Ce-doping effect on modulation of spin-exchange interaction and dielectric behaviour of nanostructured LaFeO3 orthoferrites

Ce-doping effect on modulation of spin-exchange interaction and dielectric behaviour of nanostructured LaFeO3 orthoferrites

Journal Pre-proof Ce-doping effect on modulation of spin-exchange interaction and dielectric behaviour of nanostructured LaFeO3 orthoferrites Javed S...

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Journal Pre-proof Ce-doping effect on modulation of spin-exchange interaction and dielectric behaviour of nanostructured LaFeO3 orthoferrites

Javed Sheikh, Smita A. Acharya, Uday P. Deshpande PII:

S0254-0584(19)31271-4

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122457

Reference:

MAC 122457

To appear in:

Materials Chemistry and Physics

Received Date:

11 June 2019

Accepted Date:

14 November 2019

Please cite this article as: Javed Sheikh, Smita A. Acharya, Uday P. Deshpande, Ce-doping effect on modulation of spin-exchange interaction and dielectric behaviour of nanostructured LaFeO3 orthoferrites, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys. 2019.122457

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Journal Pre-proof Ce-doping effect on modulation of spin-exchange interaction and dielectric behaviour of nanostructured LaFeO3 orthoferrites Javed Sheikh1, Smita A. Acharya1* and Uday P. Deshpande2 1Advanced materials Research Laboratory, Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur-440033, M.S. India 2UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore452001, M.P. India. *Email: [email protected] ABSTRACT In present attempt, Ce-doped LaFeO3 (LCFO) system is explored as single phase room temperature multiferroics (MFs). Nano-crystalline LCFO system with various compositions of Ce [La1-xCexFeO3 (0  x  0.1)] are synthesized by microwave-assisted gel combustion method. Detailed Rietveld analysis of X-ray diffraction data revealed that the systems retain distorted orthorhombic phase with space group Pbmn for x < 0.10; However, impurity phase related to CeO2 is detected for x = 0.1 indicates solid solubility limit of Ce in LFO is x ≤ 0.08. New phonon mode related to Ce translational vibration with several kinks above 300 cm-1 in Fourier transform infrared spectra, is a clear indication of structural modification of LFO by Ce. Enhanced magnetization with Ce doping content may be attributed to spin exchange interaction Ce3+-O-Fe3+ and Fe3+-O-Fe3+. M-H curves exhibit partial ferromagnetic nature with G-type antiparallel spin arrangement of LCFO. Temperature driven dielectric and magnetic behaviour exhibit transition near Neel temperature evidence for magneto-dielectric coupling in LCFO systems. Remarkable enhancement in the magnetization, coercivity, and remanant magnetization with reduction in dielectric constant, loss and leakage current of LFO by Ce-doping suggests that LCFO ferrites are promising materials as room temperature multiferroic. Key-words: Ce-doped nanostructured LaFeO3, Magneto-dielectric coupling, XRD, FTIR 1. Introduction

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Journal Pre-proof In the last few decades the multiferroics (MF) or Magnetoelectric (ME) material (coexistence of magnetic and electric order) have been extensively studied with the intention (i) to understand the rich physics and (ii) to consider promising technological applications such as multistage memory devices, spintronics devices, magnetically modulated transducers, ultrafast optoelectronic devices and sensors etc [1-2]. Furthermore, their properties can be tailored and/ or enhanced by varying their dimension, size, composition, structure, etc [3]. However, there are few single phase systems which display the MF properties [4]. Most of these systems have transition temperature far away or below room temperature (e.g.BiMnO3 TC = 750 K and TN =150 K) which cause weak ME coupling at room temperature and no practical use [3]. BiFeO3 (BFO) is one of the extensively studied room temperature MF materials [5]. However, functioning of BFO is limited by high leakage current, high coercive field which makes it difficult to use in device applications [6]. There is need of strategic and systematic efforts to search out or develop new materials having high magnetization and polarization. Doping is one of the approaches to tailor magnetic and dielectric behaviour of the materials. Consequently, properties can be altered or modified precisely by the choice of suitable dopant at A or B site in ABO3. Very recently, LaFeO3 (LFO) has been intensively explored as room temperature single phase multiferroic system with promising additional applications in solid oxide fuel cell, catalysis, chemical sensors etc, [6-8]. LFO is a member of orthoferrite family and crystallise in orthorhombic symmetry of space group Pbnm. Within the Pbnm symmetry, Fe cation preserves the central symmetry and La cation prefers off-centre position [9]. According to Glazer’s notations this tilting can be represented by a+b-b-. The rotation of the perovskite allows two distinct Fe3+-O2--Fe3+ superexchange linkage angles: 1 = one Fe3+-O1-Fe3+ along the [001] direction, and 2 = one Fe3+-O2-Fe3+ in the plane perpendicular to the [001] direction. The exchange interactions Fe3+-O2--Fe3+ are strong giving rise to G-type 2

Journal Pre-proof antiferromagnetic (AFM) ordering of the iron spins. Since each iron cation is coupled antiferromagnetically to six iron cations, the very high Neel temperature (TN ~ 750 K) is observed in the result [7, 8, 10]. However, its functionality is limited due to low magnetisation and high dielectric loss. The renewed interest is generated in LFO due to low oxygen vacancy content in the ferrite. As a result it can be a model insulator to manipulate functional properties for possible applications as compared to BFO with unwanted large oxygen vacancy content [10]. Further, the substitution of La3+ and /or Fe3+ site is also observed to improve the physical properties. Various types of (divalent and trivalent) dopant substitution have been studied, for example, LaxSr1-xFeO3- [11], LaxZn1-xFeO3- [12], LaxAl1xFeO3-

[13-14], LaxCa1-xFeO3- [15], LaxNa1-xFeO3- [16], La(1-x)GdxFeO3- [17]. La(1-

x)CexFeO3-

[18]. Also it has been found that dopant substitution with different ionic radii at

La3+ ion in LFO cause mixed valence state of Fe2+, Fe3+ and Fe4+ and/or vacancy of oxygen to preserve charge neutrality [16, 18-25]. The lattice distortion due to dopant-size mismatch is also responsible to change A-O and B-O bond distances, bond angles as well as Fe-O-Fe bridging angles with affecting FeO6 octahedron. It modulates physical properties of LFO mainly dielectric constant, dielectric loss, electrical conductivity, thermal stability, ferroelectricity and ferromagneticity. Therefore, the substitution effect of divalent or trivalent ions into the La or Fe sub-lattices has been specially investigated to improve potential ability of this material. [8, 26-27]. In most recent report [20, 28-31], nano-composite strategies have been widely explored to tune magnetic, dielectric and multiferroic behaviour of LFO and BFO systems. However, present study is specially focused on modification of magnetic and dielectric behaviours of single phase pervoskite (orthoferrites) system LaFeO3 by dopant effect.

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Journal Pre-proof The thrust of the present study is to partially substitute non-magnetic

La3+ by

magnetic Ce3+ to introduce new exchange interactions (such as Ce3+-O2--Fe3+ and Ce3+- O2-Ce3+ with Fe3+- O2--Fe3+). These may give rise to some interesting magnetic and dielectric properties in LFO system. It makes curious to develop ultra-fine LaxCe1-xFeO3- powders (0  x  0.1) and investigate its magnetic, dielectric and electrical behaviour to search out the potential as room temperature multiferroic. The choice of Ce3+/4+ ions at La3+ site is mainly to create charge imbalance and thus spin-charge discrepancies to introduce magnetic and dielectric anomalies. 2. Experimental Polycrystalline LaxCe1-xFeO3- systems having compositions (0  x  0.1) were synthesized by the microwave-assisted gel combustion method using glycine as a fuel. Detail of synthesis procedure is followed as reported elsewhere with modifying mode of heating [10]. In the present attempt, we have used microwave mode of heating in place of conventional heating to shorten time and save energy of synthesis reaction. The assynthesized powder was calcined at 600oC for 3 hours (optimized condition) to obtain the final products as polycrystalline pure and doped LFO. Microstructural feature of assynthesized doped LFO system was investigated by transmission electron microscopy (TEM). The polycrystalline powder was compacted into cylindrical pellet by using hydraulic press and sintered at 1300oC for 5 h (optimize condition) for dielectric measurement. To study the dielectric properties of the compounds, both flat surfaces of the pellets were polished with airdrying conducting silver paste. Thereafter, the pellets were fired at 500oC for 30 minutes for proper binding of silver on their surfaces, and then cooled to room temperature before taking dielectric measurement.

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Journal Pre-proof The prepared samples were studied by X-ray powder diffraction (XRD) using BRUKER D8 Advanced diffractometer with Cu Kα source using 1-D position sensitive detector [LynxEye] based on silicon drift detector technique. The XRD patterns were fitted with Rietveld refinement technique using full Prof suite software and 3D view of crystal structure was generated by VESTA program. Infrared spectra were recorded with a FTIR, Bruker, Germany, Model-Vertex 70 spectrometer using a KBr pellet for the 700-100 cm-1 region. The magnetisation measurement with respect to magnetic field and temperature were carried out using SQUID-VSM magnetometer of Quantum Design. The dielectric measurement of pure and doped LFO systems were done using Wyne-Kerr made LCR meter [model 4100]. Leakage current was measured by P-E loop tracer (Marine India). 3. Results and discussion 3.1 Structural Analysis 3.1.1

X-ray diffraction Analysis of crystallographic phase present in the LCFO systems was carried out by

Rietveld method using Fullprof suite (Fig 1). Rietveld fitted XRD patterns of LCFO are indexed with orthorhombic phase having Pbnm space group and found good match with standard JCPDS file no. 74-2203. However, close look of the spectra show a trace of impurity peak near 26o for LCFO-10 system, which is found to be matched with CeO2 phase (JCPDS file no.810792); indicate solid solubility limit of Ce in LFO is x ≤ 0.08. The refined unit cell and position parameter are summarized in Table-1 and considered relatively good fit based on lower χ2 values. Variation of lattice parameters (a and c) with Ce concentration (x) is displayed in Fig 1(b). The mutation is observed for x > 0.06 for both the lattice parameters. The lattice parameters ‘a’ and ‘c’ are found decreasing with increasing ‘x’ (for samples x = 0, 0.02, 0.04) while anomalous around x = 0.06. However, the unit cell volume is decreasing with increasing Ce doping concentration (See inset Fig 1(b)); the lattice contraction is 5

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Figure 1: (a) Rietveld refined XRD data file (b) Ce-content vrs lattice parameter ‘a’ and ‘c’ (c) Ce-content vrs spontaneous orthorhombic strain of LCFO systems.

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Journal Pre-proof edoping level in LFO

Lattice parameter

a (Ao)

Rietvel d fitted goodne ss of fit χ2

B (Ao)

C (Ao)

Volume of unit cell

Tolera nce factor

(Ao)3

Sponta neous orthor hombi c strain

Lattice distortion

Bond length

Bridging angle

(Ao)

a/c

b/c

Fe-Oa (Ao)

Fe-Ob (Ao)

Fe-Oc (Ao)

(in degree)

Fe-O-Fe

0

5.5688(2)

5.5349(4)

7.8756(5)

2.21

244.6906

0.9788

0.3432

0.7084

0.7071

2.01(1)

2.00(3)

2.02(5)

155.9(6)

2

5.5349(3)

5.5367(4)

7.8290(6)

1.46

240.9203

0.9776

0.3433

0.7072

0.707

1.96(2)

2.07(2)

2.12(6)

163.03(4)

4

0.5321(2)

5.5487(5)

7.8261(3)

1.04

240.2297

0.9762

0.3435

0.709

0.7069

1.82(1)

2.01(3)

2.37(3)

163.7(3)

6

5.5284(6)

5.5417(3)

7.8418(5)

1.40

240.2471

0.9748

0.3461

0.7067

0.705

1.92(3)

2.04(1)

2.43(4)

165.5(6)

8

5.5299(4)

5.5495(2)

7.8216(4)

1.01

240.0307

0.9735

0.3432

0.7095

0.7070

1.86(4)

2.01(5)

2.01(4)

163.3(6)

10

5.5394(3)

5.5455(7)

7.8161(4)

1.15

240.1007

0.9721

0.3409

0.7095

0.7081

1.70(1)

2.07(2)

2.01(2)

161.5(3)

Table-1: Rietveld refined structural data, lattice parameters, volume of unit cell, tolerance factor and spontaneous orthorhombic strain of LCFO. expected due to difference in their ionic radii of La3+(1.216 A0) and Ce3+ (1.034 A0)/Ce4+ (0.97 A0) [32-33]. The contraction in unit cell volume causes strong distortion in tilting of FeO6 octahedra. The octahedra buckled in order to fit inside the reduced volume. However dopant content induces mutation in lattice parameters (a and c, while b is almost constant) indicates non-uniformity in octahedral tilting. The dopant mediated mutation in ‘a’ and ‘c’ parameters for x > 0.06 can be due to increase of Ce4+ density for high content of Ce and charge unequilibrium is compensated by Fe3+ Fe2+ reduction. To get more clarification of the results, the spontaneous orthorhombic strain ‘s’ is calculated by the formula s = 2(ba)/(b+a). The increase of strain parameter ‘s’ for sample having x = 0, 0.02, 0.04, 0.06 and drop down for x = 0.08 and 0.1 (see Fig 1 (c)) evidence for anomalous structural distortion and FeO6 octahedral tilting for x > 0.06. The 3D view of crystal structure of pure and Ce-doped LFO systems were produced from XRD data by using VESTA program. A Schematic presentation of 3D structure of a LCFO cell is shown in the Inset Fig 1(a). The quantitative structural information like bond length, bond angle and bridging angle etc are estimated. The accuracy of these quantitative 7

Journal Pre-proof estimations of the structural data by VESTA are limited, but provided data trends; which can be used to understand the role of Ce in LFO. It has distorted orthorhombic phase with two equivalent site of O1 and O2 in the FeO6 corner sharing tilted octahedral with La ion occupying space between the octahedron (see Inset Fig 1 (a)). Geometrical coupling between the unit cell shape and three –dimensional steric rotation of the FeO6 octahedron as a rigid unit is reflected as rotostrictive coupling between strain and octahedron rotation. The distortion in FeO6 octahedron leads to change in bond length, bond angle and bridging angle. The substitutional effect of Ce in place of La in LFO exhibits decrease in bond length Fe-Oa and Fe-Oc; while mutation is noticed for x > 0.06. Its consequently affect on Fe-O-Fe bridging angle. This is attributed to change in co-valency of Fe-O and La-O and higher orthorhombic distortion in case of Ce doping. To confirm nano-dimensions of the LCFO system, TEM image of as-calcined LCFO-6 is shown in Fig 1(d); depicts average particles size of LCFO is in range of 10-12 nm. 3.1.2 Fourier Transform Infrared Spectroscopy (FTIR): FTIR spectra of LCFO systems are investigated in order to reveal lattice contraction effect on local vibration of LFO. The infrared absorption spectra of La1-xCexFeO3 are shown in Fig 2. According to symmetry calculations, the orthorhombic Pbnm phase for LFO should possess 25 IR active optical phonon modes (9Blu + 7B2u + 9B3u) in the range 50-645 cm-1. In the present study, it has been found that only eight are properly resolved. The disagreement in the numbers of bands with the group theory predictions can be explained by the fact that some bands are located very close to each other and hence cannot be resolved due to their overlapping. The lattice dynamics calculations for the isostructural orthoferrites show that modes detected below 200 cm-1 (1 and 2) can be assigned to translation motion of La3+ ions relative to FeO6 octahedra. 500-250 cm-1 region can be related to the bending Fe-O vibration 8

Journal Pre-proof and modes above 500 cm-1 correspond to stretching vibrations of Fe-O bonds [34-35]. In more detail, the strong absorption peak at 170 cm-1 (2) is associated with an external phonon mode, which arises due to the vibrations of La3+ ions against FeO6 octahedra. The absorption bands between 200-300 cm-1 (3 and 4) is correlated with oxygen octahedral tilting modes. The broad absorption peak (5) near 350 cm-1 is correlated with Fe3+-O2- bending vibrations. The transverse optic (TO) mode Blu (6) observed at 410 cm-1 is associated with O-Fe-O deformation vibration of the pervoskite LFO and the band (TO, B3u) at 540 cm-1 (7) is related

Figure 2: FTIR spectra of LCFO system to iron-oxygen (Fe-O) stretching vibrations. A small shoulder (8) appears at 600 cm-1 is characteristic features of rare earth orthoferrites.

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Journal Pre-proof It can be clearly observed that with progressive addition of Ce, the intensity of the observed band increases significantly while width decreases. Close view of the spectra depicts few significant modifications caused by Ce doping effect in LFO: (i) outset of satellites band near 220 cm-1 (see Inset Fig 2(a)) can be assigned to translational motion of Ce relative to FeO6 octahedra, (ii) split of (3) bands into three parts (see Inset Fig 2(b) and (iii) appearance of a satellite peak near 410 cm-1 (See inset Fig 2(c)). All these spectral changes indicate strong orthorhombic distortions in LFO perovskite structure. These modes greatly affect the changes of both Fe-O bond length and Fe-O-Fe bridging angle. Ce-induced structural distortions in LFO in terms of bond length, bond angle and bridging angle are ensured by FTIR spectra. In order to estimate oxygen content and concentrations of differently charged cations Fe2+ and Fe3+ cations in La1-xCexFeO3, iodometry titration method was used. However content of Ce3+ and Ce4+ are unabled to detect by chemical titration method accurately. X-ray absorption spectroscopy may be helpful, but it is not in the scope of present study.

Compound

Oxygen stoichiometry

Content (%)

Chemical formula

La1-xCexFeO3

Rietveld

Titration

Fe3+

Fe2+

x=0

3.02

3.04

94

6

La3+Fe3+0.94Fe2+0.06O2-3.02

x = 0.02

2.97

2.98

94

6

La3+0.98Ce3+0.02Fe3+0.94Fe2+0.06O2-2.97

x = 0.04

2.97

2.98

94

6

La3+0.96Ce3+0.04Fe3+0.94Fe2+0.06O2-2.97

x = 0.06

2.98

2.99

93.8

6.2

La3+0.94Ce3+0.059Ce4+0.001Fe3+0.938Fe2+0.062O2-2.98

x = 0.08

2.96

2.94

93.7

6.3

La3+0.92Ce3+0.075Ce4+0.005Fe3+0.937Fe2+0.063O2-2.96

x = 0.1

2.96

2.95

93.7

6.3

La3+0.9Ce3+0.09Ce4+0.01Fe3+0.937Fe2+0.063O2-2.96

Table-2: Estimated values of oxygen stoichiometry and content of Fe3+/Fe2+

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Journal Pre-proof In case of iodometry titration, the principle of electrical neutrality can be achieved as a result of both changes in valence of iron and oxygen index with the appearance of anion vacancies. The method of iodometry titration allows us to determine the oxygen index in LaFeO3-, which contains variable valence 3d-ions in Fe with an accuracy of  ± 0.02 per formula unit. Such a systematic error in the determination of the oxygen content and the corresponding was 0.02/3 relative error allow to calculate the filling factors for the La and Fepositions in a real LaFeO3- structure with an accuracy up to 1 %. The presence of oxygen vacancies are also estimated by the Rietveld Refinement of the structural data. The obtained values of oxygen stoichiometry and content of Fe3+ and Fe2+ are tabulated in table -2. 3.2 Dielectric Studies Frequency dependent dielectric constant (’) and dissipation factor (tan  ) are measured in the frequency range 20 Hz to 200 KHz and displayed in Figs 3(a) and (b). It can be noticed that all the samples exhibit dielectric dispersion where the values of both dielectric constant and loss decreasing rapidly with increasing frequencies. This behaviour is generally observed in dielectric material and consistent with combined response of orientational relaxation of dipoles and conduction of charge carrier. It can be understood as an intra well hopping probability of charge carrier dominates and dipole are unable to follow the field reversal in such small interval of time at high frequencies. The appreciable high value of real dielectric constant is obtained for pure LFO; while drop down by two orders by Ce-doping. This can be associated with electronic structure of Ce and Ce-O bond. Though, the 4f electron resides within the cerium atom core. It spatially extends 4f electron wavefunction and can hybridize with 2p state of oxygen. Depending upon the hybridization strength oxidation state of cerium (Ce3+/Ce4+) will be settled on. The hybridization strength is connected with the overlap of the Ce-4f electron wavefunction with 2p electron wavefunction of oxygen and thus

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Journal Pre-proof distance between Ce-O ions. As is known, Ce tends to be tetravalent (Ce4+) in compounds with Fe, Ni, Co, etc.; and in oxides, Ce can be trivalent (Ce3+) or tetravalent (Ce4+). So the existence of the both oxidation state of cerium (i.e. Ce3+ and Ce4+ ) in LFO can be realized. The substantially high dielectric constant in LFO system can be attributed to electron hoping between Fe2+ and Fe3+ leads to dipolar polarization. Substitution effect of La3+ by Ce4+ in LFO may produce partial suppression of Fe3+ content with formation of metastable Fe2+; leads to decrease in dipoles per unit volume and thus dipolar polarization. In order to understand whether the dielectric response of the samples is actually due to non-linear dielectric (e.g. ferroelectric) nature or by some artifact effect, we have analyzed the room temperature frequency dependent data in the light of universal dielectric response (UDR) model [36]. According to this model, localized charge carriers hopping between

Figure 3: Room temperature frequency dependent (a) dielectric constant, (b) dissipation factor, (c) plot between log (f) vrs (log f) of LCFO. spatially fluctuating lattice potentials not only produce conductivity but also give rise to dipolar effect. If it is true, there should be a linear behaviour of log f versus log (f) plot. It 12

Journal Pre-proof means that UDR phenomenon is responsible for dielectric response of these samples at given frequency regime. Fig 3 (c) shows log f versus log (f) plots of LCFO samples; exhibits linear behaviour and confirms UDR phenomenon is responsible for dielectric response. Dielectric loss plot of LCFO systems (Fig 3(b)) exhibit decrease in dielectric loss by about two orders of magnitude by Ce-doping in LFO. This behaviour is attributed to Ce induce suppression in the number of dipoles per unit volume aligned against the applied field. These results are consistent with the dielectric response of the corresponding materials. To probe the dielectric response in the proximity of the magnetic transition temperature of LCFO nanoparticles. Dielectric response was mapped in a frequency range from 20 to 105 Hz at 1 V (rms) and temperature range from 30 to 500oC (273 to 773 K). Figs 4 (a and b) show the temperature dependence of dielectric constant and loss of LCFO at constant frequency 10 KHz, respectively; Inset Figs 4(a) and (b) display the value for pure LFO system. Dielectric constant and dielectric loss are found increasing with temperature and anomalous around the magnetic transition temperature (~750 K) for almost all samples. The results clearly signify a coupling between dielectric and magnetic properties. The dielectric anomaly of pure LFO (see inset Figs 4a & b) is detected about 743 K, close to Neel temperature (TN = 750 K) where the magnetic phase transition from AFM to paramagnetic take place. For doped LFO system, dielectric anomaly is found to be shifted by around 30 K temperature (see Inset Fig 4(a)). The dissipation factor (tan ) curves of LCFO systems also display sharp anomaly near Neel temperature of LFO. It is clear evidence for magnetodielectric coupling of Ce-doped LFO system. However, dissipation factor of pure LFO system shows (see inset of Fig 3(b)) two relaxation peaks around 550 and 673 K, respectively. These can be associated with Debye like dipolar relaxation response due to intrinsic dipoles, which are suppressed by Ce doping effect. The dielectric response of pure LFO is closely matched with earlier report by Bhargav et al [37]. 13

Journal Pre-proof The dielectric constants as well as loss both are found increases with temperature due to thermally activated processes. The electron hopping between Fe3+ and Fe2+ ions present on the octahedral site is thermally activated by increasing temperature. This hopping causes total displacements in the direction of external applied electric field. This in turn, enhances their contribution to the space charge polarization leading to an increase in the value of ’. Overall decrease in tan  is observed in the Ce-substituted samples for any particular frequency and temperature; it decreases DC conductivity of the materials. 3.3 Magnetic studies Magnetization (M) versus temperature (T) profile of the samples for field cooling (FC) and zero field cooling (ZFC) conditions are displayed in Fig. 4(c & d, respectively). Magnetic transitions are detected at around 750 K for almost all samples and for both i.e. ZFC and FC measuring conditions. The temperature is closely related to Neel temperature of LFO system. The dielectric peak (’) versus (T) and (Tan ) versus (T) are closely matched with the magnetic transition temperature of the LCFO systems; evidence for magneto-dielectric coupling. These results demonstrate that paramagnetic as well as paraelectric transitions outset in the systems as temperature (T) approaches to 750 K. As the temperature decrease below 750 K, a magnetic order starts setting in the samples and thus systematic enhancement in magnetization. Ce-doping effect in LFO is found to enhance magnetization by two orders at any given temperature. In rare earth orthoferrites system, the magnetic moments of iron and rare earth atom are the source of magnetic properties. In Ce- doped LFO system, the La3+ is non-magnetic, however Ce3+ is magnetic. It induces exchange interaction Ce3+-O-Fe3+ with Fe3+-O-Fe3+ which seems to be governing the magnetization patterns in LCFO systems; whereas the magnetization in LFO is only due to Fe3+-O-Fe3+ interaction. While, La3+-O-Fe3+ and Ce4+-O-Fe3+ interactions are non-magnetic. So in Ce-doped LFO system, magnetization

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Figure 4: Temperature profile of (a) dielectric constant, (b) dissipation factor and (c) Magnetization curve of LCFO @500 Oe.

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Figure 5: (a) Room temperature M-H curve, (b) Coercivity and remanant magnetization curve and (c) dopant content vrs room temperature magnetization of LCFO

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Journal Pre-proof arising from Ce3+-O-Fe3+ and Fe3+-O-Fe3+ in coupled with lattice contraction effect attribute to the two order enhancement in magnetization. Figs 5 (a-f) show M-H loops of LCFO systems at 300 K between  7 KOe. The shape of hysteresis curves show that the magnetization does not saturates up to the applied field. Experimental values of coercivity, remnant magnetization and room temperature magnetization exhibit (see Fig 5(g and h)) significant enhancement in the magnetization of doped systems. These results are evidence for enhancement of ferromagnetic contribution of LFO by Ce doping. The increase in magnetization and ferromagnetism could be associated with disordering of antiparallel spin arrangement. In G-type anti-ferromagnetic spin arrangement of LaFeO3, the anti-parallel spin disordering in Fe3+-O2--Fe3+ linkages is due to change in Fe3+-O2- distance and increase in bridging angle as detected from Rietveld results with progressive substitution of Ce into LFO. Both Fe-Oa and Fe-Oc bond length decrease and corresponding Fe3+-O2--Fe3+ bond angle increased from 155 to 165. In addition to that Ce3+ O2--Fe3+ attribute constructively to the spin alignment. This accounts for the increased of magnetization and ferromagnetism in the system. Such an observation is consistent with analysis of interaction for Fe3+-O2--Fe3+ linkages by Gilleo et al. [38]. 3.4

Electrical behaviour Leakage current is key technical limitation; as it deteriorates performance of the

multiferroic system; so system having minimum leakage current is preferable. Leakage current (J) as a function of applied electric field (E) for LCFO systems at 300 K are studied (see Fig 6). The pure LFO sample has a higher leakage current density and linear decrease in J with an increase in Ce content. Mutation in J value is observed at x > 0.06. At an applied field 3.5 KeV, the leakage current density of this samples are 2.94 X 10-4, 8.82 x 10-5, 5.29 x

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Journal Pre-proof 10-5, 5.29 x 10-5, 2.64 x 10-5, 4.85 x 10-5, 7.05 x 10-5 for x = 0.0, 0.02, 0.04, 0.06, 0.08, 0.1, respectively. The decrease in J with increasing x value might be explained as the reduction

Figure 6: Leakage current as a function of applied electrical field in La1-xCexFeO3 of charge defects oxygen vacancies and the transition from Fe3+ to Fe2+. To prove technical feasibility of LCFO system as single phase room temperature multiferroics, we compare leakage current of pure LFO as well as LCFO systems with BiFeO3 (BFO) system [39-40], there are two order decrease in leakage current of LCFO over BFO. 3.5 Correlation of the observed magnetic properties to changes in structural features: Dopant induced increase in magnetic susceptibility in LFO could be understood from structural features. Origin of AFM in LFO system is associated with spin configuration of Fe3+ ions, which is described by 4(Gx Ay Fz), where Gx represents the basic AFM spin arrangement of the Fe3+ ions along the a-axis and Fz the ferromagnetic (FM) spin arrangement along c-axis due to canting of the Gx spins. Ay also represents the AFM spin-arrangement along b-axis due to canting of the Gx spins. The super-exchange interaction that is coupling of magnetic moments of Fe3+- with oxides increases as the Fe3+-O2- distance decreases and as the Fe3+-O2- -Fe3+ angle approaches to 180oC. Rietveld results showed that with the progressive substitution of Ce into LFO proceed with unit cell contraction and thus change in octahedral 18

Journal Pre-proof tilt. It consequent with Fe-Oa bond distances decreased (from 2.01 to 1.70 Ao); while Fe-Oc bond distance increased (from 2.02 to 2.43 Ao) and corresponding bond angle Fe3+-O2 - - Fe3+ increased from 155.9 to 165o. It modifies anti-parallel spin alignment along a-axis and canted spin along c-axis. This accounts for the two order increase of magnetization and ferromagnetism of Ce-doped LFO as compared to LFO. However, magnetization, coercivity and remanant magnetization are varying non-linearly with dopant content.

The dopant

induced structural non-linearity (i) lattice parameter deviation from Vegard’s rule, (ii) spontaneous orthorhombic strain, which is highest for x = 0.06 and (iii). Dopant induced variation in oxygen non-stoichiometry and charge density Fe3+/Fe2+ as confirmed by Rietveld refinement and iodometric titration; which is due to enhancement in density of non-magnetic Ce4+ over magnetic Ce3+ at high doping level, all these are responsible for variation trends of magnetization, coercivity, dielectric behaviour with reduction in leakage current of LCFO. However, magnetic ordering temperature contradict this trends, which is highest for LCFO-4 system amongst all the studied compositions of Ce; to understand the effect of dopant on magnetic ordering temperature, we need to do further experimentation. It is the part of our next publication. Conclusion In summary, we have successfully prepared pure and Ce-doped LFO system by microwave assisted gel combustion method. Structural studies by XRD confirmed the formation of highly crystalline single phase of La1-xCexO3 systems for x ≤ 0.08; while a impurity trace related to CeO2 phase is detected for x = 0.10. So, solid solubility of Ce in LFO is x < 0.10. Unit cell visualization of pure and Ce doped LFO are also carried out using Rietveld refined XRD data by VESTA program. The obtained structural parameters from Reitveld and VESTA show non-linear variation of bond length, bond angle and lattice 19

Journal Pre-proof parameter with dopant concentration and anomalous around x = 0.06. FTIR spectra confirmed dopant induced structural defects. New phonon modes related to Ce3+ translational vibration with FeO6 octahedra and strong octahedral distortion are demonstrated from the FTIR study. Room temperature dielectric behaviour of pure and doped LFO indicated decrease in dielectric constant with Ce doping concentration. Magneto-dielectric coupling are ensured from the temperature derived dielectric constant, dielectric loss factor and magnetization behaviour. Isothermal magnetization curve shows increase in magnetisation of Ce- doped LFO system with increase in remanent magnetisation and coercivity. M-T curve shows decrease in magnetisation with temperature which indicates ferromagnetic nature of LCFO system. The prospective of the work is that Ce doping enhances magnetization and reduced dissipation factor of LFO system by around two orders. The correlation between the dielectric and magnetic properties with low leakage current for LCFO systems prove the potential to be used as room temperature ME materials. Acknowledgement: SAA and JS acknowledge UGC-DAE-CSR Indore for financial funding and experimental support to carry out the research work by project CSR-IC/CRS-81/2111. References 1. A.G. Zhdanov, T.B. Kosykh, A.P. Pyatakov, A.K.Zvezdin D.Viehland, Peculiarities of incommensurate–commensurate phase transitions in multiferroics, J Magn. and Mag. Materials, 300 (2006) e437–e439 2. A.K. Zvezdin, A.M. Kadomtseva, S.S. Krotov, A.P.Pyatakov, Yu.F.Popov, G.P.Vorob’ev, Magnetoelectric interaction and magnetic field control of electric polarization in multiferroics, J Magn. and Mag. Materials 300 (2006) 224–228 3. M.P. Singh , W. Prellier, L. Mechin, Ch. Thin Solid Films Simon, B. Raveau, Can multiferroics be synthesied by superlattice approach?, 515 (2007) 6526–6531 20

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Journal Pre-proof Table Caption: 1. Rietveld refined structural data, lattice parameters, volume of unit cell, tolerance factor and spontaneous orthorhombic strain of LCFO. 2. Estimated values of oxygen stoichiometry and content of Fe3+/Fe2+

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Declaration of interests  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Shraddha Shirbhate

Smita Acharya

Uday Deshpande

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Highlight:     

Ce-doped LaFeO3 system is explored as single phase room temperature multiferroics. Ce in LFO enhances magnetization of LFO by around two orders. Temperature driven dielectric and magnetic ordering outset at around 750 K Magneto-dielectric couplings are confirmed in the study. LCFO system shows potential to be used as room temperature multiferroic.