Enhanced magnetization in multiferroic BiFe03 through structural distortion and particle size reduction

Journal of Magnetism and Magnetic Materials 483 (2019) 59–64

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Enhanced magnetization in multiferroic BiFe03 through structural distortion and particle size reduction Toshi Bagwaiyaa, Hilal A. Reshia, Poonam Khadea, Shovit Bhattacharyab, Vilas Shelkea, a b

T

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Department of Physics, Barkatullah University, Bhopal 462026, India Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

A B S T R A C T

BiFeO3 is widely known multiferroic compound with high ferroelectric polarization but low magnetization values due to antiferromagnetic ordering. We used allied strategy of particle size reduction and structural modulation to improve the magnetization. Calcium doped Bismuth Ferrite nanoparticles were synthesized by sol-gel route. The Rietveld refinement of X-Ray Diffraction patterns indicates single phase formation with distorted rhombohedral (space group R3c) structure. The average crystallite size varies from 72 nm to 27 nm. The Unit cell volume decreases with increase in Ca concentration. The structural distortion is confirmed by Raman spectra through line width broadening and shifting of phonon frequencies. X-ray photoelectron spectroscopy revealed Fe+3 oxidation state and slight shifting of Bi 4f peak indicating the substitution of Ca2+ at Bi-site. The field dependent magnetization measurements show remnant magnetization up to 0.265 emu/gm and coercivity up to 543 Oe in Ca substituted samples.

1. Introduction In the present era of technology, multifunctional properties of materials play a major role to envisage novel concepts and to design new devices. Multiferroic is one such material that shows more than one ferroic orders such as (anti)ferromagnetic, (anti)ferroelectric, (anti) ferroelectric simultaneously [1]. Multiferroics has a wide range of potential applications in the field of wave attenuation, EMI shielding, gas sensor, resistive switching and data storage devices [2–5]. Bismuth ferrite (BFO) is one of the most promising single-phase multiferroic materials. It has rhombohedral symmetry with the R3c space group [6]. It shows G-type canted antiferromagnetic ordering and a ferroelectric ordering above room temperature (Tc ∼ 1103 K and TN ∼ 643 K) [7,8]. The spins of Fe+3 ions are canted on a cycloid with the periodicity of 62 nm along the [1 1 0] axis [9,10]. The magnetism and ferroelectricity are due to the local spins and off-center structure distortion, respectively [11]. A suppression of spiral spin structure can be achieved by decreasing particle size (< 62 nm). Doping of rare earth materials (La, Nd, Sm, etc.) at the Bi+3 site can modify the spin cycloid structure, reduce leakage current and stabilize pure phase [12,13]. Recently, Yotburut et al. [14] and Sharma et al. [15] reported the effect of A-site rare-earth doping on magnetic properties of BFO. The first principle study also showed possible increase in magnetic moment through Bi site substitution [16]. The Ca substituted BFO was studied for microwave, dielectric and magnetic properties [17]. The alkaline-earth metal substitution in BFO nanoparticles reduced Neel temperature [18]. Weak

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ferromagnetic ordering induced due to A site substitution in BFO nanoparticles has been reported widely [19–23]. In spite of plethora of studies, the structure-physical properties relationship in BFO material is far from understanding. It shows significant variation in multiferroic properties in thin film, bulk and nanostructure forms. We reported the manipulation of crystal and domain structures of BFO thin films through substrate induced strain and orientations [24–26]. In bulk material, we studied magnetic behavior through interface-mediated mechanism [27–29]. Thus, the structure, strain and grain boundary interface have strongly correlated influence on the multiferroic properties of BFO compound. Therefore, the objective of the present work was to explore the combined effect of such variation on magnetic behavior of BFO compound. The divalent Ca substitution at Bi site is expected to alter the crystal and band structure to some extent. Concomitantly, the nano-structuring can impact crystallite strain and multiple grain boundary interfaces. The reduced grain size also implies termination of spin cycloid of AFM ordering. We synthesized nanostructured Bi1−xCaxFeO3 (0 ≤ x ≤ 0.3) compounds using chemical route. The phase formation and crystal structure are explored from Rietveld analysis of X-ray diffraction data. The reduction in particle size is evident from XRD data and scanning electron microscopy images. The Raman spectra confirm the structural distortion as a consequence of Ca substitution at Bi site. The aliovalent Ca2+ substitution results in Bi valence modification as revealed from X-ray Photoelectron Spectroscopy. The structural distortion, valency alteration and particle size reduction collectively influence the magnetic

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

https://doi.org/10.1016/j.jmmm.2019.03.092 Received 31 August 2018; Received in revised form 7 March 2019; Accepted 21 March 2019 Available online 22 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials 483 (2019) 59–64

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substitution [17,21]. Indeed, ABO3 type Perovskite structures are known to exhibit variety of lattice symmetries [32]. A quantitative parameter to define structural symmetry is Goldschmidt tolerance factor, t = [ < rA > + rO]/[√2(< rB > + rO)], where < rA > , < rB > are average ionic radii of A, B site respectively and rO is ionic radius of oxygen. The calculated values of ‘t’ are 0.8870, 0.8853, 0.8835 and 0.8818 for BFO, BCF10, BCF20 and BCF30 samples respectively. Since the ionic radii mismatch between Bi3+ and Ca2+ ions is very small (∼0.05 Å), the variation in tolerance factor is at third decimal place. Such small variation may impart structural distortion but may not be sufficient for structural transformation. The reported dual phase behavior [17,21] may be the attribute of additional factors like oxygen vacancies, excess crystalline strain during phase formation, etc. Fig. 2 displays the surface morphology observed through Scanning Electron Microscopy. The microstructure images show different sizes and shapes of grains homogeneously distributed all over the surface. The grains are agglomerated to make an appearance of larger grain. The average grain size for all the samples is given in Table 1. The grain size distribution histograms are given in Supporting information. The average grain size variation (30–50 nm) is similar to that determined from XRD data. The reported grain sizes for similar systems are 45–90 nm [21], 150 nm [30], 100–500 nm [17]. In chemical route synthesis, the grain growth depends on several parameters like chemical reagents, chelating agent, pH value, sintering temperature/ duration, etc. The Raman spectra of BFO and Ca-doped BFO samples are shown in Fig. 3(a). Group theory predicts 13 (4A1 + 9E) Raman-active modes in the rhombohedral (R3c) BiFeO3 at room temperature where A1 modes are polarized along the z-axis and E modes in the x-y plane [33]. Bi atoms are responsible for low wave number modes (up to 167 cm−1); Fe atoms are responsible for the modes between 152 and 262 cm−1 and the modes above 262 cm−1 indicate oxygen atoms motion [34]. The Raman spectra are deconvoluted into 10 modes for pure and 8 modes for doped samples. The peak positions obtained by deconvolution through Gaussian function are listed in Table 2. In the pure sample, four prominent peaks E−1, A1−1, A1−2, A1−3 identified at 74.01, 131.98, 168.09, 217.12 cm−1 are associated with the BieO bond. Five weak intensity peaks at 92.17, 267.17, 335.15, 536.79, 599.80 cm−1 and one low-intensity A1 mode at 466.13 cm−1 are observed. All the peaks are well matched with the previously reported data [23,35]. The intensity of all the modes is decreasing with slight shifting for Ca substitution. It indicates structural disorder and/or internal stress [36]. The suppression of A1 modes in doped nanomaterial samples may be associated to enhanced coupling of magnetic, ferroelectric and structure order parameters [37].

behavior of Ca substituted samples. 2. Experimental details Nanostructured Bi1-xCaxFeO3 with x = 0 (BFO), 0.1 (BCF10), 0.2 (BCF20), 0.3 (BCF30) were prepared by sol-gel method as reported elsewhere [3]. Bismuth nitrate penta-hydrate (Bi(NO3)3·5H2O), ferric nitrate nano-hydrate (Fe(NO3)3·9H2O), calcium carbonate (CaCo3), ethylene glycol ((CH2OH)2) were used as starting materials. The nitrate precursors dissolved in ethylene and carbonate precursor dissolved in DI water were continuously stirred at 80 °C on a hot plate. The solution turned into viscous brown gel and then into brown colored ash. The ash was ground for several hours to make a fine powder. The as-prepared powder was pressed into pellets and then sintered at 600 °C for five hours in air ambient. The phase and crystal structure investigation was carried out by using X-ray diffractometer with CuKα radiation (proto maker AXRD). The microstructure of the samples was observed using scanning electron microscopy (Nova, FESEM). The Raman spectroscopy measurement was performed on Horiba JY HR800 micro Raman set up. The valence state was investigated by X-ray photoelectron spectroscopy (XPS) with Mg Kα source and DESA-150 electron analyzer (Staib Instrument, Germany). XPSPEAK 4.1 software was used to fit the narrow scan of each element using Shirley background. A field dependent magnetization was measured at 10 K using Quantum Design vibrating sample magnetometer (VSM). 3. Results and discussion 3.1. Structure and microstructure analysis: The Rietveld refined X-ray diffraction patterns of Bi1−xCaxFeO3 (0 ≤ x ≤ 0.3) samples are shown in Fig. 1(a). All the samples are in single phase with rhombohedral R3c space group. The XRD peaks (1 0 4) and (1 1 0) around 2Θ value ∼ 31° are separated in x = 0 sample. These doublet peaks merge into a single peak in Ca substituted samples. Moreover, the diffraction peaks shift slightly towards higher 2Θ value with increasing Ca concentration. The shifting and merging of the peaks indicate structural modulation which may be attributed to Asite ionic radii mismatch and the decrease in crystallite size. The average crystallite size calculated using Williamson-Hall plot are 72 nm, 34 nm, 30 nm, 27 nm for BFO, BCF10, BCF20, BCF30 samples respectively. The reduction in crystallite size may be due to the restriction of crystal growth when Ca is substituted at Bi site. Quantitative phase analysis was done through Rietveld refinement using the FULLPROF software to check the possibility of structural transformation or the presence of other symmetries along with R3c symmetry. Although, we tried to fit doped samples with Pnma, Pbnm and double phase refinement, the samples showed appropriate fit to rhombohedral R3c space group with reasonable refinement parameters. It confirms that Bi+3 ions are replaced by Ca+2 ions in the rhombohedral structure without structural transformation. The lattice and structure parameters obtained from the refinement are listed in Table 1. The unit cell volume decreases gradually with ‘Ca’ substitution. The unit cell volume of nanostructured BFO sample is smaller than that reported previously for bulk sample [28]. Such decrease is the consequence of crystalline strain generated during nanostructure formation. A further decrease in unit cell volume is caused with Ca substitution due to smaller radius of Ca2+ ion (1.12 Å) than Bi3+ ion (1.17 Å). The volume reduction is accompanied by the decrease in FeeOeFe bond angle and slight elongation of Fe-O bond length. The goodness of fit (χ2) values below 1.23 reflects the fine quality of Rietveld analysis. The schematic representations of R3C structure based on Rietveld refined atomic positions for BFO and BCF30 samples are shown in Fig. 1b). Previous reports indicated structural distortions [30,31] or gradual structural transformation to Pnma space group with Ca

3.2. Composition and valency identification XPS measurement was performed to verify the oxidation states, elementary compositions and chemical shifting of pristine and Ca doped BFO. The fitted narrow scan spectra of Bi 4f, Fe 2p and Ca 2p lines for all the samples are shown in Fig. 3(b)–(d) respectively. Carbon C 1s peak position is used to calibrate binding energies of different XPS spectra. Characteristic peak for Bi 4f7/2 (159.6 eV), Bi 4f5/2 (164.9 eV), Fe 2p3/2 (710.3 eV), O 1s (529.6 eV and 531.1 eV) are identified in BFO sample [38]. There is a possibility of mix valance state to balance the charge neutrality created due to replacement of trivalent (Bi+3) ion by divalent (Ca+2) ion. As seen in Fig. 3(b) the main Bi 4f peaks of Ca doped BFO are accompanied by shoulder peaks around 160.4 eV and 165.7 eV. These peaks can be attributed to Bi +5 oxidation states [39]. Fig. 3(c) and (d) show XPS core level spectra of Fe 2P doublet and Ca 2p respectively. The fitted spectra indicate Fe presence in +3 oxidation state without +2 state. In Ca-doped BFO samples, the peaks located at 710.1 eV and 723.6 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, with spin-orbital splitting energy of 13.5 eV. As 60

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Fig. 1. a) X-ray Diffraction patterns for Bi1−xCaxFeO3 samples with experimental data (red circles), Rietveld refinement (black line), Bragg positions (green tick marks) and difference values (blue line); b) Schematic comparison of R3C structure generated through Rietveld refinement for BFO and BCF30 samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of Bi or creation of oxygen vacancies. The thermodynamic and kinetic conditions during synthesis protocol decide the end product. Nevertheless, the presence of Fe mixed valence and oxygen vacancies along with structural changes make the interpretation of magnetic interactions quite complicate. The usual Fe3+–O2−Fe3+ antiferromagnetic interaction with superimposed Dzyloshinsky-Moria (DM) exchange interaction will be influenced by the extra Fe2+ ions and deficient O2− ions. In our previous study with Neutron Diffraction and XPS data of BFO bulk composite samples, we have categorically refuted the possibility of Fe mixed valence and oxygen deficiency [28].

Table 1 Structural and Rietveld refinement parameters for Bi1-xCaxFeO3 samples. Parameters

BFO (x = 0)

BCF10 (x = 0.1)

BCF20 (x = 0.2)

BCF30 (x = 0.3)

a (Å) c (Å) V (Å3) Bi Fe O

5.561 13.799 369.586 (0, 0, 0.816) (0, 0, 0.091) (0.567, 0.559, 0.358) 157.56 2.012 1.15 72

5.570 13.673 367.429 (0, 0, 0.497) (0, 0, 0.221) (1.097, 0.745, 0.304)

5.546 13.578 361.759 (0, 0, 0.426) (0, 0, 0.703) (1.056, 0.607, 0.416)

5.543 13.543 360.410 (0, 0, 0.645) (0, 0, 0.041) (1.229, 0.912, 0.458)

154.8 2.019 1.23 34

149.3 2.028 1.12 30

149.0 2.030 1.16 27

51

33

30

30

Fe-O-Fe (°) Fe-O (Å) χ2 Crystallite Size (nm) Grain Size (nm)

3.3. Magnetic behavior The magnetic hysteresis (M–H) behavior of all the samples was studied at 10 K over a magnetic field range of ± 50 KOe. The bulk BFO is known to exhibit robust antiferromagnetic (AFM) ordering with very low magnetization value [35]. Comparatively, the pristine nano-BFO shows slightly higher magnetic moment with slanted M–H loop as seen in Fig. 4(a). The remnant magnetization (Mr) and coercivity (Hc) are still small. Significant ascent in entire M–H loops is seen with increasing Ca concentration. The magnified view of hysteresis loops shown in Fig. 4(b) indicates widening in loops with enhanced value of remnant magnetization and coercivity. The magnetization values do not show saturation behavior up to the maximum (50 KOe) applied magnetic field. The observed values of maximum magnetization (M50), remnant magnetization (Mr) and coercivity (Hc) for all samples are given in

compared to BFO, Fe 2P peaks in Ca-doped samples shifted towards lower binding energy by 0.2 eV. Satellite peak at718.4 eV confirms the + 3 oxidation state of Fe element. Ca 2p spectra are deconvoluted into two peaks of 2P3/2 ∼ 347.2 eV and 2P1/2 ∼ 350.6 eV. These findings are different from the previous report [21] in which, instead of mixed valency of Bi, mixed valency of Fe and huge oxygen deficiency was reported. The substitution of divalent Ca at trivalent Bi site can create charge imbalance. It may be balanced by higher valence 61

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Fig. 2. SEM images of a) BFO, b) BCF10, c) BCF20, and d) BCF30 samples indicated agglomerated grains. The average grain size was determined using ImageJ software.

Fig. 3. a) Raman spectra with deconvolution lines and XPS Core level- b) Bi 4f, c) Fe 2p, d) Ca 2p spectra with XPSPEAK 4.1 fitted lines for Bi1-xCaxFeO3 samples. 62

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magnetization 0.265 emu/gm and coercivity 543 Oe indicating weakly ferromagnetic ordering in otherwise antiferromagnetic BFO system. The previously reported maximum values of remnant magnetization for similar system are around 0.09 emu/gm [21], 0.035 emu/gm [17] and 0.153 emu/gm [31]. These research papers discussed various factors like Fe mixed valence, oxygen vacancies, structural modulations, spin cycloid, etc. to account for the improvement in remnant magnetization. It is clearly understood that the localized magnetic moments Si and Sj interact with each other through the interaction energy Eij = dij (Si × Sj), where dij is DM vector [28]. The energy minimization criterion imparts canting of magnetic moments. Moreover, the interaction between neighboring Fe3+ ions through oxygen 2p orbitals is in accordance with Goodenough-Kanamori-Anderson (GKA) rules [28]. Since FeeOeFe bond angle determines the orbital overlap between Fe and O, the structural distortions play vital role in magnetization behavior. In our nanoparticles system, there is combination of several factors behind enhancement of magnetization. (i) The substitution of small ionic radius ion at the Bi site results in the tilting of the FeO6 octahedron and reduction in Fe-O-Fe bond angle. (ii) The helical spin order is suppressed due to the reduction in particle size. (iii) Uncompensated spins at the surface of nanoparticles, due to large surface to volume ratio or reduce particle size contribute to the net magnetization (iv) Lattice strain may increase the spin canting [40]. The tilting of the octahedral and reduction of FeeOeFe bond angle from 157.5° to 149° is evident from Rietveld analysis of XRD data also. The change in angle leads to increased superexchange between the two antiferromagnetically aligned Fe+3 ion and decreased anti-symmetric magnetic interaction (D-M interaction) that suppress the spiral spin structure [31]. The particles size observed in all the doped samples is in the range of 35–40 nm. The modulation in the cycloidal spin structure occurs due to the decrease in particle size or large surface to volume ratio [37,41].

Table 2 Raman Active modes for Bi1−xCaxFeO3 samples. Modes (cm−1)

BFO

BCF10

BCF20

BCF30

A1 A2 A3 A4 E E E E E E E E E

131.98 168.09 217.12 466.13 74.012 92.17 – 267.17 – 335.15 536.79 599.80 –

112.79 140.36 173.97 422.10 76.91 91.98 209.08

124.91 163.02 222.79 491.78 75.92 92.75 –

– – 236.19 490 79.17 95.89 –

291.76 – 525.22

298.43 –

638.39

644.05

285.78 359.42 540.45 – 650.12

4. Conclusion We synthesized nanostructured Bi1−xCaxFeO3 (0 ≤ x ≤ 0.3) samples through sol-gel technique. Rietveld refinement of the XRD data reveals single phase nature in rhombohedral symmetry with the R3c space group. The unit cell volume and FeeOeFe bond angle are decreased significantly with Ca concentration. Reduction in particle size is confirmed from XRD data as well as SEM images. The Raman spectra show decrease in peak intensity and shifting towards lower wave number demonstrating structural distortion. XPS spectra indicate that the charge is compensated by formation of Bi+5 ions in Ca doped sample and Fe+3 valence remain unaffected. The nanostructured Ca doped BFO samples exhibit remnant magnetization values up to 0.265 emu/gm as a result of combined effect of structural distortion and particle size reduction. Acknowledgments Fig. 4. a) Magnetization (M–H) behavior of Bi1−xCaxFeO3 samples measured at 10 K, b) magnified view of hysteresis loops.

The authors are thankful to Department of Atomic Energy, Board of Research in Nuclear Sciences, Mumbai for financial support. A part of the work was carried out at UCG-DAE Consortium for Scientific Research, Indore. We acknowledge Drs. Alok Banerjee, Vasant Sathe and Mukul Gupta for providing characterization facilities.

Table 3 Observed magnetic parameters for BFO samples. Sample

M50 (emu/gm)

Mr (emu/gm)

Hc (Oe)

BFO BCF10 BCF20 BCF30

0.52 1.064 1.27 1.56

0.088 0.167 0.228 0.265

163 463 479 543

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.03.092. References

Table 3. There is clear ascending trend in all these parameters with increasing Ca concentration. The sample BCF30 shows remnant

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