Influence of Fe substitution on structural and magnetic

Journal of Magnetism and Magnetic Materials 452 (2018) 120–128

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Research articles

Influence of Fe substitution on structural and magnetic features of BiMn2O5 nanostructures Vishwajit M. Gaikwad a, Saveena Goyal a, Premakumar Yanda b, A. Sundaresan b, Suvankar Chakraverty a,⇑, Ashok K. Ganguli a,c,⇑ a b c

Institute of Nano Science & Technology, Habitat Centre, Sector – 64, Phase X, Mohali, Punjab 160062, India Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru, Karnataka 560064, India Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India

a r t i c l e

i n f o

Article history: Received 26 September 2017 Received in revised form 17 November 2017 Accepted 24 November 2017 Available online 6 December 2017 Keywords: Magnetic Structural Nanostructure Frustrated

a b s t r a c t Nanostructures of complex oxides [BiFexMn2xO5 (x = 0, 1, 2)] have been designed to study their structural, optical and magnetic behaviour. X-ray diffraction data (XRD) revealed orthorhombic phase with Pbam space group. Noticeable expansion in unit cell parameters has been found from BiMn2O5 (x = 0) to BiFe2O4.5 (x = 2). The observed structural changes via tuning of B-site (x = 0–2) played an important role in overall magnetic properties. Transmission electron microscopic images confirm that the average particle size of all the materials are in nano domain range with different morphologies. From optical studies, it has been found that the observed energy band gap values are strongly related to 3d electron numbers. These values appear to be larger than that reported for bulk. Isothermal magnetization plots (at 5 K) show increase in coercivity (Hc) from x = 0 to x = 2. Temperature dependent magnetization studies implied anti-ferromagnetic interactions for BiMn2O5, frustrated magnet for BiFeMnO5 and ferromagnetic behaviour for BiFe2O4.5. Ferromagnetic state of nanostructured BiFe2O4.5 is in contrast with its bulk counterparts. Ó 2017 Published by Elsevier B.V.

1. Introduction Significant research is currently devoted to multiferroic materials due to their fundamental aspects and potential applications (sensors, memories, transducers) [1]. Most of the known multiferroic compounds shows weak magneto-electric (ME) output [2,3]. Continuous efforts has been given to search out new class of multiferroics in which multiferroicity/ferroelectricity can be induced by charge ordering that is required for high ME coefficient [3–5]. Recent studies demonstrated that RMn2O5 (R = rare earth, Y or Bi) based oxides are most promising for magnetic and electrical properties. RMn2O5 are type II multiferroics, in which ferroelectricity occurs only in magnetically ordered state [6]. In RMn2O5, Mn has mixed valence state (+3, +4) which decides the magnetic behaviour via superexchange interaction through oxygen [7,8]. Among RMn2O5 family, BiMn2O5 (BMO) is capable of showing significant polarization due to presence of highly polarisable Bi3+ ions [9]. BMO crystallizes in orthorhombic structure (space group: Pbam). It shows commensurate antiferromagnetic (AFM) ordering below ⇑ Corresponding authors at: Institute of Nano Science & Technology, Habitat Centre, Sector – 64, Phase X, Mohali, Punjab 160062, India (A.K. Ganguli). E-mail address: [email protected] (A.K. Ganguli). https://doi.org/10.1016/j.jmmm.2017.11.101 0304-8853/Ó 2017 Published by Elsevier B.V.

40 K [10–13]. In BMO, Mn3+ and Mn4+ are coupled through AFM superexchange interactions, but due to odd number of spins, all spins cannot be compensated leading to uncompensated magnetic structure [10]. The magnetic structure of BMO can be tuned by replacing Mn3+ and Mn4+ by Fe3+. It is interesting to study the structural and magnetic aspects when Mn3+ sites are partially or fully substituted by Fe3+, since such substitution might induce ferromagnetism in the system [2]. In the present work, we have designed Mn-rich (BiMn2O5), Fe-rich (BiFe2O4.5), and Fe-Mn (BiFeMnO5) materials at nanoscale and investigated the influence of size as well as atomic substitution on their structural, optical and magnetic properties.

2. Experimental details Complex metal oxides can be synthesized by several routes [1,14– 18]. Nanoparticles of these oxides can be synthesized also by low temperature route and details may be found elsewhere [19]. Nanostructured complex oxide systems [BiFexMn2xO5 (x = 0, 1, 2)] were prepared by co-precipitation route. For preparation of BiMn2O5 (BMO) nanostructures; 0.1 M bismuth nitrate [Bi(NO3)35H2O, Aldrich, 99.999%], and 0.2 M manganese chloride [MnCl24H2O,

V.M. Gaikwad et al. / Journal of Magnetism and Magnetic Materials 452 (2018) 120–128

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Himedia, 97%] were dissolved in 2-methoxyethanol (TCI, 99.0%) . The solution was stirred at 80 °C for 2 h. pH of solution was maintained at 10.5 by drop-wise addition of aqueous ammonia [NH4OH, Merck]. Resultant dark brown colour solution was centrifuged to obtain a precipitate. Thereafter, the precipitate was mixed with absolute ethanol and further reaction was carried out in ethanol medium at room temperature for 24 h. After completion of reaction, precipitate was separated by centrifugation and washed several times (5–6 times) with double distilled water in order to remove water soluble by-products. Afterwards, resulting precipitate was dried at 90 °C in oven. The final dark brown powder was calcined at 600 °C to obtain pure crystalline phase of BiMn2O5 (BMO). Similar methodology was opted for the synthesis of BiFeMnO5 (BFMO) and BiFe2O4.5 (BFO). X-ray powder diffraction of samples were performed using a Bruker D8 Advance X-ray diffractometer equipped with a copper target (CuKa 1.5406 Å). Diffraction data was collected in the range 10°–90° with scanning rate of 0.0206° per step. The raw data was refined using Rietveld refinement method with the help of Full Prof suite. Room temperature Fourier Transform Infrared Spectroscopic studies were carried out by Bruker Cary 600 series spectrometer from Agilent technologies. FTIR measurements were taken in IR range from 400 to 1000 cm1. Optical behaviour of samples was checked by UV–Visible Diffuse Reflectance (UV–Vis DRS) spectra performed using a Shimadzu UV-2600 spectrophotometer. Morphology of prepared samples were recorded using Transmission Electron Microscope (JEOL JEM-2100) at an accelerating voltage of 200 kV. Field and temperature dependent magnetization measurements were performed using Quantum Design’s Superconducting Quantum Interference Device (SQUID) magnetometer. Magnetic field sweeping rate was kept 100 Oe/s for field dependent magnetic measurements.

3. Results and discussion Raw X-ray diffraction patterns of BMO, BFMO and BFO were refined with Rietveld method using initial structural model [20]. Rietveld refined XRD data (Fig. 1) indicate single phase orthorhombic (Pbam) structure. Refined XRD patterns are indexed with orthorhombic phase which shows good agreement with standard powder diffraction files; PDF-00-027-0048 (for BMO and BFMO) and PDF-00-025-0090 (for BFO). Using Scherrer formula, average crystallite sizes were calculated as 40 ± 3, 34 ± 4, and 24 ± 2 nm for BMO, BFMO and BFO, respectively. Refined structural parameters are tabulated in Table 1. Along ‘a’ and ‘c’ – crystallographic axes, increase in lattice cell constants are found with respect to Fe-substitution. The order of increase in cell volume is represented as; BMO [370.66(4) Å3] < BFMO [378.85(5) Å3] < BFO [405.24(5) Å3]. This increase in cell volume is attributed [21,22] to anti-site disorder and decrease in Jahn-Teller distortion of Mn3+. The distortion due to Jahn-Teller cation (Mn3+) decreases with Fe substitution from BMO to BFO. The structural (unit cell) models are illustrated using Visualization of Electronic and Structural Analysis (VESTA) software [23,24] [inset of Fig. 1]. BMO crystallizes in orthorhombic system (space group: Pbam) in which Mn atoms have two different surroundings. Mn4+ occupy 4f site whereas Mn3+ occupy 4 h site. Mn4+ are co-ordinated to six oxygen atoms forming Mn4+ O6 octahedra whereas Mn3+ are bonded to five oxygen atoms forming Mn3+ O5 square pyramids. These octahedra are interconnected by sharing edges via O3 atoms and form infinite chains along c-axis. Two square pyramids (Mn3+ O5) linked via O1 atoms may be viewed as Mn2O10 dimer unit. These pyramidal dimer units are interconnected to Mn4+ O6 octahedra through O2 and O3 atoms. In case of BFMO, Fe3+ ions prefer to occupy Mn3+ pyramidal sites leading to contraction of the square pyramidal units, due to shorter bond lengths of Fe-O1 and Fe-O3 bonds as compared to Mn3+-O1/

Fig. 1. Rietveld refinement of X-ray diffraction patterns of (a) BiMn2O5 (BMO) (b) BiFeMnO5 (c) BiFe2O4.5 with inset showing unit cell representation.

O3 bonds (Table 1). The changes in bond distances observed within square pyramids cannot be correlated to effective ionic radii, because Mn3+ and Fe3+ have same effective ionic radii (0.58 Å) in view of 5-fold coordination [25]. However, the contraction in pyramidal units can be correlated to Jahn Teller character of Mn3+, due to which, Mn3+O5 pyramids have longer axial bond lengths [Mn2-O1 1.74(1) Å, Mn2-O2 1.95(3) Å] as compared to Fe3+O5 [Fe-O1 1.70(1) Å, Fe-O4 1.85(1) Å]. The overall expansion in unit cell from BMO (x = 0) to BFO (x = 2) [BiFexMn2xO5 (x = 0, 1, 2)] is attributed to the large expansion in

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Table 1 Structural parameters extracted from X-ray diffraction (XRD) studies. Sample

Lattice parameters (Å)

Cell volume (Å3)

GOF (v2)

Crystallite size (nm)

Bond distances (Å)

Bond angles (°)

BMO

a = 7.5440(4) b = 8.5320(5) c = 5.7584(3) a = b = c = 90°

370.66 (4)

3.04

40 ± 3

Mn1-O2(x2)1.82(5) Mn1-O3(x2)1.97(3) Mn1-O4(x2)1.89(1) Mn2-O1(x2)1.74(1) Mn2-O2(x2)1.95(3) Mn2-O3(x1)1.78(4)

Mn1-O(3)-Mn2140.7(3) Mn1-O(2)-Mn2118.8(4) Mn2-O(1)-Mn2122.0(6) Mn1-O(3)-Mn175.0(5)

BFMO

a = 7.6408(6) b = 8.5069(5) c = 5.8294(4) a = b = c = 90°

378.90 (5)

2.17

34 ± 4

Mn-O2(x2)2.18(3) Mn-O3(x2)2.59(5) Mn-O4(x2)1.89(1) Fe-O1(x2)1.70(1) Fe-O3(x1)1.40(4) Fe-O4(x2)1.85(1)

Mn-O(3)-Mn54.4(3) Fe-O(1)-Fe131.2(6) Mn-O(3)-Fe111.8(1) Mn-O(4)-Fe127.6(5)

BFO

a = 7.9555(1) b = 8.4709(1) c = 6.0133(9) a = b = c = 90°

405.24 (1)

1.28

24 ± 2

Fe1-O2(x2)2.42(1) Fe1-O4(x2)2.27(3) Fe1-O5(x2)1.96(8) Fe2-O4(x2)1.67(2) Fe2-O1(x1)1.83(7) Fe2-O3(x1)1.76(3) Fe2-O5(x1)1.54(6)

Fe1-O(5)-Fe2130.1(2) Fe1-O(4)-Fe2120.1(8) Fe1-O(5)-Fe193.4(3)

octahedral units. In BFMO, with the introduction of Fe3+, expansion is found in Mn4+O6 octahedral units. Average bond distances increase from [Mn1-O2 1.82(5)Å, Mn1-O3 1.97(3)Å: BMO] to [Mn1-O2 2.18(3) Å, Mn1-O3 2.59(5) Å: BFMO] within Mn4+O6 octahedra. This noteworthy increase is due to the fact that some Fe3+ are introduced at Mn4+O6 octahedral sites. Inclusion of cations with higher ionic radii (Fe3+ 0.645 Å) than (Mn4+ 0.53 Å) in view of six fold coordination [25] is responsible for large octahedral expansion, hence the increase in unit cell. In case of BFO, octahedral units only encloses Fe3+ (Fe3+O6) gives more pronounced expansion in octahedra (Table 1). The order of increase in octahedral expansion is from x = 0 (BMO) to x = 2 (BFO). Thus, the overall expansion in unit cell is found from BMO (x = 0) to BFO (x = 2). All notable changes in bond distances and angles (from x = 0 to 2) are summarized in Table 1. There is no significant change of bond distances compared to bulk samples. To probe the local structural distortion, we have studied Fourier Transform Infrared (FTIR) Spectra of BMO, BFMO and BFO. Fig. 2 shows FTIR spectra recorded for (a) BMO, (b) BFMO, and (c) BFO at room temperature. BMO crystallizes orthorhombic structure with space group Pbam (D92h ). According to group theory analysis, BMO should have 48 Raman and 36 Infrared (IR) active modes [26]. 36 IR active modes are distributed as CIR ¼ 8B1u þ 14B2u þ 14B3u ; 9Au are silent modes; and three B1u þ B2u þ B3u are acoustic modes [26]. For BMO, we observed 7 IR active modes in the range of 400–1000 cm1.The number of observed modes is always lower than group theoretical predictions, since large number of modes of different irreducible representations cannot be separated within the resolution of the FTIR spectrometer. Modes are observed between 550 and 650 cm1 corresponding to Mn-O stretching vibrations within Mn4+O6 octahedra in ab plane [Fig. 2(a)]. Two low intense modes appear at 417 and 475 cm1 attributed to Mn-O bending vibrations of Mn3+O5 square pyramids [26,27]. For BFMO, Mn-O stretching modes are flattened and broader with the introduction of Fe3+ [Fig. 2(b)]. This is the signature of expansion of distorted Mn4+O6 octahedra. Mn-O bending mode which is observed at 417 cm1 (for BMO) [Fig. 2(a)], vanishes completely (for BFMO) [Fig. 2(b)]. It indicates extremely low occupation of Fe3+ at octahedral unit, whereas fair occupation of Fe3+ is observed at pyramidal/tetrahedral unit (as specified by Fe-O stretching mode at 820 cm–1). BFMO shows higher wave No. shift (Dm 10 cm1) for Fe-O stretching mode with respect to BFO (810 cm1) [Fig. 2 (c)] [28–30]. The shift is caused by anti site disorder of Fe3+ result-

ing into local distortion at pyramidal/tetrahedral site. FTIR spectra of BFO [Fig. 2(c)] possesses an intense mode at 810 cm1 suggesting dominant Fe-O stretching vibrations at pyramidal/tetrahedral units [30]. Due to nonexistence of Mn sites, Mn-O stretching and bending modes are absent, instead of them we observed Fe-O stretching and Fe-O-Fe bending modes due to internal vibrations of Fe3+O6 octahedra (500–650 cm1) [Fig. 2(c)]. Thus, we found that FTIR modes are very sensitive to B-site variations [BiFexMn2xO5 (for x = 0, 1, 2)]. Possible structural changes has been observed and discussed via FTIR study [from BMO (x = 0) to BFO (x = 2)]. For nanostructured BMO, BFMO and BFO materials systems, Bsites (Mn, Fe/Mn, Fe) are totally responsible for deciding magnetic behaviour of sample. The observed structural changes via tuning of B-site (x = 0–2) might be very effective to modify their magnetic characteristics. To compare the optical behaviour of prepared nano structures (BMO, BFMO, BFO), UV–visible diffuse reflectance (UV–Vis DRS) spectra were studied. UV–Vis DRS is commonly used to determining the optical band gap. Spectra are plotted in terms of KubelkaMunk (K-M) function F(R) versus photon energy (E) [Fig. 3(a)]. F(R) is used to estimate optical absorption from reflectance (R) 2 using formula [31]; FðRÞ ¼ ð1RÞ . Energy band gap values are esti2R mated using Tauc relation [31]; F(R)*hm = B(hm Eg)n, where, h is Planck constant, m is frequency of light, F(R) is K-M function, n = 1/2 for direct transition, Eg is energy band gap. Optical direct band gaps (Eg) are extracted by extrapolating E versus [hm*F(R)]2 graph. Average band gap values are obtained as; 1.61, 1.60 and 2.11 eV for BMO, BFMO and BFO, respectively. The observed band gap values are higher than reported values for bulk systems [3,29,32–34]. BMO spectra [Fig. 3(a)] shows strong absorption edge near 770 nm (1.61 eV) which corresponds to p-d charge transfer (CT) transition (O 2p ? Mn 3d) in octahedral (Mn4+ O6) centres of BMO [35,36]. This CT transition occurs near 775 nm (1.60 eV) for BFMO. Inclusion of Fe3+ at Mn3+ site is responsible for slight shifting absorption edge from 770 to 775 nm (1.60 eV). Two absorption edges [587 nm (2.11 eV) and 810 nm (1.53 eV)] are observed for BFO, since BFO is multiband semiconductor [18]. First absorption edge (587 nm) correlated to n-p or p-p electronic transitions whereas second absorption edge (810 nm) assigned to d-d electronic transitions of Fe3+ as shown in earlier reports [18,34,37]. From Fig. 3(b), it may be seen that energy band gap values for BMO (Eg 1.61 eV) and BFO (Eg 2.11 eV) systems are found to be higher than for their bulk counterparts. Higher band


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Fig. 2. Room temperature FTIR spectra of (a) BiMn2O5 (BMO) (b) BiFeMnO5 (BFMO) (c) BiFe2O4.5 (BFO).

Fig. 3. UV-DRS spectra of (a) BiMn2O5 (BMO), BiFeMnO5 (BFMO), BiFe2O5±d (BFO) for the estimation of Eg (b) Variation of Eg with 3d electron number and their comparison with bulk values.

gap values ascribed to smaller particle size [BMO 40 ± 3 nm (Eg 1.61 eV), BFO 24 ± 2 nm (Eg 2.11 eV)]. In bulk, a large number of atoms are involved, whose orbitals overlap with each other. The energy bands of bulk are formed by the merger of bunch of

energy levels of the overlapping atomic orbitals. With decreasing particle size and hence, decreasing number of atoms, width of energy bands decreases due to reduction of energy levels forming the energy bands, resulting the increase in band gap for nanoparticles [38–41]. 3d electron numbers are calculated by considering average number of 3d electrons at B site. For BMO; Mn3+ (3d4), Mn4+ (3d3), 3d electron number = 3.5, for BFMO; Mn4+ (3d3), Fe3+ (3d5), 3d electron number = 4, for BFO; [Fe3+ (3d5), Fe3+ (3d5), 3d electron number = 5]. Oscillator strength of charge transfer transition steadily decreases as the 3d-electron number increases as unoccupied 3d states decreases [42], but well known exception is Fe (3d5). Fe has larger CT energy gap than Mn though 3d electron number for Fe is greater. This is because of stable 3dn (n = 5) configuration via exchange interaction for Fe-system [42]. Thus, experimentally observed energy band gap has been proved to be a function of 3d electron number as well as particle size [38,42,43]. Transmission electron microscopic (TEM) images of calcined samples of (a) BMO, (b) BFMO and (c) BFO are displayed in Fig. 4. BMO [Fig. 4(a)] shows rectangular cuboid like nanocrystals with average size around 60±4 nm which is approximate with crystallite size (40 ± 3 nm) obtained from XRD. Regular lattice fringes (shown by yellow mark) within cuboid with interplanar d-spacing of 0.32 nm which corresponds to (1 2 1) lattice plane of orthorhombic phase of BMO. BFMO illustrates spherical morphology with an average particle size around 50 ± 6 nm [34 ± 4 nm obtained by XRD]. Smaller non uniform crystals of BFO are shown in Fig. 4(c). Average particle size for BFO is found to be around 15 ± 4 nm [24 ± 2 nm (XRD)]. Crystalline domain shows interplanar spacing of 0.33 nm which is correlated to (1 2 1) Miller plane of BFO. Although the synthesis route was same for all three systems (BMO, BFMO, BFO), we got different morphology (rectangular cuboid for BMO, spheres for BFMO). This is due to growth kinetics involved with varying compositions during crystallization. Average particle size obtained from TEM analysis shows good agreement with crystallite sizes obtained from X-ray diffraction (XRD).

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Fig. 4. Particle size determination using transmission electron microscope (TEM) (a) BMO (b) BFMO and (c) BFO.

Isothermal magnetization curves at 5 K for BiFexMn2xO5 (x = 0, 1, 2) are shown in Fig. 5(a). Linear dependence of magnetization with no hysteresis on applied magnetic field is observed for BMO and can be attributed to anti-ferromagnetic (AFM) nature of BMO sample which is consistent with earlier reports [7,10,44]. In BMO, Mn3+ and Mn4+ are coupled through AFM superexchange interactions which generates global anti-ferromagnetic structure. However, hysteresis with coercivity is observed for Fe-substituted sample (BFMO) [Fig. 5(a)]. It indicates the presence of some weak ferromagnetic components in the sample [2,21]. Fe3+ substitution preferentially occurs at pyramidal site. Fe3+ moments in pyramidal dimer units (Fe2O10) are ferromagnetically coupled, which is responsible for weak ferromagnetism. But, these weak ferromagnetic interactions cannot be overcome by anti-ferromagnetically coupled Mn4+-O2-Fe3+ sublattices [45], hence, it does not allow the saturation of magnetization. Thus, BFMO shows dominant anti-ferromagnetic interactions along with weak ferromagnetic

components. BFO exhibits small coercivity representing weak ferromagnetic behaviour [Fig. 5(a)]. Basically, bulk BFO has antiferromagnetic character [46,47]. In present system of nanosize BFO (average particle size 24 nm), the surface to volume ratio increases due to nanosize effect. This leads to formation of uncompensated canted spins on the surface of the nanoparticles that gives rise to weak ferromagnetism as compared to the antiferromagnetism in its bulk counterparts [38,48]. We observed systematic increase in coercivity values with the introduction of Fe3+ at B-lattice site. The order coercivity is; BMO (200 Oe) < BFMO (1006 Oe) < BFO (1309 Oe). Fe3+ substitution increases the strength of Mn4+-O-Fe3+-O-Mn4+ interaction (for BFMO). Even more stronger interactions (Fe3+-O-Fe3+-O-Fe3+) occur in fully substituted Fe-system (BFO). Hence, BFO achieves larger coercivity than BFMO and BMO [inset of Fig. 5(a)]. The order of strength of interaction is; Mn4+-O-Mn3+-O-Mn4+ < Mn4+-O-Fe3+-O-Mn4+ < Fe3+-O-Fe3+-O-Fe3+. As the strength of interaction increases, higher field is required for


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the stability and hence coercivity. We have samples with typical particle size 30 nm. This is reasonably large enough size to slower down the relaxation considerably and hence increase the stability/coercivity [50–54].The observed effective magnetic moments are close to bulk values [Fig. 5(c)]; however, for BFO, meff achieved lower value than bulk [46,55]. This prominent difference may be originating from finite size effect due to which surface canting of spins exists and leads to lowering of overall meff. Fig. 6 shows temperature dependent magnetization studies (from 2 to 390 K) carried out on BMO, BFMO and BFO samples at 100 Oe in zero field cooled (ZFC) and field cooled (FC) condition. BMO shows ZFC-FC bifurcation appearing at 269 K which might be due to spin canting at surface [Fig. 6(a)]. ZFC shows peak around 41 K corresponding to antiferromagnetic ordering temperature (TN) which agrees well with earlier reports [1,11,56]. Rise in FC magnetization below TN is attributed to spin canting due to surface effect at low temperatures. In case of Fe substituted sample (BFMO), bifurcation is seen at 345 K (slightly above room temperature), both FC and ZFC magnetization increases with decrease in

Fig. 5. (a) Isothermal magnetization curves at 5 K for BMO, BFMO and BFO with inset showing magnified plots to point out difference in coercivities (b) Variation of Hc and meff with 3d electron number and their comparison with bulk.

de-alignment of spins in antiferromagnetic (Mn4+-O-Fe3+-O-Mn4+) and ferromagnetic (Fe3+-O-Fe3+) chains which lead to high coercivity value. Coercivity decides the strength of magnetic field for overcoming anisotropy to flip the magnetic moments [49]. From Fig. 5 (b), it has been observed that coercivity increases with 3d-electron number. The probable reason is that coupling at magnetic site is stronger for 3d5 system (Fe3+-Fe3+; BFO) as compared to 3d4 system (Mn4+-Fe3+; BFMO) and 3d3.5 (Mn4+-Mn3+). Another possible cause is surface anisotropy. Coercivity of nanostructured materials has also a contribution from surface anisotropy. It is important to note that coercivity of present nanostructured samples are significantly higher in comparison to the bulk reported values, this suggests significant higher magnetic stability for nanosize samples [20,21,44,48] [Fig. 5(b)]. Magnetic materials undergo magnetic multi domain to single domain state with decreasing particle size. Single domain nanoparticle follows Neel relaxation (s), that depends on particle size (V) and anisotropy energy per unit volume (K),

KV

s ¼ s0 ekB T . Slower the relaxation of the nanomagnet higher is

Fig. 6. Temperature dependent magnetization plots (a) BMO (b) BFMO, and (c) BFO at 100 Oe.

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temperature. ZFC shows maximum near TN 34 K which is indicative of magnetic transition. Nature of temperature dependent magnetization plots suggest weak ferromagnetic moments in BFMO. BFO shows ZFC-FC splitting at 390 K. The observed magnetic transition temperature (TN 180 K) is lower than usual (260 K) [28,47,57]. To evaluate effective magnetic moment, Curie-Weiss law [v = C/(T-h)] is applied to the paramagnetic region. Weiss temperature (h) and Curie constant (C) are extracted out by extrapolating the paramagnetic region of susceptibility inverse (v1) versus temperature (T) curves [inset of Fig. 7]. Weiss temperatures are obtained as 70, 150, and 200 K for BMO, BFMO and BFO, respectively, whereas TN are 41 K, 34 K and 180 K for BMO, BFMO and BFO, respectively (see Fig. 7). This suggests a transition from pure antiferromagnetic to weak ferromagnetic. It implies normal AFM interactions for BMO (h 70 K TN), frustrated AFM (h 150 K TN) for BFMO and weak FM for BFO (h 200 K) [58]. Experimentally observed effective magnetic moments are meff (obs) 6.46 mB (for BMO), 7.35 mB (for BFMO) and 4.93 mB (for BFO). Observed values are found closer to theoretical values [meff 6.24 mB (BMO) calculated using formula: meff = [meff (Mn4+)2 + meff (Mn3+)2]1/2; meff 7.07 mB (BFMO) calculated from : meff = [meff (Mn4+)2 + meff (Fe3+)2]1/2; meff 5.92 mB (BFO) [Fe3+ high spin value]. The difference between experimental and theoretical values might be the consequence of distribution of magnetic ions affected by anti-site disorder and finite size effect [2]. ZFC-FC magnetization values for Fe- substituted systems

(BFMO, BFO) are lower than BMO. According to GoodenoughKanamori-Anderson (GKA) rules [59–61]; 90° angle of cation (d5)-anion-cation (d5) gives FM interaction whereas 90° angle of cation (d3)-anion-cation (d5) suggests dominant AFM ordering. In case of BFMO, Fe3+(d5)-O-Fe3+(d5) 131.2(6)° (Table 1) resulting into FM short range ordering below transition. Mn4+-O-Mn4+ 54.4(3)° (Table 1) leading to weaker AFM coupling. Subsequently, AFM contribution is from Mn4+(d3)-O-Fe3+(d5)-O-Mn4+(d3) interactions is dominant over Fe3+-O-Fe3+ and Mn4+-O-Mn4+ interactions. Thus, Fe-substituted (BFMO) sample has less ZFC-FC magnetization than BMO. Whereas, in case of BFO, Fe3+(1)-O-Fe3+(1) 93.4(2)°, Fe3+(2)-O-Fe3+(2) 130.1(2)° and 120.1(8)° [Table 1] leading to weak FM interaction. Magnetic interaction through Fe3+-O-Fe3+O-Fe3+ chains contribute to FM interactions. As a result of this, below transition temperature, FC magnetization of BFO is found to be higher than BFMO. Fig. 7 illustrates the variation of TN and h with 3d electron number for BiFexMn2xO5 (x = 0, 1, 2). With increase in 3d electron number TN slightly decreases from 41 to 34 K for BFMO (3d4) and then rises for BFO (3d5) [TN 180 K]. Initial decrease is the consequence of frustrated anti-ferromagnetic order observed in BFMO. However, higher TN is usually observed for Fe- based (3d5) systems [18,24,32,46,62]. Complementary nature of magnetism is observed for BFO in comparison with bulk. Basically, bulk BFO is purely antiferromagnetic with Weiss temperature [46,55] (h 1468 K), whereas the present case nanosized BFO is showing weak ferromagnetism (h 200 K).

Fig. 7. Variation of magnetic parameters (TN and h) with 3d electron number and their comparison with bulk values. Inset shows inverse of susceptibility versus T plot.


4. Conclusion In summary, single phase nanostructured oxides [BiFexMn2xO5 (x = 0, 1, 2)] were designed via simple co-precipitation route. Overall expansion in unit cell was found from BMO (x = 0) to BFO (x = 2). The band gap increases with increasing number of d-electron in the system. The band gap estimated for these nanostructures appear to be larger than that reported for their bulk. In contrast to bulk, an antiferro to ferromagnetic transition through frustrated magnetic state is observed as we go from BMO to BFO through BFMO. Magnetic stability of these nanostructures enhanced in comparison to bulk materials. The observed structural changes via tuning of B-site (x = 0–2) were correlated with magnetic features. Magnetic stability measured in terms of coercivity of these nanostructures have been increased in comparison to their bulk phase. This higher magnetic stability as well as appearance of ferromagnetic state in nanostructured BFO may pave a way to realize materials with higher functionality such as multiferroicity in nanostructured AB2O5 materials. Acknowledgments The authors acknowledge DST-SERB, India for financial support through research grant [File no. EMR/2015/001716]. VMG and SG wants to acknowledge INST, Mohali for providing financial assistance through Post Doctoral and Doctoral Fellowship, respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jmmm.2017.11.101. References [1] R.-J. Jia, J.-T. Han, X.-J. Wu, C.-L. Wu, Y.-H. Huang, W. Huang, Controllable synthesis and magnetic property of BiMn2O5 crystals, Mater. Res. Bull. 43 (2008) 1702–1708, https://doi.org/10.1016/j.materresbull.2007.07.023. [2] K.S. Kumar, C. Venkateswaran, Effect of Fe substitution on the magnetic properties of BiMn2O5, J. Phys. D. Appl. Phys. 44 (2011) 325001, https://doi.org/ 10.1088/0022-3727/44/32/325001. [3] N. Li, K. Yao, G. Gao, Z. Sun, L. Li, Charge, orbital and spin ordering in multiferroic BiMn2O5: density functional theory calculations, Phys. Chem. Chem. Phys. 13 (2011) 9418, https://doi.org/10.1039/c0cp02252g. [4] N. Das, M.A. Nath, G.S. Thakur, M. Thirumal, A.K. Ganguli, Monoclinically distorted perovskites, A2ZnTiO6 (A = Pr, Gd): Rietveld refinement, and dielectric studies, J. Solid State Chem. 229 (2015) 97–102, https://doi.org/ 10.1016/j.jssc.2015.05.003. [5] N. Das, R. Singh, A. Das, L.C. Gupta, A.K. Ganguli, Structural, magnetic and dielectric properties of a new double perovskite Pr2CoTiO6, J. Solid State Chem. 253 (2017) 355–359, https://doi.org/10.1016/j.jssc.2017.06.024. [6] D. Khomskii, Classifying multiferroics: mechanisms and effects, Physics (College. Park. Md) 220 (2009), https://doi.org/10.1103/Physics.2.20. [7] A.F. García-Flores, E. Granado, H. Martinho, R.R. Urbano, C. Rettori, E.I. Golovenchits, V.A. Sanina, S.B. Oseroff, S. Park, S.W. Cheong, Anomalous phonon shifts in the paramagnetic phase of multiferroic RMn2O5 (R = Bi, Eu, Dy): possible manifestations of unconventional magnetic correlations, Phys. Rev. B Condens. Matter Mater. Phys. 73 (2006) 3–8, https://doi.org/10.1103/ PhysRevB.73.104411. [8] T.A. Tyson, Z. Chen, M.A. DeLeon, S. Yoong, S.W. Cheong, Local structure of multiferroic RMn2O5: important role of the R site, J. Magn. Magn. Mater. 321 (2009) 1714–1718, https://doi.org/10.1016/j.jmmm.2009.02.017. [9] Y. Liu, I. Zhitomirsky, Electrochemical supercapacitor based on multiferroic BiMn2O5, J. Power Sources 284 (2015) 377–382, https://doi.org/10.1016/j. jpowsour.2015.03.050. [10] D.K. Shukla, S. Mollah, R. Kumar, P. Thakur, K.H. Chae, W.K. Choi, A. Banerjee, Effect of Ti substitution on multiferroic properties of BiMn2O5, J. Appl. Phys. 104 (2008) 1–10, https://doi.org/10.1063/1.2964072. [11] A. Muñoz, J.A. Alonso, M.T. Casais, M.J. Martínez-Lope, J.L. Martínez, M.T. Fernández-Díaz, Magnetic structure and properties of BiMn2O5 oxide: a neutron diffraction study, Phys. Rev. B 65 (2002) 144423, https://doi.org/ 10.1103/PhysRevB.65.144423. [12] K.S. Kumar, D.P. Joseph, S.P. Raja, P. Manimuthu, C. Venkateswaran, Synthesis and characterization of BiMn2O5 ceramics, AIP Conf. Proc. 1349 (2011) 1155– 1156, https://doi.org/10.1063/1.3606273.

127

[13] V.T. Santana, L. Walmsley, I. Fier, R.A. Eichel, P. Jakes, I. Chumak, A. Ozarowski, J. Van Tol, O.R. Nascimento, Antiferromagnetic resonance of polycrystalline BiMn2O5, Solid State Commun. 207 (2015) 40–43, https://doi.org/10.1016/j. ssc.2015.02.007. [14] C.N.R. Rao, J. Gopalakrishnan, K. Vidyasagar, A.K. Ganguli, A. Ramanan, L. Ganapathi, Novel metal oxides prepared by ingenious synthetic routes, J. Mater. Res. 1 (1986) 280–294, https://doi.org/10.1557/JMR.1986.0280. [15] C.N.R. Rao, J. Gopalakrishnan, Synthesis of complex metal oxides by novel routes, Acc. Chem. Res. 20 (1987) 228–235, https://doi.org/10.1021/ ar00138a004. [16] T.T. Kodas, Generation of complex metal oxides by aerosol processes: superconducting ceramic particles and films, Adv. Mater. 1 (1989) 180–192, https://doi.org/10.1002/adma.19890010602. [17] J. Zhang, R.D. Averitt, Dynamics and control in complex transition metal oxides, Annu. Rev. Mater. Res. 44 (2014) 19–43, https://doi.org/10.1146/ annurev-matsci-070813-113258. [18] Y. Li, Y. Zhang, W. Ye, J. Yu, C. Lu, L. Xia, Photo-to-current response of Bi2Fe4O9 nanocrystals synthesized through a chemical co-precipitation process, New J. Chem. 36 (2012) 1297, https://doi.org/10.1039/c2nj40039a. [19] A.K. Ganguli, A. Ganguly, S. Vaidya, Microemulsion-based synthesis of nanocrystalline materials, Chem. Soc. Rev. 39 (2010) 474–485, https://doi. org/10.1039/b814613f. [20] K. Liu, D. Sallagoity, M. Josse, O. Toulemonde, On the anomalous magnetic behavior and the multiferroic properties in BiMn2O5, Inorg. Chem. 52 (2013) 7853–7861, https://doi.org/10.1063/1.2964072. [21] M. Retuerto, M.J. Martínez-Lope, K. Krezhov, M.T. Fernández-Díaz, J.A. Alonso, Structural and magnetic characterization of BiFexMn2–xO5 oxides (x = 0.5, 1.0), J. Solid State Chem. 184 (2011) 2428–2433, https://doi.org/10.1016/j. jssc.2011.07.009. [22] A. Muñoz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis and study of the crystallographic and magnetic structure of HoFeMnO5, Eur. J. Inorg. Chem. (2007) 1972–1979, https://doi.org/10.1002/ejic.200601144. [23] K. Fujii, H. Kato, K. Omoto, M. Yashima, J. Chen, X. Xing, Experimental visualization of the Bi–O covalency in ferroelectric bismuth ferrite (BiFeO3) by synchrotron X-ray powder diffraction analysis, Phys. Chem. Chem. Phys. 15 (2013) 6779, https://doi.org/10.1039/c3cp50236h. [24] V.M. Gaikwad, S.A. Acharya, Perovskite-spinel composite approach to modify room temperature structural, magnetic and dielectric behavior of BiFeO3, J. Alloys Compd. 695 (2017) 3689–3703, https://doi.org/10.1016/ j.jallcom.2016.11.367. [25] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A A32 (1976) 751–767, https://doi.org/10.1107/S0567739476001551. [26] F.M. Silva Júnior, C.W.A. Paschoal, R.M. Almeida, R.L. Moreira, W. Paraguassu, M.C. Castro Junior, A.P. Ayala, Z.R. Kann, M.W. Lufaso, Room-temperature vibrational properties of the BiMn2O5 mullite, Vib. Spectrosc. 66 (2013) 43–49, https://doi.org/10.1016/j.vibspec.2013.01.010. [27] S.-H.F.R. Song, H.-J. Wang, Solvothermal preparation of Mn3O4 nanoparticles and effect of temperature on particle size, Chem. Res. Chinese Univ. 28 (2012) 577, https://doi.org/10.1016/j.jcrysgro.2003.11.117. [28] M.M.M. Mangir Mursheda, Gwilherm Nénert, Manfred Burianek, Lars Robbena, Manfred Mühlbergc, Hartmut Schneider, Reinhard X. Fischer, Temperaturedependent structural studies of mullite-type Bi2Fe4O9, J. Solid State Chem. 197 (2013) 370–378, https://doi.org/10.1016/j.matchemphys.2011.03.011. [29] A. Kirsch, M.M. Murshed, M. Schowalter, A. Rosenauer, T.M. Gesing, Nanoparticle precursor into polycrystalline Bi2Fe4O9: an evolutionary investigation of structural, morphological, optical, and vibrational properties, J. Phys. Chem. C 120 (2016) 18831–18840, https://doi.org/10.1021/acs. jpcc.6b04773. [30] Y. Liu, R. Zuo, S. Qi, Surfactant-free solvothermal synthesis and optical characterization of Bi2Fe4O9 in mixed H2O/EtOH solvent, Powder Technol. 254 (2014) 30–35, https://doi.org/10.1016/j.powtec.2014.01.013. [31] Rosendo López, Ricardo Gómez, Band-gap energy estimation from diffuse reflectance measurements on sol-gel and commercial TiO2: a comparative study, J. Sol-Gel Sci. Technol. 61 (2012) 1–7, https://doi.org/10.1007/s10971011-2582-9. [32] H. Yang, J. Dai, L. Wang, Y. Lin, F. Wang, P. Kang, A novel approach to prepare Bi2Fe4O9 flower-like spheres with enhanced photocatalytic performance, Sci. Rep. 7 (2017) 768, https://doi.org/10.1038/s41598-017-00831-3. [33] J. Zhang, B. Xu, X.F. Li, K.L. Yao, Z.L. Liu, Origin of the multiferroicity in BiMn2O5 from first-principles calculations, J. Magn. Magn. Mater. 323 (2011) 1599– 1605, https://doi.org/10.1016/j.jmmm.2010.12.040. [34] Q. Zhang, W. Gong, J. Wang, Size-dependent magnetic, photoabsorbing, and photocatalytic properties of single-crystalline Bi2Fe4O9 semiconductor nanocrystals, J. Phys. Chem. C (2011) 25241–25246, https://doi.org/10.1021/ jp208750n. [35] A.S. Moskvin, R.V. Pisarev, Optical spectroscopy of charge transfer transitions in multiferroic manganites, ferrites, and related insulators, Low Temp. Phys. 36 (2010) 489–510, https://doi.org/10.1063/1.3455721. [36] S. Chakraverty, K. Yoshimatsu, Y. Kozuka, H. Kumigashira, M. Oshima, T. Makino, A. Ohtomo, M. Kawasaki, Magnetic and electronic properties of ordered double-perovskite La2VMnO6 thin film, Phys. Rev. B. 84 (2011) 132411, https://doi.org/10.1103/PhysRevB.84.132411. [37] Y. Liu, R. Zuo, Morphology and optical absorption of Bi2Fe4O9 crystals via mineralizer-assisted hydrothermal synthesis, Particuology 11 (2013) 581–587, https://doi.org/10.1016/j.partic.2012.11.002.

128


[38] A.K. Sarfraz, S.K. Hasanain, Size dependence of magnetic and optical properties of Co3O4 nanoparticles, Acta Phys. Pol. A 125 (2014) 139–144, https://doi.org/ 10.12693/APhysPolA.125.139. [39] P.A.R. Banerjee, R. Jayakrishnan, Effect of the size-induced structural transformation on the band gap in CdS nanoparticles, J. Phys. 12 (2000) 10647–10654. [40] T. Lin, N. Wan, J. Xu, L. Xu, K. Chen, Size-dependent optical properties of SnO2 nanoparticles prepared by soft chemical technique, J. Nanosci. Nanotechnol. 10 (2010) 4357–4362, https://doi.org/10.1166/jnn.2010.2203. [41] C. Binns, Introduction to Nanoscience and Nanotechnology, 2010 ISBN 978-0471-77647-5. [42] T. Arima, Y. Tokura, J.B. Torrance, Variation of optical gaps in perovskite-type 3d transition-metal oxides, Phys. Rev. B 48 (1993) 17006–17009, https://doi. org/10.1103/PhysRevB.48.17006. [43] K.-F. Lin, H.-M. Cheng, H.-C. Hsu, L.-J. Lin, W.-F. Hsieh, Band gap variation of size-controlled ZnO quantum dots synthesized by sol–gel method, Chem. Phys. Lett. 409 (2005) 208–211, https://doi.org/10.1016/j.cplett.2005.05.027. [44] D.K. Shukla, R. Kumar, S.K. Sharma, P. Thakur, R.J. Choudhary, S. Mollah, N.B. Brookes, M. Knobel, K.H. Chae, W.K. Choi, Thin film growth of multiferroic BiMn2O5 using pulsed laser ablation and its characterization, J. Phys. D. Appl. Phys. 42 (2009) 125304, https://doi.org/10.1088/0022-3727/42/12/125304. [45] A.K. Ganguli, J. Gopalkrishnan, Magnetic properties of Ca2Fe2-xMnxO5, Proc. Indian Acad. Sci. 97 (1986) 627–630. [46] T.-J. Park, G.C. Papaefthymiou, A.R. Moodenbaugh, Y. Mao, S.S. Wong, Synthesis and characterization of submicron single-crystalline Bi2Fe4O9 cubes, J. Mater. Chem. 15 (2005) 2099, https://doi.org/10.1039/b501552a. [47] M. Liu, H. Yang, Y. Lin, Y. Yang, Influence of Co doping on the magnetic properties of Bi2Fe4O9 powders, J. Mater. Sci. Mater. Electron. 25 (2014) 4949– 4953, https://doi.org/10.1007/s10854-014-2256-9. [48] Z.M. Tian, S.L. Yuan, X.L. Wang, X.F. Zheng, S.Y. Yin, C.H. Wang, L. Liu, Size effect on magnetic and ferroelectric properties in Bi2Fe4O9 multiferroic ceramics, J. Appl. Phys. 106 (2009) 1–5, https://doi.org/10.1063/1.3259392. [49] L. Ben Tahar, M. Artus, S. Ammar, L.S. Smiri, F. Herbst, M.J. Vaulay, V. Richard, J. M. Grenèche, F. Villain, F. Fiévet, Magnetic properties of CoFe1.9RE0.1O4 nanoparticles (RE = La, Ce, Nd, Sm, Eu, Gd, Tb, Ho) prepared in polyol, J. Magn. Magn. Mater. 320 (2008) 3242–3250, https://doi.org/10.1016/j. jmmm.2008.06.031. [50] C. Ma, J.Q. Yan, K.W. Dennis, R.W. McCallum, X. Tan, Size-dependent magnetic properties of high oxygen content YMn2O5±d multiferroic nanoparticles, J. Appl. Phys. 105 (2009), https://doi.org/10.1063/1.3077263.

[51] Y. Qu, H. Yang, N. Yang, Y. Fan, H. Zhu, G. Zou, The effect of reaction temperature on the particle size, structure and magnetic properties of coprecipitated CoFe2O4 nanoparticles, Mater. Lett. 60 (2006) 3548–3552, https://doi.org/10.1016/j.matlet.2006.03.055. [52] D. Xue, G. Chai, X. Li, X. Fan, Effects of grain size distribution on coercivity and permeability of ferromagnets, J. Magn. Magn. Mater. 320 (2008) 1541–1543, https://doi.org/10.1016/j.jmmm.2008.01.004. [53] Y. Mei, S. Wu, Morphology control of YMn2O5 nanocrystals by hydrothermal synthesis and their magnetic properties, RSC Adv. 3 (2013) 11888, https://doi. org/10.1039/c3ra41671b. [54] S. Chakraverty, M. Bandyopadhyay, Coercivity of magnetic nanoparticles: a stochastic model, J. Phys. Condens. Matter. 20 (2008) 219803, https://doi.org/ 10.1088/0953-8984/20/21/219803. [55] A.A. Zatsiupa, L.A. Bashkirov, I.O. Troyanchuk, G.S. Petrov, A.I. Galyas, L.S. Lobanovsky, S.V. Truhanov, Magnetization, magnetic susceptibility, effective magnetic moment of Fe3+ ions in Bi25FeO39 ferrite, J. Solid State Chem. 212 (2014) 147–150, https://doi.org/10.1016/j.jssc.2014.01.019. [56] Z.H. Sun, B.L. Cheng, S. Dai, K.J. Jin, Y.L. Zhou, H.B. Lu, Z.H. Chen, G.Z. Yang, Z.H. Sun, B.L. Cheng, S. Dai, K.J. Jin, Y.L. Zhou, H.B. Lu, Z.H. Chen, Effect of Ce substitution on magnetic and dielectric properties of BiMn2O5, J. Appl. Phys. (2014), https://doi.org/10.1063/1.2190716 84105. [57] Z.M. Tian, Y. Qiu, S.L. Yuan, M.S. Wu, S.X. Huo, H.N. Duan, Enhanced multiferroic properties in Ti-doped Bi2Fe4O9 ceramics, J. Appl. Phys. 108 (2010) 64110, https://doi.org/10.1063/1.3478705. [58] A.P. Ramirez, Strongly geometrically frustrated magnets, Annu. Rev. Mater. Sci. 24 (1994) 453–480, https://doi.org/10.1146/annurev.ms.24.080194.002321. [59] J.B. Goodenough, Theory of the role of covalence in the perovskite-type manganites [La, M(II)]MnO3, Phys. Rev. 100 (1955) 564–573, https://doi.org/ 10.1103/PhysRev. 100.564. [60] J. Kanamori, Superexchange interaction and symmetry properties of electron orbitals, J. Phys. Chem. Solids. 10 (1959) 87–98, https://doi.org/10.1016/00223697(59)90061-7. [61] P.W. Anderson, Antiferromagnetism. Theory of superexchange interaction, Phys. Rev. 79 (1950) 350–356, https://doi.org/10.1103/PhysRev. 79.350. [62] V.M. Gaikwad, S.A. Acharya, Novel perovskite–spinel composite approach to enhance the magnetization of LaFeO3, RSC Adv. 5 (2015) 14366–14373, https://doi.org/10.1039/C4RA11619D.