Multiferroicity in La1/2Nd1/2FeO3 nanoparticles

Solid State Sciences 37 (2014) 55e63

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Multiferroicity in La1/2Nd1/2FeO3 nanoparticles Sadhan Chanda a, *, Sujoy Saha a, Alo Dutta a, A.S. Mahapatra b, P.K. Chakrabarti b, Uday Kumar c, T.P. Sinha a a

Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata 700009, West Bengal, India Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan 713104, India c Department of Physical Science, Indian Institute of Science Education and Research, Kolkata, Mohanpur 741252, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 April 2014 Received in revised form 25 July 2014 Accepted 13 August 2014 Available online 27 August 2014

Nano-sized La1/2Nd1/2FeO3 (LNF) powder is synthesized by the solegel citrate method. The Rietveld refinement of the X-ray diffraction profile of the sample at room temperature (303 K) shows the orthorhombic phase with Pbnm symmetry. The particle size is obtained by transmission electron microscope. The antiferromagnetic nature of the sample is explained using zero field cooled and field cooled magnetisation and the corresponding hysteresis loop. A signature of weak ferromagnetic phase is observed in LNF at low temperature which is explained on the basis of spin glass like behaviour of surface spins. The dielectric relaxation of the sample has been investigated using impedance spectroscopy in the frequency range from 42 Hz to 1 MHz and in the temperature range from 303 K to 513 K. The ColeeCole model is used to analyse the dielectric relaxation of LNF. The frequency dependent conductivity spectra follow the power law. The magneto capacitance measurement of the sample confirms its multiferroic behaviour. © 2014 Elsevier Masson SAS. All rights reserved.

Keywords: Nanostructures Solegel chemistry Antiferromagnetic Impedance spectroscopy Dielectric properties Multiferroicity

1. Introduction Multiferroicity in perovskite oxides has been investigated extensively in recent years due to their potential application in solid oxide fuel cells, non-volatile magnetic memory devices and ultrasensitive magnetic read-heads of modern hard disk drives [1e4]. Among these multiferroic perovskites, the rare-earth orthoferrites, RFeO3 (R ¼ rare earth) have been investigated for their usefulness in a wide variety of applications such as in gas sensors [5,6] and photo-catalysis [7]. They have also been potentially used for data storage applications due to their antiferro
el temperature ranging from 620 K to magnetic properties with Ne 740 K [8]. The high value of the local internal electric field produces the spontaneous polarization leading to the on-set of ferroelectric ordering, and the interaction between Fe-cations via oxygen anion induces super-exchange interaction leading to the on-set of magnetic ordering in the same system [9]. The studies of the rare earth transition metal oxides have revealed many fascinating aspects. Among all the rare earth orthoferrites, the LaFeO3 is an orthorhombically (Pbnm) distorted

* Corresponding author. Tel.: þ91 033 23031191; fax: þ91 033 23506790. E-mail addresses: [email protected], [email protected] (S. Chanda). http://dx.doi.org/10.1016/j.solidstatesciences.2014.08.006 1293-2558/© 2014 Elsevier Masson SAS. All rights reserved.

antiferromagnetic (AFM) material having the highest Neel temperature, TN ¼ 740 K [10]. It has been widely investigated to characterise the verities of properties such as multiferrocity [11], exchange bias [12], colossal dielectric response [13] etc. Some attempts have been made to increase the value of the magnetization by doping Al3þ and Sb3þ at La3þ in LaFeO3 [14,15]. Wu et al. have investigated the hysteresis and magnetic behaviour of RFeO3 (R ¼ La, Nd) nanocrystalline powder [16]. NdFeO3 has an orthorhombically distorted perovskite-type structure with space group Pbnm. In NdFeO3, there exists three major magnetic interactions (FeeFe, NdeFe and NdeNd) [17]. These competing interactions determine their interesting magnetic properties and lead to a number of applications. In this work, a single phase nano-sized La1/2Nd1/2FeO3 (LNF) powder is prepared by solegel citrate method and its structural, dielectric, magnetic and multiferroic properties are investigated. La3þ has no magnetic moment while Nd3þ has magnetic moment of ~3.3e3.7 mB. Nd3þ ion is chosen for two way improvement of the multiferroic behaviour of LaFeO3: (i) the enhancement of the magnetisation for the nanoparticles and (ii) the increment of the electrical properties by increasing the value of the dielectric constant and decreasing the dielectric loss. To the best of our knowledge, no report on the structure, magnetic and dielectric properties and/or the multiferroic behaviour of Nd3þ doped LaFeO3 is found in the literature.

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It is to be mentioned that most of the reported RFeO3 powders are prepared from high temperature solid-state reaction from their corresponding pure oxides, which sometimes affect atomic stoichiometric ratio of the materials due to oxygen vacancy, and formation of undesirable phases [18]. To prepare phase pure nanopowders solegel citrate method [6] has been used. 2. Experimental details 2.1. Sample preparation The perovskite oxide LNF in powder form was synthesized by the solegel citrate method [6,19]. At first, reagent grade of La(NO3)3.9H2O (Alfa Aesar), Nd(NO3)3.6H2O (Alfa Aesar) and Fe(NO3)3.9H2O (Alfa Aesar) were taken in stoichiometry ratio and separately dissolved in de-ionized water by stirring with a magnetic stirrer. The obtained clear solutions were then mixed together. Citric acid and ethylene glycol (EG) were added to this solution drop wise according to the molar ratio of {La3þ þ Nd3þ, Fe3þ}:{citric acid}:{ethylene glycol} ¼ 1:1:4 to form a polymericmetal cation network. The solution was stirred at 348 K using a magnetic stirrer for 4 h to get a homogeneous mixture of brown colour and then the solution was dried at 393 K to obtain the gel precursor. In the next step, the gel was ground to get a fine powder. The powder was calcined at 973 K in air for 3 h to get the powder of LNF. The calcined sample was pelletized into discs using polyvinyl alcohol as binder. Finally, the discs were sintered at 1023 K and cooled down to room temperature (RT ~ 303 K) by cooling at the rate of 1 K/min. 2.2. Sample characterization The determination of lattice parameters and the identification of the phase was carried out using a X-ray powder diffractometer having Cu-Ka radiation (Rigaku Miniflex II) in the 2q range of 15e100 by step scanning at 0.02 per step at RT. The refinement of crystal structure was performed by the Rietveld method with the Fullprof program [20]. The background was fitted with 6-coefficients polynomial function, while the peak shapes were described by pseudo-Voigt profiles. Throughout the refinement, scale factor, lattice parameters, positional coordinates (x, y, z) and thermal parameters were varied and the occupancy parameters of all the ions were kept fixed. The particle size and selected area electron diffraction (SAED) pattern of the sample were studied by the high resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2, 200 KV). A small piece of sintered pellet was used to measure the magnetic parameters. The vibrating sample magnetometer (VSM; Lakeshore) was used to measure the magnetization in the temperature range from 300 K to 800 K at an applied magnetic field of 1 kOe. To investigate the magnetic behaviour of the sample at low temperature, magnetization measurement was performed in the temperature range from 5 K to 300 K in zero field-cooled (ZFC) and field cooled (FC) modes at an applied magnetic field of 1 kOe by a Quantum Design SQUID magnetometer. Hysteresis loop for the sample was taken at 5 K, 300 K and 793 K. For electrical measurements, both the sides of sintered pellet were polished. Two thin gold plates were used as electrodes. The impedance, conductance and phase angle were measured using an LCR meter (HIOKI-3532) in the frequency range from 42 Hz to 1 MHz at the oscillation voltage of 1.0 V. The measurements were performed over the temperature range from 303 K to 513 K using an inbuilt cooling-heating system. Each measured temperature was kept constant with an accuracy of ±1 K. The complex dielectric constant ε* (¼1/juCoZ*) was obtained from the temperature

dependence of the real (Z0 ) and imaginary (Z00 ) components of the complex impedance Z* (¼Z0 p þffiffiffiffiffiffi jZffi 00 ), where, u is the angular frequency (u ¼ 2pn) and j ¼ 1: Co ¼ εoA/d is the empty cell capacitance, where A is the sample area and d is the sample thickness. The polarization vs. electric field (PE) loop was observed by a PE loop tracer (Multi-ferroic Precission Premier-II supplied by Radient Technologies, USA), where the maximum applied electric field was 20 kV/cm. The CeV loop was measured at 1 kHz using an LCR meter (HIOKI-3532) with an external dc bias voltage unit (HIOKI-9268). DC IeV characteristic of the sample was measured by using a source measuring unit (Keithley SMU-236). To measure the magnetoelectric coupling of the sample, dielectric constant was measured at RT in the presence and absence of magnetic field. 3. Results and discussion 3.1. Structural analysis The room temperature XRD profile of LNF is shown in Fig. 1. The Rietveld refinement of the XRD profile is performed in the space group Pbnm ðD16 Þ: The unit cell parameters, reliability factors and 2h crystallographic positions of constituent atoms are listed in Table 1. The good agreement between the observed (symbols) and refined profile (solid line) of the XRD pattern indicates the single phase formation of the material with Pbnm space group. A schematic presentation of the LNF cell is shown in Fig. 2(a), with the following distribution of ions in crystallographic positions: La3þ and Nd3þ ions in (4b), Fe3þ ion in (4c) and O2 ions in (4c) and (8d). Fe3þ ions are surrounded by six O2 ions, constituting FeO6 octahedra. The orientations of the FeO6 octahedra in LNF can be described by the aacþ tilting Glazer system [21] which indicates that the octahedra rotate along the Cartesian axes x and y in consecutive layers in opposite directions and along the z-axis in the same direction (for space group Pbnm) as shown in Fig. 2(b). The octahedral tilts {(180  4)/2} for two oxygens in Pbnm symmetry can be calculated from the FeeOeFe bond angles (4). The tilt values obtained from the XRD data are a ¼ 11.15 and cþ ¼ 16.88 . The crystallite sizes (d) of LNF nanoparticles are also estimated by DebyeeScherrer's equation: d ¼ 0.94l/Bcosq where, d is the crystallite size, l is the wavelength of radiation used, q is the Bragg angle and B is the full width at half maxima (FWHM) on 2q scale. The crystallite sizes are found to be in the range from 30 to 70 nm. The HRTEM micrographs of the sample are shown in Fig. 3(aec). The particles are found to be segregated as observed from Fig. 3(a).

Fig. 1. Rietveld refinement plot of LNF at room temperature.

S. Chanda et al. / Solid State Sciences 37 (2014) 55e63 Table 1 Structural parameters for LNF as obtained from Rietveld analysis of XRD pattern. Atoms

Site

x

La/Nd 4c 0.504(1) Fe 4b 0.0000 OI 4c 0.403(7) OII 8d 0.781(6) ¼ 149.52 ¼ 155.74

y

z

Biso (Å2) Bond length (Å)

0.5367(4) 0.5000 0.010(5) 0.204(5)

0.2500 0.0000 0.25 0.037(2)

0.45 0.35 0.69 0.48

FeeO1 (x2) ¼ 2.022 FeeO2 (x2) ¼ 2.056 FeeO2 (x2) ¼ 1.946

Cell parameters: a ¼ 5.5013(4) Å, b ¼ 5.5664(4) Å, c ¼ 7.8057(6) Å. Reliability factors: Rp ¼ 4.99, Rwp ¼ 6.32, Rexp ¼ 5.44, c2 ¼ 1.30.

The sizes of the particles are estimated to be in the range from 40 to 75 nm. Fig. 3(b) indicates the homogeneous orientation of the lattice planes with interplanar spacing of 0.27 nm which correlates with the d spacing value of (112) plane of the system. The SAED pattern (Fig. 3(c)) shows bright spots indicating that particles are well crystalline in nature. 3.2. Magnetic properties Fig. 4(a) shows the temperature dependence of magnetization (M) of the sample from 300 to 800 K under an applied magnetic field of 1 kOe. The value of M decreases slowly with the increase of temperature and shows an antiferromagnetic to paramagnetic
el temperature, TN). Fig. 4(b) shows the transition at 728 K (¼Ne MH curves of LNF at 300 K and 793 K. The lack of the saturation of the magnetization at 300 K supports the antiferromagnetic behaviour of LNF. Above TN, a paramagnetic region is observed which is justified by linear nature of MH loop at 793 K. The ZFC and FC magnetizations of the sample are shown in the temperature range from 5 K to 300 K in Fig. 4(c). It is observed that the magnetization (M) first increases slowly with the decrease of temperature and then increases sharply below 35 K. The ZFC magnetic hysteresis loop of LNF at 5 K is shown in Fig. 4(d). The observed linear MeH loop at 5 K (Fig. 4(d)) indicates the presence of a predominate AFM interaction. Moreover, a weak FM

57

component is also observed in the loop. The sharp increase of magnetization below 35 K (Fig. 4(c)) may be due to the weak FM nature of the material. Orthoferrite LNF has two magnetic sublattices (Fe3þ and Nd3þ) which are coupled through a negative exchange interaction [22,23]. The iron sublattice is ordered into a slightly canted antiferromagnetic structure with a weak ferromagnetic moment which is perpendicular to the antiferromagnetic axis [24], while the magnetic moments of Nd ions have negative temperature coefficient. Below 35 K, the magnitude of Nd moments is larger than that of Fe cations and ferromagnetic-like situation which is also revealed by ZFC curve may occur [25]. However, the moments of Nd ions decrease with increasing temperature which induces monotonical drop in magnetization up to 35 K. Thereafter, the antiferromagnetism of Fe sublattice becomes prominent. In the inset of Fig. 4(d) the expansion of small region between 2.5 and 2.5 kOe of the hysteresis loop is shown. It can be seen that the values of coercivity HC1 (¼880 Oe) and HC2 (¼þ620 Oe) are not equal. This indicates the presence of an exchange bias in LNF. The strength of exchange bias effect can be understood in terms of exchange field HE which is defined as [26]:

H þ HC2 ; HE ¼  C1 2

(1)

where HC1 and HC2 are the left and right side field values respectively when magnetization becomes zero in hysteresis loop. The value of HE for LNF is found to be 130 Oe. The presence of exchange bias effect and the observed open hysteresis loop (shown in the inset of Fig. 4(c)) even at high field 30 kOe is a signature of uncompensated surface spins. Due to the large surface/volume ratio in nanoparticles, the effect of surface disorder is large which may give rise to spin glass like behaviour of surface spins. In principle, the spin glass like behaviour of the surface spins in nanoparticles comes due to the magnetic frustrations and disorder, which is different from the geometrical frustration occurring in systems with special symmetries and no disorder in spin [27]. Besides the magnetic frustrations and disorder of the surface spins, the exchange bias anisotropy in LNF (observed from the

Fig. 2. (a) Schematic presentation of the LNF orthorhombic unit cell. The Fe atoms are located at the centres of the FeO6 octahedra. The OI oxygen atoms located along z axis and the OII oxygen atoms located in the planes parallel to xy plane. (b) A view looking down the z axis of two adjacent layers of octahedral for LNF (aacþ tilt system).

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in ε0 (in Fig. 5(a)) and the respective relaxation peaks in ε00 (as shown in Fig. 5(b)) may be ascribed to the dipole polarization effect, which appears at radio-frequency range. The polarization mechanism in the material depends solely on the two effects, the frequency of applied electric field and temperature. At fixed temperature, if an alternating field is applied then the polarization may fully develop at sufficiently low applied frequencies (the polarization and the field are in phase) but when the frequency of the applied field is very high, the field is reversed before the polarization has responded and no response will be the result. The magnitude of the polarization thus drops off as the frequency is increased. At low temperature, the electric dipoles freeze through the relaxation process and there exists decay in polarization with respect to the applied electric field, which is responsible for the sharp decrease in ε'. When the temperature is high, the rate of polarization formed is quick, and thus the relaxation occurs in high frequency as shown in Fig. 5(b) for LNF. At high temperature, ε' increases in the low frequency range ( 0 signifies that the relaxation has a distribution of relaxation times, which gives a broader peak shape than a Debye peak. Moreover, when the electrical conductivity is dominated at the low frequency range for the sample, a contribution term by electrical conduction is generally added to the relaxation equation. The modified ColeeCole equation incorporating the conductivity term is given as [29,31]

presence of an asymmetry in the remanence magnetization) can be explained by the deviation of the surface spins from the collinear arrangement of the core spins due to spin canting which leads to the uncompensated surface spins. This effect enhances the weak FM interaction. The enhanced FM coupling facilitates the formation of ferromagnetic interaction between the surface spins in LNF nanoparticles. The positive value of HE also supports the ferromagnetic nature of surface spins [12]. Thus, the LNF nanoparticles can be supposed to be composed of an AFM core and FM spin glass like surface shell.

where s* (¼s1 þ js2) is the complex conductivity. From the above relation, the complex permittivity can be decomposed into the real and imaginary parts

3.3. Dielectric relaxation

and

The angular frequency u (¼2pn) dependence of the real (ε0 ) and imaginary (ε00 ) parts of complex dielectric constant (ε*) for LNF are shown in Fig. 5 as a function of temperature. The step-like decrease

ε ¼

ε* ¼ ε∞ þ

εs  ε∞ 1a

1 þ ðjutÞ



j

s* ε o us

n

 o 1a ðutÞ 1 þ sin ap=2 0 s   ε ¼ ε∞ þ þ 2s 1 þ 2ðutÞ1a sin ap=2 þ ðutÞ22a εo u

00

εs  ε∞

(2a)

ðεs  ε∞ ÞðutÞ1a cosðap=2Þ s   þ 1s 1a 22a ε u o sin ap=2 þ ðutÞ 1 þ 2ðutÞ

(3)

(4)

S. Chanda et al. / Solid State Sciences 37 (2014) 55e63

59

Fig. 4. (a) Temperature dependence of the magnetization of LNF measured at 1 kOe from RT to 800 K, (b) Hysteresis curves at RT and 793 K, (c) Temperature dependence of the magnetization of LNF after ZFC and FC measured at 1 kOe from 5 K to RT, (d) Hysteresis curve at 5 K.

Here s1 is the conductivity due to the free charge carriers (dc conductivity) and s2 is the conductivity due to the space charges (localized charges) and s is a dimensionless exponent (0 < s < 1). Equations (3) and (4) show the contributions of conductivity to the dielectric constant (ε0 ) and dielectric loss (ε00 ). The first term of Equation (4) is the part of the dielectric relaxation loss due to permanent dipole orientation or other motions which do not involve long-range displacement of mobile charge carriers, while the second term is the part of the losses associated with long-range migration of carrier response. The above equations indicate that the localized charge carriers at defect sites and interfaces (s2) have large contribution to the dielectric permittivity, whereas the free charge carrier (s1) contributes to the dielectric loss. The experimental data for both ε0 and ε00 have been fitted well by using Equations (3) and (4) in the temperature range from 393 K to 453 K over the entire range of frequencies as shown by the solid lines in Fig. 5(a) and (b) respectively. Thus, it can be concluded that the two main factors, dipolar and conductivity relaxations are responsible for the dielectric relaxation in LNF. All the parameters used in the fitting of Fig. 5 are listed in Table 2. It is also important to mention that the values of s tend to increase (s / 1) with increasing temperature, indicating that the carrier polarization mechanism is weakly dispersive at higher temperature, which might be attributed to some barrier height extracted. To get an idea about the type of relaxation response in these materials, it is necessary to find out the activation energy of relaxation process. At a temperature T, the most probable relaxation time corresponding to the peak position in ε00 vs. log u is proportional to exp(Ea/kBT) (Arrhenius law). The linear fit of the experimental data as shown by the solid line in the inset of Fig. 5(b) gives the activation energy of 0.32 eV. Such a value of activation energy indicates that the conduction mechanism in the samples may be due to the polaron hopping based on the electron carriers. In hopping process, the electron disorders its surroundings, by moving its neighbouring atoms from their equilibrium positions causing structural defects in the B perovskite sites of the system. Fig. 6 shows the complex impedance plane plots of the material at different temperatures. At each temperature, impedance plane

plots show two semicircular arcs, a larger one at low frequency and a smaller one at the higher frequency side. The smaller arcs are magnified and shown in the inset of Fig. 6(b) for the material. The two semicircular arcs in well resolved condition have been shown in the inset of Fig. 6(b) for the temperature 423 K. The appearance of two arcs in impedance plane plots at each temperature indicates the presences of two types of relaxation phenomena with two different relaxation times. Both the semicircular arcs have been found to be depressed with their centre below the real axis indicating the heterogeneity and deviation from the ideal behaviour [32]. In order to correlate the electrical properties of LNF with its microstructure, an equivalent circuit model consisting of two parallel RQ circuits connected in series (as shown in the inset of Fig. 6(a)) has been employed to interpret the nature of impedance plane plots [33]. Here R and Q are the resistance and the constant phase element for grain interiors (Rg, Qg) and grain boundaries (Rgb, Qgb) respectively. The constant phase element (CPE) is used to explain the non-ideal behaviour of the capacitance which may have its origin in the presence of more than one relaxation process with approximately similar relaxation times. The capacitance from the concept of the CPE is given by the following relation, C ¼ Q1/nR(1n)/ n , where the parameter n estimates the deviation from ideal capacitive behaviour, n is zero for pure resistive behaviour and is unity for capacitive one. The high resistive nature of grainboundaries may be attributed due to the presence of dangling bonds and non-stoichiometric distribution of oxygen on the grain boundaries, which can act as carrier traps and form the barrier layer for charge transport. The capacitance of this region also becomes high because of the inverse proportionality between the capacitance and the thickness (d) of the grain-boundary layer (C f 1/d). The response of grain boundaries, due to their higher resistance and capacitance, lies at lower frequencies compared to the response of grains. So, we can conclude that the larger (low frequency) and smaller (high frequency) semicircular arcs are due to grain boundary and grain interior respectively. The fitted parameters Rg,, Rgb, Cg, Cgb, ng and ngb are listed in Table 3 for the material. The decrease in the resistance of grains and grain boundaries may be

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S. Chanda et al. / Solid State Sciences 37 (2014) 55e63

sðuÞ ¼ sð0Þ þ Aun

(5)

where s(0) is the dc conductivity and A depends on temperature and exponent n is temperature and material dependent. The term Aun denotes the frequency dependence and characterizes all dispersion phenomena. The exponent n has been found to behave in a variety of forms as a constant, decreasing and increasing with temperature, but always varying within 0 < n < 1. However, in general, the frequency dependence of conductivity does not follow the simple power law as given above but well described by the double power law given as [35]

sðuÞ ¼ sð0Þ þ A1 un1 þ A2 un2

Fig. 5. Frequency (angular) dependence of ε0 (a) and ε00 (b) at various temperatures for LNF. Solid lines are the fitting of experimental data using Eqs. (3) and (4). Inset is the Arrhenius plot of most probable relaxation time where the symbols are the experimental data points and the solid line is the least-squares straight-line fit.

due to the thermal activation of the localized charges. Two types of thermal activations, i.e., carrier density in the case of band conduction and carrier mobility in case of hopping, are responsible for the reduction in the resistance with temperature. In order to understand the effect of the conductivity on the dielectric properties of the nanoceramic, the frequency dependence of ac conductivity has been characterized over the temperature from 303 K to 513 K. The angular frequency dependent s (in Fig. 7) shows two plateaus, one at low frequency and other at high frequency region. Although at low temperature two plateaus are observed, at high temperature the plateaus are moved to the high frequency side and the high frequency plateau has gone out from the experimental investigation range. The low-frequency plateau represents the total conductivity whereas the high-frequency plateau represents the contribution of grains to the total conductivity. The presence of both the high and low-frequency plateaus in conductivity spectra suggests that the two processes are contributing to the bulk conduction behaviour in LNF. One of these processes relaxes in the high-frequency region and the contribution of the other process appears as a plateau in the high-frequency region. The total conductivity at a given temperature can be expressed by the power law [34]

(6)

The experimental data in high temperature side are fitted by single power law with single n value for a temperature whereas the data in low temperature region are fitted with double power law with two n values as shown by the solid lines in Fig. 7. The exponent n in the lower frequency region is found to be in the range of 1.5e2 and increases with increase of temperature. In the high-frequency region the values of n are found to be in between 0.3 and 0.4. The lower and higher frequency regions represent the non-diffusing and localized diffusing modes respectively, and based on this theory Ishii et al. [36] have predicted that the exponent n is greater than 1 and tends to attain 2 with increasing temperature in lower frequency region whereas it is less than 1 in the higher frequency region. Fig. 8(a) illustrates the room temperature ferroelectric hysteresis loop (PE) of the sample observed at 100 Hz frequency for different applied electric field. The tendency of saturation increases with the increase of maximum applied electric field. The values of maximum polarization (Ps), remanent polarization (Pr) and the electric coercivity (Ec) are found to be 0.17 mC/cm2, 0.029 mC/cm2 and 2.46 kV/cm, respectively. The observed loop indicates the ferroelectric ordering of dipole moment configuration due to spontaneous polarization of the sample. We have also recorded the PE loops of the sample at RT by changing the time window of measurements i.e., the frequency of the measuring system which are shown in Fig. 8(b). The value of maximum polarization of the sample decreases with the increase of frequency and the shape of hysteresis loops changes with frequency. This change in the shape and polarization is quite likely due to the change of ferroelectric response of the sample with the variation of measuring time window. The ferroelectric behaviour of LNF at RT is also confirmed by its capacitanceevoltage (CeV) characteristic as shown in Fig. 9. The ''butterfly'' nature of the CeV curve suggests a weak ferroelectric behaviour at room temperature. This nature is also observed for other multiferroic material [37]. The hysteretic currentevoltage (IeV) characteristic of LNF at RT as shown in the inset of Fig. 9 (which is found to be asymmetric with respect to the polarity of applied voltage) may be due to the ferroelectric resistive switching effect [38]. The observed IeV characteristic supports the weak ferroelectric behaviour of LNF at RT. In order to demonstrate the coupling between electric and magnetic polarizations in LNF nanoceramics, we have carried out the in-situ measurement of changes in capacitance with the

Table 2 Fitting parameters of modified ColeeCole equation obtained using Eqs. (3) and (4). Temp (K)

Dε ¼ (εs  ε∞)

a

s

umax (104 rad s1)

s1 (107 S m1)

s2 (107 S m1)

Ea (eV)

393 423 453

133 133 143

0.12 0.10 0.10

0.59 0.59 0.59

45 85 150

4.6 0.6 0.9

0.25 0.3 0.35

0.32

S. Chanda et al. / Solid State Sciences 37 (2014) 55e63

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Fig. 7. Frequency (angular) dependence of the ac conductivity (s) of LNF at various temperatures where the symbols are the experimental points and the solid lines represent the fitting to experimental data using Eq. (6).

Fig. 6. (a) The complex impedance plane plots between Z00 and Z0 for LNF at different temperatures. Inset shows the equivalent circuit model. (b) Magnified plot showing circle grain effect at high frequency. Inset shows the well resolved two semicircular arcs at 423 K.

[39] is based on symmetry arguments, which are not yet available for our novel antiferromagneticeferroelectric compound, the essence of magneto-electric coupling may be envisaged as follows. When a magnetic field is applied to a magneto-electric material, the material is strained. This strain induces a stress on the piezoelectric (all ferroelectrics are piezoelectrics), which generates the electric field. This field could orient the ferroelectric domains, leading to an increase in polarization value. Such magneto-electric coupling and the large dielectric constant observed in the present system may be useful in device applications.

application of a magnetic field. In case of multiferroic (MF) materials, the dielectric constant is affected by the onset of magnetic ordering produced by an applied magnetic field. So, we have measured the capacitance of the sample in the presence and absence of magnetic field and calculated the dielectric constant for both the cases. In Fig. 10, we have shown the variation of the magneto-capacitance (MC) effect at RT which is calculated by the formula.

. MC ¼ ½ε0 ðH; T Þ  ε0 ð0; T Þ ε0 ð0; T Þ; where ε0 (H,T) and ε0 (0,T) are the dielectric constants in the presence and absence of magnetic field (H), respectively. The value of the ε0 increases with the increase of the applied magnetic field which indicates the existence of positive coupling between magnetic and electric order parameter in the sample at RT. Since magneto-electric coupling is observed at the higher frequency (10 kHz) where only the bulk effect of the sample is present, it establishes that MC effect is the intrinsic property of LNF. The bulk effect at 10 kHz can be substantiated by the complex impedance plane plot (Z00 vs Z0 ) at RT as shown in the inset of Fig. 10, where the first semicircle in this plot due to bulk effect corresponds to this frequency range. Although the theory of magneto-electric media

Table 3 Fitting parameters of complex plane impedance plots. Temp (K)

Rg (104 U)

Cg (pF)

ng

Rgb (106 U)

Cgb (pF)

ngb

363 393

4.4 2.65

54.47 57.73

0.69 0.67

52 39.5

59.71 67.89

0.53 0.59

Fig. 8. PeE loops observed at RT for LNF under different (a) applied voltages and (b) frequency.

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electron microscope and found to be ranging from 40 to 75 nm. The zero field-cooled and field cooled magnetizations of the sample are measured from 5 K to 800 K in an applied magnetic field of 1 kOe. A signature of weak ferromagnetic phase observed in LNF is explained on the basis of spin glass like behaviour of surface spins. Ferroelectric hysteresis loop observed at room temperature indicates the presence of ferroelectric ordering in LNF. The dielectric relaxation of the sample has been investigated using impedance spectroscopy in the frequency range from 42 Hz to 1 MHz and in the temperature range from 303 K to 513 K. The ColeeCole model is used to analyse the dielectric relaxation of LNF. The frequency dependent conductivity spectra follow the power law. The observed magneto capacitance of the sample confirms its multiferroic behaviour. The sample LNF is concluded to be a novel single phase multiferroic material. Acknowledgements Fig. 9. CeV characteristics acquired at 1 kHz at RT. The ac probing signal was 1 V. Inset shows dc IeV characteristics.

Sujoy Saha acknowledges the financial support provided by the UGC New Delhi in the form of JRF. Alo Dutta thanks the Department of Science and Technology of India for providing the financial support through DST Fast Track Project under grant no. SR/FTP/PS032/2010. The authors acknowledge Dr. P. Singha Deo of S. N. Bose National Center for Basic Sciences for taking TEM micrographs. References

Fig. 10. Variation of MC (@10 kHz) with magnetic field at RT of LNF. Inset shows the complex impedance plane plot between Z00 and Z0 for LNF at RT.

It is to be mentioned that the value of magnetization of 0.64  104 emu/g/Oe at RT of LNF nanoparticle is found to be more than that of LaFeO3 (0.3  104 emu/g/Oe) [16] and NdFeO3 (0.45  104 emu/g/Oe) [16] nanoparticle samples. The enhanced value of magnetization in LNF may be due to the presence of three major magnetic interactions (FeeFe, NdeFe and NdeNd) whereas for LaFeO3 only FeeFe interaction contributes to the magnetization. The coercive field for LNF is found to be ~880 Oe which is very high (6 times) if compared with LaFeO3 (~137 Oe) [16]. The dielectric constant (ε0 ) has been increased in LNF with respect to that of LaFeO3. ε0 is found to be ~80 and 105 at 105 Hz at RT for LaFeO3 [13] and LNF respectively. The ac conductivity (s) increases from 2.36  103 U1 m1 for the LaFeO3 to 4.21  103 U1 m1 for the LNF at T ¼ 553 K and at frequency 1 MHz [15]. Thus the sample LNF may be concluded to be a novel single phase multiferroic material. 4. Conclusions The multiferroic behaviour of nano-sized La1/2Nd1/2FeO3 powder, synthesized by the solegel citrate method, has been investigated. The Rietveld refinement of the X-ray diffraction profile of the sample at room temperature shows the orthorhombic phase with Pbnm symmetry. The particle size is obtained by transmission

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