, microstructural, vibrational, electrical, magnetic properties; their enhanced magneto capacitance (MC) effect have also been carried out. Reitveld refinement studies of X-ray diffraction data shows composition-driven structural phase transformation from rhombohedral (R3c) to tetragonal (P4mm). Two phonon scattering Raman modes were observed for the higher wavenumber which supports the crystal structural transition in the BFO-CFO. Ferroelectric polarization shows that the polarization increased with increasing CFO concentration, which describes the changes of the polar symmetry of the crystal structure from rhombohedral (R3c) to tetragonal (P4mm). In Further, the maximum efficiency of energy density (η = 68.65%), reversible energy density of 0.138 J/cm3 and the strong magneto capacitance was observed in 0.9BFO-0.1CFO, which belongs to the morphotropic phase boundary (MPB) region near to the BiFeO3-rich region. The magnetic response analysis has shown, the saturation magnetization (Ms) values of 83 emu/gm and 139 emu/gm for pure CFO and 0.7BFO-0.3CFO composite, respectively, and their magnetic behaviours were also confirmed with Arrott–Belov–Kouvel (ABK) plot.

www.nature.com/scientificreports/ has exhibited a single phase of rhombohedrally distorted perovskite structure with R3c space group at room temperature. This distorted perovskite structure with FeO 6 octahedra paves way for coupling of magnetism and ferroelectricity 9 . However, pure BFO is limited for the enhancement of ME coupling. It possesses high leakage current by oxygen vacancy, high dielectric loss, weak antiferromagnetic order, and structural instability due to the formation of impurities phases like Bi 2 Fe 4 O 9 and Bi 25 Fe 2 O 39 because of the kinetics of formation. These drawbacks can be overcome by substituting suitable rare elements at Bi site, transition elements at Fe site and co-doping with both sites of BFO to make composite system. Recent studies on BFO-CFO composite have shown that the composite system play an important role in modifying the crystal structure which leads to enhance the electrical, magnetic, and ME coupling properties. Khalid et al. 10 successfully prepared (1 − x)BiFe 0.95 Mn 0.05 O 3 -xCoFe 2 O 4 (0.2 ≤ x ≤ 0.7) core-shell nano-composites and observed structural transformation from rhombohedral (R3c) to rhombohedrally distorted cubic-type perovskite (R3m) in 0.8BFMO-0.2CFO which gives maximum polarization, highest efficiency of energy density and enhanced the magneto capacitance indicating that these composites have a potential for application in energy storage devices. The enhancement of ferromagnetism and ferroelectricity of BFO-CFO system have been ascribed to several factors including contributions of valence fluctuations of the Fe ion 11,12 . If Fe 3+ is reduced to Fe 2+ , the system can then compensate ionically by forming oxygen vacancies (V O •• ) or electronically with holes (h•). These reactions are thus described in Kröger-Vink notation: It has been generating the electronic charge carriers that contribute to leakage current density or oxygen vacancy in the BFO. The results suggest that the presence of secondary phases like Bi 2 Fe 4 O 9 and Bi 24 Fe 2 O 39 has been known to affect the electrical properties.
On substituting Co 3+ and Fe 3+ ions, point defects with an effective negative charge is created. These defects are equalized by defects of effective positive charge intending to conserve overall charge neutrally which may possibly be electron holes or oxygen vacancies. By using Kröger-Vink notation, these two possible substitutions can be written as: These lattice defect change will affect the electrical and magnetic properties. For this reason, CFO has been chosen for making BFO-CFO composites at different molar concentrations as it exhibits high magnetic response, coercivity, and magneto crystalline anisotropy. It also has potential applications in data storage devices, ferrofluid technology, magnetic resonance imaging, optoelectronics, and in targeted drug delivery 13,14 . Thus, CFO is assumed to be a suitable ferrite in designing composites with BFO to improve the ME coupling. In addition to that CFO has a high magnetic nature which is expected to enhance the magnetic response of the composites.
In this work, a systematic study on the structural phase transformation in BFO-CFO ceramics has been made by varying the CFO concentration in order to investigate the first-time peculiarities of multiferroic behaviour has been done and their compositional driven microstructural, electrical, magnetic and energy storage density properties and enhanced magneto capacitance effect have also been carried out.

Experimental details
A series of (1 − x) BFO-xCFO (CFO, x = 0.00, 0.05, 0.10 and 0.30) ceramics along with pure BFO and CFO powders were synthesized by the conventional solid-state reaction method using high purity analytical grade of Bi 2 O 3 , Fe 2 O 3 and Co 3 O 4 reagents (Sigma Aldrich) as starting materials and the stoichiometric proportions of precursor reagents were calculated for each sample and well mixed and ground with agate mortar. These powders were calcinated at 840 °C for 3 h. The calcined ceramic powders were uniaxially pressed (90 MPa) to fabricate into pellets with 8 mm diameter and 0.90 mm thickness. Then, the prepared pellets were sintered at 980 °C for 5 h.
The crystal structures of powders were identified with X-ray diffraction (XRD) analysis by using a Philips X′Pert/MPD diffraction system (CuΚ α radiation, λ = 1.5406 Å) and their phase fractions and crystal structure information were calculated by Rietveld refinement using General Structure Analysis System (GSAS) software. The micro structural features of the micro grains in sintered pellets were studied by Scanning Electron Microscopy (SEM, JEOL JSM-6700) and lattice fringes were observed by Transmission Electron microscopy (TEM, JEM-2010-200 kV). Raman spectra is measured using argon laser (532 nm) with a Kaiser holographic edge filter (JASCO Hololab 5000). Ferroelectric polarization was measured with a P-E loop tracer (Radiant Technologies, USA). For magnetic behavior of the samples were measured with a 5 T mini vibrating sample magnetometer (VSM) from Cryogenic Ltd. The magneto capacitance (MC) effect is measured with LCR meter (model HIOKI 3532-50, Japan) by applying a different magnetic field ranging from 0 Oe to 7 kOe.

Results and discussion
Structural analysis. XRD patterns of (1 − x) BFO-xCFO (CFO, x = 0.00, 0.05, 0.10 and 0.30) ceramics and BFO and CFO powders calcined at 840 °C for 3 h as shows in Fig. 1. The XRD pattern of BFO shows peaks corresponding to the BiFeO 3 rhombohedral structure (R3c, JCPDS 71-2494) and the impurity phases of *Bi 24 Fe 2 O 39, #Bi 2 Fe 4 O 9 which was formed due to the kinetics of formation 15,16 . The XRD pattern of CFO indicated the cubic  Fig. 1b, the 0.95BFO-0.05CFO reserved the rhombohedral structure for relatively lower CFO concentration. However, with increasing CFO concentration to x = 0.1 and x = 0.3, new peak has appeared at 2θ = 31.5° which suggests that this new peak is corresponding to tetragonal structure with a space group of (P4mm) [17][18][19][20] . In addition to that, the X-ray diffraction peaks are shifted to the higher angle with increasing CFO concentration, which indicates the shrinkage of the BFO unit cell due to the substitution of Co 3+ (0.685 Å) cation at the site of Fe 3+ (0.785 Å). According to literature reports, the splitting of the (101) and the (101)/(104) reflections of the coexistence of rhombohedral (R3c) and tetragonal (P4mm) phases into triplets is usually taken as an evidence for a tetragonally distorted phase. Singh et al. 17 , reported that BiFe 1-x Co x O 3 transforms from rhombohedral R (R3c) to tetragonal T (P4mm) crystal structures as the cobalt concentration increases and both phases (R and T) coexist for Co content between x = 0.10 and 0.20. These morphotropic phase boundary (MPB) samples may potentially display the magneto capacitance (MC) effect with a variance in the macroscopic magnetization because only rhombohedral domains exist for Co concentration below x = 0.20. Azuma et al. 18 studied the (1-x) BiCoO 3 -xBiFiO 3 composite system consisting tetragonal (P4mm) to rhombohedral (R3c) phase boundary between x = 0.6 and x = 0.8 at room temperature. Nakamura et al. 19 reported the (1 − x) BiFeO 3 -xBiCoO 3 thin films which shows the rhombohedral and tetragonal phases coexisting in the range of x = 0.075 and x = 0.15. Oka et al. 20 reported tetragonal (P4mm) to rhombohedral (R3c) phase transition at high temperature in BiCo 1−x Fe x O 3 (x = 0.7) and a monoclinic phase is found at room temperature in the vicinity of this composition x = 0.8. Based on these research lines, we attempted three different structural models of R3c, P4mm, Fd3m to fit the XRD patterns of the prepared (1 − x) BFO-xCFO composite powders with concentration of CFO up to x = 0.30 sample. The Rietveld refinement results carried out using X-ray diffraction data for all the samples are shown in Fig. 2. The refinement is revealed that using the rhombohedral crystal symmetry (R3c) for BFO and (1 − x) BFO-xCFO (x = 0.05) ceramic were well matched with experimental data. The crystal structural information obtained from Rietveld refinement results are summarized in Table 1.
As shown in the Fig. 3a,b, the Bragg profiles could be fitted reasonably well with the tetragonal structural model (101), along with rhombohedral structure (101)/(104), which clearly confirms the coexistence of both structures in the composites. From the refinement results, we have observed the slightly variation in lattice parameter, volumes and phase fractions according to CFO concentration increasing into BFO. It has been observed structural phase transformation from rhombohedral (R3c) to other structural phases started at x = 0.10, which can be considered as a morphotropic phase boundary.
Microstructural analysis. SEM images of (1 − x)BFO-xCFO (CFO, x = 0.00, 0.05, 0.10 and 0.30) ceramics, BFO and CFO pellets sintered at 980 °C for 5 h are shown in Fig. 4. As it can be seen, semi-rectangular agglomerated BFO particles with average grain size of 1.6 µm are observed in BFO sample. In composite powders, CFO particles are distinguishable and composite nature of their microstructure is evident. However, the white grains are belonging to BFO and dark grains are belong to CFO. The average grain size is calculated for (1 − x) BFO-xCFO (x = 0.05, 0.10 and 0.30) composites and the samples are 2.0, 1.36 and 1.19 µm, respectively. This decreasing trend with the increasing of CFO concentration in the composites might be due to the role of the impurity ions which is to suppress the growth of the grains in the heterogeneous nucleation sites 21 .
HRTEM images of calcined BFO-CFO powders consist of clear internal crystal lattice structures, which confirm their high crystallinity as shows in      www.nature.com/scientificreports/ in Fig. 6a. According to the group theory, rhombohedrally distorted crystal structure with space group of R3c having 13 Raman-active modes are predicted, which is given as formula 22 : Here, the A 1 and E modes are representing transverse and the longitudinal optical modes, respectively as Raman active modes. For BFO samples have been observed 4A 1 and 5E modes are 134, 166, 210, 265, 326, 343, 416, 463, and 525 cm -1 , which matches with the previous literature reports 23 . the low-frequency range Raman modes are mostly associated with the Bi-O vibration and the higher frequency modes are associated with Fe-O vibrations 24 . Thus, the origin of ferroelectricity is due to the stereochemical activity of the Bi-6s 2 lone pair electron and changes in the BFO covalent bonds. In the present work, there are six characteristics of Raman modes (i.e., E1, A 1 -1, A 1 -2, A 1 -3, A 1 -4 and E 2 ) these are assisted in inducing ferroelectricity of BFO. For 0.95BFO-0.05CFO sample, 4A 1 and 5E Raman modes were observed at 133, 168, 211, 265, 330, 342, 416, 463 and 528 cm −1 , while E 7 Raman mode almost vanished. For 0.9BFO-0.1CFO sample, Raman modes were observed at 170, 275, 426, and 550 cm −1 modes that were slightly shifted to higher wavenumber, which may be attributed to the lower atomic weight of Fe 3+ (55.845 g/mol) than that of Co 3+ (58.93 g/mol) and E 7 modes appeared with high intensity, this may be due to the introduced new crystal structure along with rhombohedral R3c or suggest a more complex  Leakage current measurement. The measured leakage current density of (1 − x)BFO-xCFO (x = 0.00, 0.05, 0.10 and 0.30) ceramic samples as a function of applied different positive electric fields (E) are shown in Fig. 7. As it can be seen, when the applied electric filed increases, the leakage current density increases for all the samples. It is noticed that the pure BFO has higher leakage current density, which may be by the presence of charged oxygen vacancies due to the reduction of Fe 3+ to Fe 2+ in BFO 27,28 . The leakage current density increased with increasing CFO concentration except 0.9BFO-0.1CFO sample, which showed the lowest leakage current density of 1.25 × 10 -6 A/cm 2 at MPB.
Ferroelectric measurement. Ferroelectric behaviour of BFO and (1 − x) BFO-xCFO (x = 0.00, 0.05, 0.10 and 0.30) sintered discs were characterized at room temperature in terms of polarization versus electric field (P-E) hysteresis loops as shown in Fig. 8. The polarization values measured from the ferroelectric hysteresis loop such as Remnant polarization (P r ) and saturated polarization (P s ) are listed in the Table 2. Rounded ferroelectric hysteresis loops for BFO, which indicates their ferroelectric characteristic and improper hysteresis loop behaviour have been observed. These kinds of ferroelectric behaviours are may be due to the higher leakage current density in the BFO 10,29 . As it can be seen at MPB region, the polarization values are high at (1 − x) BFO-xCFO (x = 0.05 and 0.30) with applied electric field 30 (kV/cm) to the samples, then compared to x = 0.10 sample. These changes in the polarization of all the ceramic samples may be attributed to the structural transition from rhombohedral to tetragonal, which is in good agreement with X-ray diffraction and Raman studies.
To investigate further the energy storage density of ceramic samples, the P r , P max and E c values were used for calculation of recoverable energy density (W R ) and total energy density (W T ) by using Eqs. (6) and (7), respectively. www.nature.com/scientificreports/ where, P max is a maximum polarization and P r is the remanence polarization. The efficiency of energy density calculated by using Eq. (8) where 'η' represents efficiency of energy density and the energy loss density (W L ) obtained by W L = W T + W R . The enhanced energy storage density was observed for BFO-CFO samples, which may be due to the improved dielectric breakdown strength E b , compared with pure BFO. In addition, the introduction of CFO leads to reduced grain size (on microscale), which is responsible for the enhanced dielectric breakdown strength E b 30 . As reported in Table 2, the recoverable energy density increased with increasing in CFO concentration upto x = 0.10, then decreased for higher CFO concentrations of x = 0.30. Hence, the maximum recoverable energy density of 0.138 J/cm 3 and efficiency of energy density (ɳ = 68.65%) was achieved at 0.90BFO-0.10CFO sample near to MPB, which is about twenty times higher than that of pure BFO (0.0066 × 10 -1 J/cm 3 ). A comparison of ferroelectric and energy storage density properties among BFO based ceramic systems are listed in the Table 3. It can be seen that the recoverable energy storage density and efficiency values are relatively matching with reported literature 10,[31][32][33] . The maximum efficiency of energy density is observed for x = 0.10, which indicates that the higher electric field is favourable for energy storage applications. Fig. 9, magnetic properties of prepared composite powders were obtained by studying their magnetic hysteresis loops and compared with those of BFO and CFO powders. The BFO sample shows a linear M-H loop, which indicates its antiferromagnetic behaviour at room temperature due  www.nature.com/scientificreports/ to the G-type of modulated spiral spin structure with long periodicity of 62 nm in the unit cell at bulk samples 34 . CFO sample shows proper. M-H hysteresis loop, which show its ferromagnetic behaviour 35 . The enhancement of magnetization of BFO-CFO samples with increasing CFO concentration can be seen as they show a well-saturated hysteresis loop so that all the BFO-CFO samples will show ferromagnetic behaviour with a large saturation magnetization. CFO exhibits a ferromagnetic behaviour with saturation magnetization (M s ) and remnant magnetization such as (M r ) of 83.17 emu/gm and 30.03 emu/gm. The obtained huge saturation magnetization (M s ) was achieved around 139.10 emu/g for x = 0.30. These magnetization values are higher than that of BFO, CFO samples which were reported earlier as nano particle and bulk ceramic materials 36,37 . The coercivity (H c ) increased sharply with increasing CFO concentration up to x = 0.30 and these results are suggesting the suitable candidates for spintronic applications. The values of M s and M r for all compositions are obtained from the hysteresis loops and are summarised in Table 2. For all the BFO-CFO samples, the enhancement of ferromagnetism can be attributed to the highly magnetic nature of CFO phase in the composite. There are some reason for enhancement of magnetisation of (1 − x) BFO-x CFO (x = 0.05, 0.10 and 0.30) samples, which may be due to some factors such as; (i) BFO has weak magnetization, it is a canted antiferromagnet, the spin orientations of the BFO grains are changed by the strain in the grain boundaries between the BFO and CFO in the composite samples and (ii) By adding CFO ions into BFO, induce a structural distortion near MPB (from rhombohedral to tetragonal phase transition), because the spin cycloid structure becomes destroyed with the formation of homogeneous spin structure. (iii) The increase of the magnetization of the composite is caused by the magnetic interaction between two compounds, i.e. the compounds are exchange coupled. Thus, the magnetic properties of the BFO can be improved by making composite with CFO 38 . Table 3. Comparison of ferroelectric and energy storage properties of recently reported BFO based ceramics samples.

Samples
Applied electric field (kV/cm) P max (µC/cm 2 ) P r (µC/cm 2 ) Energy storage density, W rec (J/cm 3 )  Magneto-capacitance measurement. In order to explain the magneto capacitance effect of (1 − x) BFO-xCFO (CFO, x = 0.00, 0.05, 0.10 and 0.30)] ceramic samples, the variation of the capacitance measured with respect to the applied magnetic field at room temperature and compared with those of BFO and CFO samples as shown in Fig. 11. In general, it is known that all ferroelectrics which are piezoelectric and piezoelectric crystals become electrically polarized under applied stress. Hence, the mechanism of magneto electric effect due to the strain will be induced by applied external magnetic field to the multiferroic materials and it becomes strained. The strain generates a stress in the material, which is generated as an electric field on the ferroelectric domain, thus modifying the electric polarization and dielectric constant of the samples. The coexistence of the  www.nature.com/scientificreports/ ferromagnetic CFO and ferroelectric BFO phases in the present composites give rise to a magneto-electric (ME) effect. This ME effect is calculated in terms of magneto-capacitance effect by using eqn.: where C P (0) and C P (H) are the capacitance in zero and H magnetic fields, respectively. It is clear from Eq. (9) that magnetoelectric coefficient depends on dielectric constant as well as magnetic moment of the sample. In the present work, it is found that the magneto capacitance effect increases with increase in applied magnetic field to the (1-x) BFO-x CFO (CFO, x = 0.00, 0.05, 0.10and 0.30) samples. BFO shows the weak magnetic response with applied magnetic field because of its antiferromagnetic nature with poor ferroelectricity at room temperature. It has been observed that the low magnetic response with applied magnetic field for CFO sample is due to the absent of ferroelectric with high magnetic moment. However, (1-x) BFO-x CFO (x = 0.05, 0.10 and 0.30) samples show a good magnetic response with applied magnetic field. The magneto capacitance effect increases with increasing CFO concentration up to x = 0.10 at near MPB, then decreases in composites with higher concentrations of x = 0.03. This indicates an improvement of magneto capacitance effect in BFO after formation of the nanocomposite. Magneto capacitance effect in a material generally originates from both intrinsic and extrinsic effects 39 . Hence, the magneto electric coupling which is caused by the magnetostriction of the CFO phase, which in turn enhancing the ME coupling in the magnetostriction of ferromagnetic domain from CFO and ferroelectric domain from BFO composites through stress mediated magnetic-mechanical-electrical transformed at the BFO-CFO composites interface 40 . Therefore, the presence of CFO phase in the composites enhances the magneto capacitance effect and makes the CFO-BFO composites as good candidates for the spintronic applications. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.