Structural, multiferroic properties and enhanced magnetoelectric coupling in Sm1−xCaxFeO3

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Structural, multiferroic properties and enhanced magnetoelectric coupling in Sm1
xCaxFeO3 K. Praveena a,n, P. Bharathi b, Hsiang-Lin Liu a, K.B.R. Varma b a b

Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2016 Received in revised form 23 May 2016 Accepted 23 May 2016

Sm1
xCaxFeO3 where x¼ 0, 0.25, 0.5, 0.75 and 1.0 was synthesized via the sol-gel method at low temperatures in steps of x¼ 0.25. The as-prepared powders were characterized by Thermogravimetric Analysis (TGA), Transmission Electron Microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The as-prepared powders were pelletized and then sintered at 1000 °C/4 h to obtain single phase material. The sintered ferrite samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS). The XRD data reveal that a pure phase with perovskite structure is obtained for all the compositions and an exothermic peak appears in the DTA curve at 700 °C for x ¼0.25, suggesting a crystallization process; a similar behaviour is found to occur for the other compositions as well. The band gap values were determined from UV reflection spectra to be in the range of 1.8–2 eV. The structural transition is further confirmed by the changes observed in Raman vibrational modes. The microstructural characteristics of all the phases show particles of different morphology and size. The dielectric constant (εr) and dielectric loss (D) were decreased with an increase of frequency (40 Hz to 110 MHz). A huge enhancement in remnant magnetization (Mr) and coercivity (Hc) is observed as Ca3 þ is increases. The improvement in the magnetic properties of present samples is due to the destruction of spin cycloid with Sm. No Fe4 þ ions are discovered upon substitution of Sm3 þ by Ca3 þ . Magnetic hysteresis loops of these samples show a significant weak ferromagnetic behaviour. Clear evidence of magneto-electric coupling is shown by the P–E loops measurement by applying an electric field. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Orthoferrites Sol-gel synthesis Dielectric properties XPS Magnetic properties

1. Introduction Multiferroics exhibit simultaneous ferroic properties with coupled electric, magnetic and structural orders. Over a past decade, there has been a resurgence of interest in understanding and applications of multiferroic materials. Many materials of technological and scientific interest have structures which derive from, or are related to the parent ABO3 perovskite structure. These are of the form Ln1
xCaxFeO3
δ or IV more strictly Ln1
xCaxFeIII 1
x þ 2δFex
2δO3
δ (Ln¼ lanthanide ion), 3þ 2þ and Ca occupying the A sites and Fe occupying the B with Ln sites. The structural characteristics of Ln1
xCaxFeO3
δ can lead to a range of useful physical properties, such as mixed electric and ionic conductivity, and ordered magnetism at elevated temperatures. While at present most of the studies of the technological applications of the Ln1
xCaxFeO3
δ family of materials have focused on their catalytic ability for carbon monoxide and methane oxidation, similar classes of n

Corresponding author. E-mail address: [email protected] (K. Praveena).

perovskite related oxide materials, such as rare earth strontium cobaltates, have been investigated to use as cathode materials for solid oxide fuel cells [1], ceramic membranes for high temperature oxygen separation, and high k-dielectric materials [2,3] and magnetic sensors [4]. For significant amount of Ca doping, charge balance is maintained by a combination of formation of Fe4 þ and oxygen vacancies (δ) [5]. Among these, SmFeO3 could be used as a novel sensing material for detecting ethanol vapour. SmFeO3 sensor has good selectivity, stability and response–recovery characteristics, but its optimal operating temperature is still unsatisfactory (around 370 °C) [6,7]. The crystal structure of the SmFeO3 was first analysed by Marezio and Dernier [8]. Although the SmFeO3 structural parameters are close to the distorted perovskite structure of CaTiO3, Sm introduces complicated new terms in the Hamiltonian for the system including those due to interactions between unlike magnetic atoms [9]. SmFeO3 is a canted anti-ferromagnet below the Neel temperature (TN) of iron. The measured temperature dependence of iron hyperfine field (proportional to Fe sub-lattice magnetization) in SmFeO3 has been compared with various

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Please cite this article as: K. Praveena, et al., Structural, multiferroic properties and enhanced magnetoelectric coupling in Sm1
xCaxFeO3, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.150i

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Fig. 2. TGA traces of the dried gels formed sol-gel process of Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1).

Fig. 1. (a) XRD patterns of Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1). (b) Lattice constants a, b and c of Sm1
xCaxFeO3 (x ¼ 0, 0.25, 0.5, 0.75, and 1).

Table 1 Data of bulk and x-ray density, porosity of Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1). x

Bulk density (dbulk) (g/cc)

X-ray density (dX-ray) (g/cc)

Porosity (%)

0 0.25 0.5 0.75 1

20.5 19.9 19.0 18.5 17.9

21.7 21.7 21.0 20.0 19.1

6 8 10 8 6

theories [10]. Jung-Hoon Lee [11] reported that SmFeO3, surprisingly as a ferroelectric with substantial degree of piezoelectricity at room temperature. And also on the basis of first-principles calculations, they concluded that the noncollinear AFM ordering with two non-equivalent spin pairs is primarily responsible for this extraordinary ferroelectricity. In addition to the magnetically induced ferroelectricity, SmFeO3 exhibits an interesting phenomenon of spontaneous magnetization reversal at cryogenic temperatures. SmFeO3 is characterized by orthorhombic Pbnm (or Pnma) structure. Several investigators have studied the solid solutions of SmFeO3 with various other rare-earth orthoferrites like GdxSm1
xFeO3 and ErxSm1
xFeO3 [12]. Recently Yuvaraj et al. reported magnetic properties of SmFeO3 by combustion method [13], and observed the saturation magnetization and coercivity values as 5.5 emu/gm and 98 Oe respectively. Kuo et al. reported SmFeO3 single crystal and studied its magnetic structure and the absence of ferroelectricity

Fig. 3. (a) FTIR spectra's of dried gels of Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1). (b) FTIR spectra's of dried gels of Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1) from 400 to 850 cm
1.

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Fig. 4. TEM images of (as prepared) of Sm1
xCaxFeO3(x¼ 0, 0.5, and 1).

[14]. Zhang et al. reported multiferroicity in SmFeO3 synthesized by hydrothermal method, and they reported the remnant polarization as 0.012 mC/cm2 at 173 K [15]. The ceramic method is one of the most commonly used; however, soft chemistry routes for the synthesis of nanomaterials such as sol–gel [16] were proved to be convenient and alternative way to obtain inorganic materials. Further, soft chemistry methods also provide a better control on stoichiometry, reduce the synthesis and sintering temperature of materials [17]. Thus, variations on the sol–gel based Pechini method [18,19] have been successfully employed for the preparation of various perovskite materials. In literature the samarium– calcium orthoferrites (0.1 rx r0.7) have been synthesized for the first time in air by the solid state reaction and analysed from a classical crystal chemistry point of view [20]. X-ray diffraction analysis on these samples indicates that these compounds are single-phase perovskite type belonging to the Pbnm space group. After going through the literature carefully, we believed that nobody has reported the ferroelectric and magnetic properties of this system and we aimed to improve the multiferroicity in SmFeO3 by adding Ca3 þ . The present study is aimed at describing the synthesis and physical and optical properties of nanocrystalline Sm1
xCaxFeO3 where x ¼0, 0.25, 0.5, 0.75, and 1.0 via sol-gel method. The X-ray results indicate that all samples consist of single phase. By introducing Ca þ 3 by weight percentage and found enhancement in the ferroelectric properties. Therefore this results indicates that these materials could be good candidates for various technological applications.

2. Experimental Polycrystalline powders of Sm1
xCaxFeO3 where x ¼0, 0.25, 0.5, 0.75, and 1.0 were prepared using sol-gel route. Stoichiometric amounts of samarium nitrate [Sm(NO3)3  6H2O], calcium nitrate [Ca(NO3)2  4H2O], and ferric nitrate Fe(NO3)3  9H2O were taken as the starting precursors and were gelated using mono hydrated citric acid solution. It is taken in such a way that the molar ratio of the total nitrate to citric acid becomes unity. With constant stirring at 60 °C/1 h, the pH of the solution is adjusted to  7 by adding ammonia solution. All these mixtures were kept for constant stirring at 60–70 °C to avoid precipitation so as to obtain a homogeneous mixture. The brown color citrate mixture obtained is a clear solution with no precipitation. After that ethylene glycol is added to the solution in proportionate to citric acid/ethylene glycol ratio of 60:40. The gel initially started to swell and filled the beaker, producing foam like precursor consisting of very light and homogeneous particles of nanosize. The resultant gels are dried at 100 °C in hot air oven for 12 h. The obtained powders were calcined at 400 °C and leached in diluted HNO3. Leaching is adopted to get a single phase nanopowder [21]. Then the powders were grounded and 2% polyvinyl alcohol (PVA) added as binder, pressed into pellets and the green specimens were sintered using conventional sintering method in air at 1000 °C/4 h. The structural studies of the samples were carried out by X-ray diffraction (XRD) using Phillips PANalytical X'pert powder diffractometer with CuKα radiation (λ ¼1.5406 Å). The theoretical density (dX-ray) of these ferrites were calculated from the values of lattice parameters by using the relation dX − ray = 8M3 g/cm3 where NV

8 is the number of formula units in a unit cell, M is the molecular

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absorption. Raman spectra measurements were carried with Nd: YAG laser with a wavelength of 532 nm in a WITec system. FE-SEM analysis is performed to study the morphological characteristics using INSPECT™ S50. Saturation magnetization (Ms), remnant magnetization (Mr) and coercivity (Hc) values were obtained using vibrating sample magnetometer (VSM) Lakeshore 665, USA up to 20 kOe. The dielectric measurements were carried out using Agilent 4284A LCR meter operating at oscillation amplitude of 1 V. XPS was measured on AXIS ULTRA (AXIS 165) at a pressure of about 2  10
9 Pa using Al Kα X-ray as the excitation source. The binding energy (BE) calibration of the spectra has been referred to carbon 1s peak, located at BE ¼284.8 eV. The ferroelectric nature of poled ceramics at room temperature was confirmed by using a radiant precision workstation.

3. Results and discussion 3.1. Crystal structure from X-ray diffraction

Fig. 5. (a) UV–vis spectra of Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1). (b) Kubelka Munk (K-M) function for Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1).

weight and N is the Avogadro number and ‘V’ is volume of the unit cell (V ¼a*b*c). The bulk density of the sample was calculated from precise measurements of the dry, saturated, and suspended weight. The weight loss of the sample in the water was measured using a digital balance with an accuracy of 0.001 mg. The bulk Mair density dbulk = M −M g/cm3 of the samples was calculated air

xylene

using the below formula: where Mair is mass of the sample in air and Mxylene is mass of the sample in xylene. With the knowledge of dbulk and dX-ray the percentage of porosity d (P%) has been estimated using the relation Porosity (P%) = (1− d bulk ) × 100 . X − ray

The Fourier transform infrared spectroscopy (FTIR) was studied on Bruker Tensor 27 DTGS TEC detector spectrophotometer from 400 to 4000 cm
1 using the KBr pellet method in transmission mode. For this purpose, 1 mg of the sample is thoroughly mixed with 150 mg of KBr in agate mortar and the mixture pressed under vacuum to produce a thin disk. The TEM was measured on JEM 2100F. The diffused reflectance spectroscopy is carried out using Analytik-Jena (Specord S600) over the spectral range 500– 2550 nm at room temperature (RT). Reflectance spectra were converted to absorbance with the Kubelka–Munk (KM) function [22]. KM function versus wavelength (nm) plot, extrapolating the linear part of the rising curve to zero provides the onset of

Fig. 1(a) shows the X-ray diffraction patterns for Sm1
xCaxFeO3 where x ¼0, 0.25, 0.5, 0.75 and 1.0 for the samples heated at 1000 °C/4 h at a heating rate of 2 °C/min. It is observed that the samples crystallized completely without the presence of secondary phases. Comparing the experimental conditions used by Li et al. [20] could be seen that the temperatures and thermal treatment times are lower in the case of sol–gel citrate route. The XRD indexed patterns represent an orthorhombic structure with a Pbnm space group, which is in good agreement with literature [23]. It is found that when Sm3 þ atoms were replaced by Ca3 þ in SmFeO3, the intensity of XRD peaks reduced; however, the displacement of the reflections was not considerable, which indicates that the structural distortion is minimum for an increased calcium content. The lattice constants a, b and c of Sm1
xCaxFeO3 where x¼ 0, 0.25, 0.5, 0.75, and 1.0 are plotted in Fig. 1(b). The lattice parameters of SmFeO3 [23] are lower for x¼ 0, compared with x¼ 1 that showing a slight increase of the cell parameter (a) with the calcium introduction. However, it can be noted that other cell parameters (c) is maximum when the Ca3 þ composition is raised, the cells parameters (b) found to fall slightly, reaching almost constant values. From Table 1 it is clear that as Ca3 þ increases porosity increases up to x ¼0.5 and thereafter again it decreases, and it is predicted from the micrographs (Fig. 7(a)). 3.2. TG analysis Fig. 2 depicts the TGA curves of the dried gels. It is observed that weight loss occurs in the TGA curves from room temperature to  700 °C for Sm1
xCaxFeO3 where x ¼0, 0.25, 0.5, 0.75, and 1.0. This effect occurs owing to the evaporation of organic compounds used during the synthesis that are physically trapped in the gel. A similar behaviour is observed for other compositions; however, the temperature decreases when the calcium content increases in the sample, when x r0.5 and the final weight loss occur at 652 °C. This suggests the crystallization process. In addition to this, the crystallization temperature diminishes when the calcium content increases in the samples. Thus sol–gel method favours for producing homogeneous nanomaterials in a uniform way, with high porosity. A determinant step in the process is the heat treatment needed to obtain the crystalline phase. 3.3. FTIR analysis Fig. 3 shows the FTIR spectra of Sm1
xCaxFeO3 where x¼ 0, 0.25, 0.5, 0.75, and 1.0. The vibrational modes of the perovskite ABO3 are mainly active in three IR spectral regions (i) in the region

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Fig. 6. (a) Raman scattering spectra for Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1). (b) Composition dependence of the peak positions of two prominent phonon modes of all the samples from fitting.

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Table 2 Band assignments of Raman scattering spectra for Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75 and 1), units are in cm
1. x

Ag (6)

B2g (5)

Ag (2)

B1g (3)

B1g (4)

B3g (3)

0 0.25 0.5 0.75 1

144.6 131.4 86.8 85.0 131.7

195.4

251.0

395.6 389.1

474.7 468.3

137.9 158.7

227.0

292.4 286.5 280.2 282.2 288.9

399.5 376.1

436.0

500–730 cm
1; (ii) in the region 250–500 cm
1 and (iii) at about 200 cm
1 (external) [24]. The first region of bands is attributed to the stretching modes of the BO6 octahedra while the second region of bands is attributed to the bending modes of the same polyhedra. Because the Fe–O bonds of the BO6 octahedral units are undoubtedly stronger than that of the 12-coordinated Sm–O units, one may predict that the BO6 units dominate the spectroscopic behaviour [25]. The FTIR spectrum for x¼ 0.5 (Fig. 3) exhibits the bands around 469, 574 cm
1 with shoulders at 619, 684, 722, and 802 cm
1. The band at 469 cm
1 is assigned to the stretching vibration of Ca3 þ –O2
bond. The main band at 574 cm
1 is assigned to antisymmetric stretching vibrations of Fe–O–Fe bonds in FeO6 octahedra. The shoulder bands (Fig. 3(b)) at 619, 684 and 722 cm
1 are assigned to the Fe–O symmetric stretching vibration of these octahedra which involves internal motion and change in Fe–O bond length [25–27]. The band at 802 cm
1 is assigned to the stretching vibration of Sm–O [28]. It is known that in GdFeO3 [29] type structure (8 A–O) bonds among the 12 bonds become shorter and the other (4 A–O) bonds become longer gradually as the size of the A cation decreases. Thus we can write the coordination number for the Sm3 þ ion; in Sm1
xCaxFeO3 as 4 þ4 þ4 which is equivalent to 8 þ4. For this reason the two bands at 869 and 793 cm
1 could be assigned to the stretching vibration of Sm–O [28]. The appearance of band at around 717 cm
1 is due to the increase of Fe–O constant force as a result, Fe ion charge increases (Fe4 þ ions formation) to conserve the charge neutrality of the perovskite structure, which is in agreement with Berger et al. [30]. 3.4. Particle size analysis Further insights into the morphology and crystal structure of the prepared samples were provided by TEM and HRTEM micrographs as shown in Fig. 4. The morphology of SmFeO3 resembles a spherical-shaped nanostructure with average size  40 nm, as in Fig. 4(a, b & d). The HRTEM micrograph was recorded from the tip of the individual spherical particle for x ¼0.5 shown in Fig. 4 (c) and the unidirectional alignment of the lattice fringes confirmed the polycrystalline nature. The regular spacing of the observed lattice plane was 0.29 nm, which is consistent with the calculated (110) lattice spacing of spherical x¼ 0.5. The selected area electron diffraction (SAED) pattern (inset of Fig. 4(c)) demonstrates the growth axis is along [110] direction for x ¼0.5. The HRTEM micrograph of the tip portion of the nanopowders shows the lattice planes with an interplanar spacing of 0.279 nm corresponding to the (121) direction. The polycrystalline nature of the nanoparticles is revealed by discrete spots in the SAED pattern, as shown in the inset of Fig. 4(c). Thus, by correlating the outcome from TEM and HRTEM micrographs, it is concluded that the observed results are in good agreement with the FESEM and XRD patterns. It should be noted that the diffraction rings are discontinuous and indicates that the nanopowders are well crystallized. According to the electron-diffraction formula, the major diffraction spots correspond to the (1 0 1), (1 2 1), (2 2 0), (2 0 2), (2 4 0), (2 4 2) and (2 0 4) diffraction planes for an orthorhombic perovskite structure.

Ag (3)

B3g (2)

Ag (1)

537.5 538.7 518.7 508.3

575.7 546.4

497.0

564.5

3.5. Band gap measurements from UV–vis spectra The structural features of Sm1
xCaxFeO3 where x¼ 0, 0.25, 0.5, 0.75, and 1 show them for potential applications. For example, the sensor applications of SmFeO3 are largely dependent on the effects of surface and quantum confinement [20]. The photo catalytic activity of SmFeO3 under visible irradiation occurs largely by photo-induced charge properties. The optical properties of Sm1
xCaxFeO3 influence its photo-induced charge properties; in this context it is important to know the bandgap of SmFeO3 and its variation with Ca doping. In order to determine the energy bandgap of Sm1
xCaxFeO3 (x ¼ 0, 0.25, 0.5, 0.75, and 1) in conjugation with structural features, we have performed diffused reflectance (DR) measurements. Fig. 5(a) shows the diffuse reflectance spectra of Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1) in the UV–vis–NIR range. The diffused reflectance data is used to calculate the absorption coefficient from the Kubelka–Munk (KM) (1 − R )2

function [22] defined as: F (R ) = 2R where R is the reflectance, KM function versus wavelength (nm) plot (Fig. 5(b)), extrapolating the linear part of the rising curve to zero provides the onset of absorption. The energy dependence of the material in the UV–vis– NIR range is further explored. The energy dependence of semiconductors near the absorption edge is expressed as: αE = K (E − Eg )η , here E is the incident photon energy (hʋ), Eg is the optical absorption edge energy, K is a constant, and the exponent ƞ is dependent on type of optical transmission as a result of photon absorption. The exponent ƞ is assigned to a value of 1/2, 3/2, 2, and 3 for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transition, respectively. For the diffused reflectance spectra, the KM function could be used instead of α for estimation of the optical absorption edge energy. It is observed that, for a plot of F(R) vs E (Fig. 5(b)) was linear near the edge for direct allowed transition (η ¼ 1/2). The intercept of the line on the abscissa (F(R) E ¼0) gave the value of optical absorption edge energy. The band gap values were determined and they are in the range of 1.8–2 eV. 3.6. Crystal structure from Raman scattering Based on the symmetry operations of the Pnma space group carried out by Smirnova et al. [31] the following assignment of the bands are shown in Fig. 6. Due to the sensitivity of the Raman vibrational modes on the crystal symmetry which controls the matrix elements of the Raman tensor [32], the structural phase transition is expected to modify the Raman phonon modes. RFeO3 has an orthorhombic (Pnma) structure. Accordingly, it has 24 zonecenter Raman active modes given by irreducible representations as 7Ag þ7B1g þ 5B2g þ5B3g [33]. In all these modes, Fe ions do not move. Sixteen of the 24 Raman-active modes involve vibrations of the FeO6 octahedra, while the remaining eight modes involve motion of R ions [34]. The phonon modes below 200 cm
1 in SmFeO3 related to lattice modes involving R atom vibrations. Region above 200 cm
1 consists of various modes involving R atom and oxygen motion. Ag(1) mode is related to the anti-stretching vibrations of FeO6 octahedra. Modes caused by R (Sm) vibrations

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Fig. 7. (a) FESEM images of Sm1
xCaxFeO3(x ¼0, 0.25, 0.5, 0.75, and 1). (b) EDAX of Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1).

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Table 3 Atomic percentage values for all compositions by EDAX analysis. x

Sm

Fe

Ca

O

0 0.25 0.5 0.75 1

39.2 34.6 28.5 7.9 –

24.3 24.2 27.0 29.9 34.6

– 10.4 13.9 22.3 26.9

32.5 30.1 32.7 39.8 38.6

are present below 200 cm
1, labelled (A). Modes between 200 and 300 cm
1 are oxygen octahedral tilt modes (T). B1g(3), Ag(2) are octahedral rotations around crystallographic y-axis B1g(4) and Ag(4) are rotations around x-axis (Pnma setting). The singlet (Ag(7)) in SmFeO3 are related R-O vibrations. B3g(3) is bending mode of FeO6 octahedra. Modes between 400 cm
1 and 450 cm
1 are oxygen octahedral bending vibrations (B) and modes above 500 cm
1 are oxygen stretching vibrations (S) [35–39]. The 620 cm
1 peak is assigned as a symmetric oxygen breathing mode. In Fig. 6(a) the low energy bands for both compounds are labelled according to the Ag and B2g symmetry of the Pnma space group. All the bands below 300 cm
1 are shifted towards higher

Sm

Fe

Fig. 9. M-H loops of Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1).

O

Sm

Ca

Fe

O

(a) x = 0

Ca

Fe

O

(c) x = 0.25

(b) x = 1

Sm

Fe

Ca

Sm

Ca

O

Fe

O

(d) x = 0.5

(e) x = 0.75

Fig. 8. EDAX mapping of Sm1
xCaxFeO3 (a) x ¼0 (b) x ¼ 1 (c) x¼ 0.25 (d) x ¼ 0.5 and (e) x¼ 0.75.

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K. Praveena et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 4 Data of saturation (Ms), remnant (Mr) magnetization, coercivity (Hc) and squareness ratio for Sm1
xCaxFeO3 system. x

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

Mr/Ms

0 0.25 0.5 0.75 1

9.5 8.11 7.15 5.87 4.76

5.32 3.78 3.54 2.52 2.17

264 213 232 201 217

0.56 0.46 0.49 0.42 0.45

9

irregular; which is a result of the sintered material being crushed until it is obtaining in the powder form. On the other hand, the image reveals that densification of the particles occurred during the sintering process (Table 1). The average size of the particles is 80–120 nm and similar characteristics are observed for other compositions. On the other hand, the substitution of Ca for rare earth Sm in Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1) may lead to either a change in electronic structure of iron or oxygen deficiency in the lattice. 3.8. Energy dispersion X-ray analysis The compositions of different Ca3 þ ion substituted samarium ferrites has been determined using the energy dispersion X-ray analysis (EDAX), and it is shown in Fig. 7(b). From the EDAX spectrum, the presence of Sm, Ca, Fe, and O are confirmed in all the samples. The quantitative analysis of EDAX spectrum revealed the relative atomic ratio of all elements stoichiometry, which is close to the expected values. The data of the EDAX analysis for all the sample are given in Table 3. Fig. 8(a-e) presents the distribution of the constituent elements on the analysed surface of the sample. EDAX elemental mapping revealed a uniform distribution of Sm, Ca, Fe, and O elements. We can clearly see the Ca3 þ doping is increases, the mapping of Sm3 þ is changed. 3.9. Magnetic properties

Fig. 10. (a) Frequency variation of dielectric constant (εr) for Sm1
xCaxFeO3 (x¼ 0, 0.25, 0.5, 0.75, and 1). (b) Frequency variation of dielectric loss (D) for Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1).

energies respect to the SmFeO3. The differences between both spectra are associated with the dissimilar distortions caused by Sm and Ca. The band assignments with composition are listed in Table 2. Fig. 6(b) shows the compositional dependence of the peak positions of representative modes in these samples. Significant changes are observed in the vibrational phonon frequencies has been observed with increasing Ca composition, indicating the changes in the crystallographic structure. The linewidth is showing blue shift upto x ¼0.75. 3.7. Microstructure analysis Fig. 7(a) depicts the microstructural characteristics of Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1) sintered at 1000 °C/4 h. The micrograph indicates that the morphology of the particles is

Investigations of room-temperature magnetic properties confirm the existence of spontaneous magnetization in the Sm1
xCaxFeO3, where x¼ 0, 0.25, 0.5, 0.75, and 1 (Fig. 9). Generally, in rare earth orthoferrites the magnetic properties arise due to the super exchange interaction of Fe3 þ –Fe3 þ , R3 þ –R3 þ and R3 þ –Fe3 þ via O2
ion [40]. Hysteresis loop shows the weak ferromagnetic behaviour at room temperature. In SmFeO3 magnetic properties come from Fe3 þ –Fe3 þ interaction, since Sm3 þ –Sm3 þ interaction induces the long range antiferromagnetic ordering below 5 K and Fe3 þ –Sm3 þ interactions start below 135 K. At this temperature, the Sm3 þ moment is opposite to canted Fe3 þ moment and the magnetic moment of Sm3 þ is higher than Fe3 þ spin. So, the 4f orbitals of Sm3 þ ions strongly interact with O2
2porbitals of oxygen which results in the Sm3 þ –O2
–Fe3 þ interaction at this temperature [40,41]. In this case, at room temperature weak ferromagnetic behaviour is possible for the following two reasons: the super exchange interaction of Fe3 þ –Fe3 þ ions is mediated by O2
ions at the position of 180° which induces the antiferromagnetic behaviour. However, at room temperature Fe3 þ –O2
–Fe3 þ hybridization is more stable relative to the hybridization of Sm3 þ and O2
ions but the weak antiferromagnetic exchange interaction of Fe3 þ –O2
–Sm3 þ results in the spin reorientation of the Fe3 þ ions, leading to the weak ferromagnetic behaviour [40]. Furthermore, Dzyaloshinsky– Moriya antisymmetric exchange mechanism, according to which the magnetic moment of Fe3 þ spins are not completely antiparallel to those of the surrounding Fe3 þ ions but rather tilted by a small angle, leads to weak ferromagnetic behaviour [42,43]. According to literature, spin reorientation occurs in between 457 and 465 K, where the magnetic moments are aligned along the crystallographic c-axis which leads to pure antiferromagnetic behaviour and above the curie temperature 674 K [44]. The magnetic ordering of SmFeO3 phase changes from antiferromagnetic to paramagnetic state due to thermal disorder. Berini et al. [45] examined the magnetic properties of SmFeO3 thin film through magneto-optic measurements. Their studies revealed that the magnetic ordering temperature was 700 K, which corroborated that above the Curie temperature it was paramagnetic and below the temperature hysteresis loops occurred due to ferromagnetic magnetization

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Fig. 11. (a-d) XPS spectra of SmFeO3 (a) Sm 3d (b) Fe 2p (c) O 1s (d) Survey. (e-h) XPS spectra of x ¼0.5 (e) Sm 3d (f) Fe 2p (g) Ca 2p (h) O 1s.

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0.5, 0.75, and 1 at room temperature in the frequency range of 100 Hz–110 MHz is investigated. The plot of log frequency versus dielectric constant is shown in Fig. 10(a). εr values of Ca doped SmFeO3 exhibit higher values than pure SmFeO3. Reducing the grain size to nanoscale will increase the dielectric constant due to the large number of grain boundaries as scattering centers. The improvement in the dielectric constant is due to the change in the structure, especially the variation of Fe–A–O bond length which favours the off-centro symmetry with Ca concentration. The dielectric constant decreases with increase in frequency and follows Debye law of frequency dispersion as it is nearly constant at very high frequencies [55]. As the frequency increases to higher values there is a crossover in the dispersion curves with increasing Ca concentration. Such crossover is observed in previous reports, La and Pb doped BiFeO3 systems [56,57]. The non-systematic variation of εr with La concentration in terms of the oxygen vacancies and change in unit cell volume. The oxygen vacancy depends crucially on the processing conditions and is determined by the microstructure of the samples as well. In our samples, though we also see a non-systematic variation of εr with introduction of Ca concentration, the specific details as a function of concentration deviate considerably from those reported by Sen et al. [56]. Fig. 10(b) represents the dielectric loss (D) with frequency. At low frequencies loss is high and as the frequency increase it attains a constant value at higher frequencies. Dielectric loss arises due to lag of polarization behind the applied alternating electric field. The loss factor can be attributed to the Maxwell–Wagner interfacial polarization, where the matching of hopping frequency with the frequency of an applied field. The absence of any loss peak in the dielectric dispersion of the material (Fig. 10(b)) suggests its behaviour to be that of quasi-dc process (QDC) [58–61]. Similar type of behaviour is observed in Mn doped EuFeO3 [62]. 3.11. Chemical bonding from XPS

Fig. 12. XPS spectra of Ca2p3/2, Ca2p1/2 for x ¼ 0.25, 0.75 and 1.

[45]. Bashir et al. [46] examined the magnetic properties of SmFeO3 synthesized by solid state reaction method and the reported magnetic moment and saturation magnetic moment was 0.21 and 0.288 mB/f.u at room temperature, which associated with the canted antiferromagnetic behaviour [47]. As the Ca3 þ concentration increases from 0.25 to 1.0, the saturation, remnant magnetization, decreases whereas coercivity decreases and then increases. These values are given in Table 4. The huge coercivity of the order of 1–2 T at room temperature is also observed in the other dopant systems [48,49]. The high HC of our samples is due to the magnetocrystalline anisotropy, arising from the deformation of Fe sublattice due to the Sm [50]. Besides magnetocrystalline anisotropy, the contribution from the magnetoelastic anisotropy [51] may also lead to the variation of HC with Sm. The squareness ratio (Mr/Ms) decreases with increasing Ca concentration. This may be explained with decreasing magnetic anisotropy depending on increasing Ca concentration [52]. According to the Stoner–Wohlfarth model [53], Mr/Ms for an assembly of non-interacting 3D random particles is given by Mr/Ms ¼0.5 for uniaxial anisotropy and Mr/Ms ¼0.832 for cubic anisotropy. In the present study we obtained high Ms compared to the earlier reports [13] as the magnetic properties depend on the method of preparation, microstructure and density [54]. 3.10. Dielectric properties The dielectric constant (εr) of Sm1
xCaxFeO3 where x ¼0, 0.25,

X-ray photoelectron spectroscopy (XPS) is used to determine the oxidation states and relative atomic ratios (at the surface) of the different elements in Sm1
xCaxFeO3 where x ¼ 0 and 0.5. For the purpose of quantification and identification of oxidation states, peaks of Sm 3d5/2, Ca 2p3/2, Fe 2p3/2 and O 1s core levels were used. Fig. 11(a-h) shows the spectra of Sm3d5/2 Ca2p3/2, Fe2p3/2 and O1s core levels for pure SmFeO3 and for Sm0.5Ca0.5FeO3. Identification of the oxidation states can be performed by comparing with the reported literature of photoelectron peak positions. XPS spectra have been charge corrected by setting the C 1s binding energy at 284.6 eV. The position of the Sm 3d5/2 peak in both the samples slightly shifted (1081.3 eV) from the nominal Sm3 þ position as measured for Sm2O3 (1082.9 eV [63]). A small shift towards lower binding energy has been previously observed for SmFeO3 (1082.4–1082.9 eV [64]) and for Sm1
xCexFeO3 (1081.9–1083.1 eV [65]). As shown in Fig. 11(a-d) shows the XPS spectra's of x¼ 0. The element of Sm3d5/2 peak does not shift with is consistent with a single oxidation state for samarium, namely Sm3 þ . The peaks for Ca3 þ 2p lines energies exhibit a very small change in the energy when going from SmFeO3 to CaFeO3 (Fig. 12). The peak position and shape of Fe2p3/2 indicate the presence of multiple oxidation states for iron. Fig. 11(b) shows the Fe2p peak. In general, the Fe2p3/2 peaks were broad together with clear shoulders (peak separation). The Fe 2p XPS spectrum is displayed in Fig. 11(b). The observed Fe 2p peak can be divided into Fe3 þ peaks (2p3/2: 710.6 eV and 2p1/2: 722.9 eV) and Fe2 þ peaks (2p3/2:709 eV and 2p1/2:724.7 eV), indicating that the Fe element exists in ionic form with valences of þ2 and þ3. Note that the Fe2p3/2 and 2p1/2 peaks have the satellite structure on the higher energy side, which is also a typical character of Fe ions with high spin divalent state. In Fig. 12 each peak corresponds to Ca2p3/2 and

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Fig. 13. Room temperature Polarization-Electric field (P-E) loops for Sm1
xCaxFeO3 (x ¼0, 0.25, 0.5, 0.75, and 1).

Ca2p1/2, Ca2p3/2 in CaFeO3 line is 345.4 eV for x ¼0.25 and shifts to 346.1 for x ¼1. 3.12. P-E measurements Fig. 13 shows polarization (P) versus the electric field (E) plot of Sm1
xCaxFeO3 where x¼ 0, 0.25, 0.5, 0.75, and 1, measured at 300 K under an applied electric field of 30 kV/cm. The plot confirms the nature of the samples are ferroelectric, we could see clearly that as the Ca content increases, the hysteresis loops broaden, correspondingly the remnant polarization (Pr) increases, which is due to the continuous increase in leakage current. XRD results show a Pnma structure, implying that the ions are essentially in their centrosymmetric positions, and the polarization

would come from small electronic rearrangements, inferring the polarization in SmFeO3 is very small. As mentioned above, the Dzyaloshinskii-Moriya interaction in YFeO3 causes an additional canting of the antiferromagnetically ordered spins. It seems more likely, that the possibility of the occurrence of ferroelectricity in such compounds is the same electronic mechanism reported in the literature [11,66].

4. Conclusions Perovskite Sm1
xCaxFeO3 particles were successfully prepared by a simple sol-gel route. XRD and FT-IR analysis confirmed the compound formation, while FESEM revealed spherical particles

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with particle size in the range of 80–120 nm. Dielectric constant is enhanced with Ca doping. Dielectric properties decreases with increasing frequency. From the VSM study, it is observed that they are weak ferromagnetic, which is the origin of canting of magnetic moments of Fe3 þ ions. Coercivity is increased as Ca3 þ increases, which may be due to the high uniaxial anisotropy (squareness ratio  0.5) in these samples. The high value of remnant polarization is observed for x¼ 0.75 and is approximately 0.17 mC/cm2. P - E loops confirms the existence of the ferroelectric phase and thus this results indicate multiferroicity at room temperature.

Acknowledgment Dr K. Praveena acknowledges the Ministry of Science and Technology of People's Republic of China under Grant No. MOST 104-2811-M-003-009 and National Science Council, Taiwan under grant no. NSC 102-2112-M-003-002-MY3 for financial support.

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