Structural, chemical and low-temperature magnetic properties of lead-Free 0.6NiFe2O4-0.4Na0.5Bi0.5TiO3 magnetoelectric composite

We report on structural, chemical and low-temperature magnetic properties of lead-free 0.6NiFe2O4-0.4Na0.5Bi0.5TiO3 composite. NiFe2O4 (NFO) and Na0.5Bi0.5TiO3 (NBTO) are seen to crystallize in inverse spinel and perovskite structure, respectively. 0.6NiFe2 O4-0.4Na0.5Bi0.5TiO3 composite exhibits both NFO and NBTO phases in appropriate composition. Zero field-cooled (ZFC) and field-cooled (FC) magnetization measurements carried out from 15 to 300 K shows a large bifurcation at room temperature. ZFC and FC magnetization measurement exhibit a hump at Tm ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}259.5 K, indicates the possible existence of competing magnetic interactions in 0.6NiFe2 O4-0.4Na0.5Bi0.5TiO3 composite. Saturation magnetization, remnant magnetization and coercivity values are observed to increase with decreasing the temperature. Temperature-dependent saturation magnetization is fit to the Bloch’s law. Magnetocrystalline anisotropy (K1) value at various temperatures are estimated and is seen to increase from 0.23 × 104 erg/cc (at 300 K) to 0.34 × 104 erg/cc (at 15 K).


Introduction
Multiferroic materials exhibit more than one ferroic property makes them highly suitable for applications in spintronics, data storage devices, processors, controllers and sensors [1][2][3]. The multiferroic magntoelectric (ME) materials show both ferroelectric and ferromagnetic ordering simultaneously. The first magnetoelectic effect was reported in single crystal Cr 2 O 3 compound in 1961 [4]. Several single phase ME materials with weak ME coupling have been reported in the literature by various research groups [5][6][7][8][9][10]. Composite ME materials have been developed by combining various oxides consisting of ferroelectric and ferro/antiferro magnetic ordering with considerable amount of coupling between their order parameters. Composites with various ratios of magnetic and electric components provide material designers to control the piezoelectric and magnetoelectirc properties tactically. The idea of composite ME materials overcome the difficulty of coexistence of ferroelectric and ferro/anti-ferromagnetic ordering in a single phase material [11,12]. For the past several years, there is a tremendous advancement in design, production and application of ME composites.
Many research groups have produced enormous lead (Pb) based (PbZr 1-x Ti x O 3 , PbTiO 3 ) ME composites, which show higher ME coefficient than those of single-phase materials at room temperature [13][14][15]. However, due to their toxic nature, Pb based materials and composites are not encouraged for practical applications. The magnetoelectric composite consisting of BaTiO 3 and Ni (Co, Mn)Fe 2 O 4 prepared by unidirectional solidification show two order high ME coefficient at room temperature than single-phase Cr 2 O 3 [16]. After the ground-breaking experiments on BaTiO 3 /CoFe 2 O 4 composite, a wide range of ferroelectric/ferrite bulk composites have been developed consisting of ferroelectric phases such as BaTiO 3 , N 0.5 B 0.5 TiO 3 , PZT and ferromagnetic phases such as CoFe 2 O 4 , NiFe 2 O 4 and CuFe 2 O 4 [17][18][19][20][21]. NiFe 2 O 4 -PbZ 0.52 Ti 0.48 O 3 exhibits resonance magnetoelectric effect in which magnetoelectric resonances are caused by electromechanical resonance in the piezoelectric phase and magneto-chemical resonance in the magnetostrictive phase [22]. Electrostatically tunable ferromagnetic resonance and large converse magnetoelectric coupling (109 Oe-cm/kV) are observed in Na 0.5 Bi 0.5 TiO 3 -CoFe 2 O 4 lead-free multiferroic composite. [23,24] 0.3Na 0.5 Bi 0.5 TiO 3 -0.7MnFe 2 O 4 composite exhibit higher ME coefficient of 240 mV/cmOe. [25] Na 0.5 Bi 0.5 TiO 3 -NiFe 2 O 4 (NBTO-NFO) composite is well studied due to the high remnant polarization of NBTO phase and large relative permittivity near room temperature. [23,26]. NFO is a soft magnetic material having high magnetostrictive coefficient with magnetic ordering temperatures above room temperature. [27][28][29]. Narendra et al. have reported the ME response of 0.14% at 1 MHz in 0.67 NiFe 2 O 4 -0.33 Na 0.5 Bi 0.5 TiO 3 [30]. Highest ME response (1122 lV/cmOe) is reported for 0.4Na 0.5 Bi 0.5 TiO 3 -0.6NiFe 2 O 4 . [31] However, detailed studies on micro-structural and magnetic interactions of 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 composite are missing in the literature. In the present case, detailed studies on structural, micro-structural, chemical and magnetic properties of bulk 0.6NiFe 2-O 4 -0.4Na 0.5 Bi 0.5 TiO 3 composite have been explored. Interestingly, the ZFC and FC magnetization measurement shows the possibility of existence of competing magnetic interactions at low temperatures and irreversibility at room temperature. Various magnetic properties are estimated at different temperatures and the temperature dependent saturation magnetization values are analyzed using Block's law. Anisotropy constant value at different temperature is estimated using single ion model. The observed temperature dependent magnetic properties, structural and chemical properties of 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 composite is discussed and reported in this manuscript.

Experimental details
Pure NiFe 2 O 4 was prepared using high-quality powders of NiO and Fe 2 O 3 (99.9% Sigma-aldrich) taken according to the stoichiometric ratio and grounded well using mortar and pestle for 2 h. The homogeneous mixture was calcinated at 800°C for 5 h. The calcined powder was finely reground for 1 h followed by 3 h sintering at 1050°C. Similarly, pure sodium bismuth titanate Na 0.5 Bi 0.5 TiO 3 was synthesized by thoroughly mixing high pure Na 2 CO 3 , Bi 2 O 3 , TiO 2 powders (99.9%, Sigma-aldrich) according to the stoichiometric ratio. The mixed powder was calcinated at 950°C for 3 h. Pure and homogeneous powders of NFO and NBTO were mixed in the ratio of 60:40 wt% respectively for 1 h to obtain the composite of the 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 . The formation of pure phase of the prepared samples and the structural analysis were performed at room temperature (RT) using X-ray diffraction (XRD) (Make: PANanalytic Xpert Pro) using Cu K a radiation (k = 1.540 Å ). Morphology and particle size were investigated using field emission scanning electron microscope (FE-SEM, Make: FEI QUANTA-200, USA) with an applied voltage of 20 kV. The micro-analysis and oxidation states of all the elements present in the NFO, NBTO and 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 are investigated using X-ray photoelectron spectroscopy (XPS) (Make: ULVAC-PHI VersaProbe III) with X-ray source of Al and photon energy of 1486.6 eV. The core elemental spectra of XPS date were fitted using mixed Gaussian and Lorentzian functions XPSPeakfit 4.1 software. Raman spectroscopy, which a powerful tool to examine the structural characteristics was employed at room temperature using a micro-Raman spectrometer (Make: LabRAM HR Evolution, HOR-IBA, France) in a backscattering configuration with the laser focused to a 1 lm 2 spot. The laser wavelength used was 532 nm with a total power of 5 mW. Temperature and field dependent magnetic measurements were carried out using vibrating sample magnetometer (VSM) (Make: Lakeshore 7400-S, USA) by varying the field up to 15 kOe in the temperature range of 15 to 300 K.  [32]. Na 0.5 Bi 0.5 TiO 3 is seen to crystallize in rhombohedral phase with the lattice parameter values of a = b = 5.4 Å and c = 13.5 Å , matches well with reported value (JCPDS card (#36-3040)) [33][34][35]. In addition to the parent phase, small amount of Na 2 CO 3 and Bi 2 O 3 phases were indentified and are indicated in the Fig. 1b. The composite 0.6NiFe 2 O 4 -0.4(Na 0.5 Bi 0.5 )TiO 3 shows the coexistence of maximum intense peaks corresponding to NiFe 2 O 4 and Na 0.5 Bi 0.5 TiO 3 phases and other common peaks confirms the presence of both phases. No extra peaks or peak splitting is observed which removes the possibility of transition from rhombohedral to tetragonal phase. Figure 2a-c shows the scanning electron micrographs (SEM) images of NiFe 2 O 4 , Na 0.5 Bi 0.5 TiO 3 and the composite 0.6NiFe 2 O 4 -0.4(Na 0.5 Bi 0.5 )TiO 3 . SEM micrograph of NiFe 2 O 4 shows aggregation of microsized particles. High dense agglomeration is observed in particles along with the considerable pores. Morphology of Na 0.5 Bi 0.5 TiO 3 sample is seen to be spherical granules. The morphology of 0.6NiFe 2-O 4 -0.4(Na 0.5 Bi 0.5 )TiO 3 composite contains of mixture of both spherical and rectangular granules from parent phases. Particle size is estimated from SEM micrographs using Image-J software. The average particle size is estimated from the histograms of NBTO, NFO and NFO-NBTO (shown in inset of Fig. 2a-c) and is found to be around 2.6 lm, 0.06 lm, and 0.7 lm, respectively. Energy-dispersive X-ray analysis (EDX) is used (Fig. 2d) to measure the elemental distribution and atomic percentage in 0.6NiFe 2 O 4 -0.4(Na 0.5 Bi 0.5 )TiO 3 composite. EXD spectra confirm the presence of all elements in the NFO-NBTO composite. However, due to high volatile nature of Bi, slight variation in its atomic percentage is observed.

Results and discussion
Raman spectra of NiFe 2 O 4 , Na 0.5 Bi 0.5 TiO 3 and 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 composites are measured at room temperature and shown in Fig. 3a-c. Small incident laser power is used to identify all the Raman bands and the de-convoluted spectra are in Fig. 3. The spectra of NiFe 2 O 4 show five Raman bands at 699, 334.5, 211.3, 485.2 and 574.5 cm -1 correspond to A 1g , E g , and 3T 2g bands respectively, which resemble the typical inverse spinel structure of NiFe 2 O 4 phase. Observed five Raman active modes matches well with the reports [36,37]. NiFe 2 O 4 crystallizes in spinel structure of Fd-3 m space group. The observed peak at 209 and 487 cm -1 (E g mode) arises due to symmetric bending of Fe atoms at A site and asymmetric bending of oxygen atoms respectively. The symmetric stretching of oxygen in Fe-O bond located at A site causes the A 1g Raman mode at 699 cm -1 . The t 2g mode at 485.2 cm -1 gives rise to symmetric stretching of Fe-O bond at B site. The Raman spectra of Na 0.5 Bi 0.5 TiO 3 show a peak at 281.1 cm -1 corresponding to A 1g mode and two other peaks at 526.2 and 579.6 cm -1 corresponding to E g mode. The peak at 280 cm -1 arises from TiO 6 vibrations and the other peaks correspond to symmetric vibrations of oxygen in Na 0.5 Bi 0.5 TiO 3 . Figure 3c shows the Raman spectra of 0.6 NiFe 2 O 4 -0.4 (Na 0.5-Bi 0.5 )TiO 3 composites, exhibiting the modes corresponding to NiFe 2 O 4 and Na 0.5 Bi 0.5 TiO 3 parent phases. Shilpa et al., have reported the absence of Raman peaks in 50% or higher doping of NFO in NFO-NBTO composite which is ascribed to accidental degeneracy of phonon frequencies. [38] However, in the present case we have found finite intensities of phonon modes correspond to ferrite phase in the composite. In composite, the A 1g mode corresponding to NBTO phase showed blue shift while E g and A 1g phonon modes from NFO phase exhibited red shift. Blue and red shift in Raman spectra indicates that there is a structural distortion in the NFO and NBTO phases with it is made into NFO-NBTO composite. The deformation of magnetic phase causes internal stress under applied magnetic field. The internal stress is mediated via interfaces and transferred to the ferroelectric phase which results to polarization [39]. Maganetoelectric coupling of the composite is directly influenced by piezoelectric and/or magnetostrictive strengths of respective components.
Chemical states of the elements and chemical bonding present in all the samples are explored using X-ray photoelectron spectroscopy (XPS). A high resolution spectra of Ni 2p, Fe 2p, O 1s, Bi 4f and Ti 2p elements of the 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 composite is carried out using Al K a radiation. The core level XPS spectra of all the elements are calibrated with the carbon binding energy of 284.6 eV. The survey spectrum is de-convoluted using mixed Gaussian and Lorentzian function (not shown). Survey spectra indicate the presence of all the elements. A core level spectrum of Ni 2p is shown in Fig. 4a. Due to the spin-orbit coupling, the 2p spectra of Ni split into two broad peaks which correspond to 2p 3/2 and 2p 1/2 energy levels centered around * 854 and * 873 eV respectively. The peaks centered at 860.9 eV correspond to the satellite peak. The deconvolution spectra of nickel reveal the presence of three nonequivalent bonds. Two bonds correspond to two types of lattice sites for cations i.e., octahedral and tetrahedral while the third one is due to the presence of oxygen vacancy in the lattice. The binding energy associated with Ni 2p 3/2 are 854.4 eV and 856.3 eV, which represents octahedral and tetrahedral bonding with oxygen and the oxidation state of 2?. The percentage of contribution in Ni 2? to octahedral Ni 2? (O h ) and tetrahedral Ni 2? (T d ) sites are 18%, and 82%, respectively. Similarly, the binding energies 873.0 and 8763 eV are correspond to octahedral and tetrahedral energies of Ni 2p 1/2 state, respectively [40,41]. The core level XPS spectrum of Fe is shown in Fig. 4b. Deconvolution of Fe 2p peak is mainly resolved in to three peaks. Two peaks corresponds to two lattice sites of Fe 3? and the third peak is associated with Fe 2? as shown in Fig. 4b. The peaks observed at 709.6 eV, 711.2 eV and 713.9 eV corresponds to Fe 2? , octahedral Fe 3? (O h ), and tetrahedral Fe 3? (T d ) bonding associated with 2p 3/2 and 724.9 eV, 726.3 eV corresponds to the state of 2P 1/2 . The broader Fe 2p peaks are observed in the present case due to the coordination of Fe ?3 with oxygen in octahedral and tetrahedral geometries. The peaks centered at 718.5 eV correspond to satellite peaks of Fe [42,43]. Appearance of Fe 2? peak indicates the presence of oxygen vacancy due to charge neutrality. [44] Quantitative analysis show the integrated peak intensities corresponding total area of Fe 3? (T d ), Fe 3? (O h ) and Fe 2? are 55%, 45% and 10%, respectively. Hence, the quantitative analysis suggests that NFO in composite have the inverse spinel structure. The core-level spectrum of O 1s is observed to be centered at 531 eV which is attributed to the lattice oxygen atoms and the peak centered at 529 eV is ascribed to the presence of oxygen vacancies (Fig. 4c) [45]. The peak observed at 531.7 eV could be due to the chemisorbed oxygen lying on the surface of the samples and may simply be an artifact of fitting of asymmetric XPS peaks with symmetric Gaussian functions [46]. The core level 1s spectrum of sodium is shown in Fig. 4d. The presence of peak at 1071.3 eV confirms the oxidation state of Na is ? 1. The XPS spectra of Bi 4f shown in Fig. 4e contains two peaks which are centered at 163.8 eV and 158.4 eV, correspond to 4f 3/2 and 4f 5/2 states, respectively. De-convoluted spectrum of Ti 2p is shown in Fig. 4f. Due to the spin orbit coupling, Ti 4? splits into two peaks correspond to Ti 2p 1/2 and Ti 2p 3/2 centered at 457.6 eV and 463.  typical ferromagnetic behavior [47]. Saturation magnetization (M s ) value of pure NiFe 2 O 4 and 0.6 NiFe 2 O 4 0.4 (Na 0.5 Bi 0.5 )TiO 3 composite at RT is seen to be 59 emu/g and 25.72 emu/g, respectively. Saturation magnetization value is seen to be lesser in the case of composite compared to that of pure NiFe 2 O 4 due to the presence of nonmagnetic Na 0.5 Bi 0.5 TiO 3 phase. It is observed that the saturation magnetization (M S ) value (at 300 K is 25.7 emu/g) increases with decreasing the temperature (28 emu/g at 15 K). Enhanced M S value at low temperatures is due to less thermal vibrations which randomizes the magnetic moments. [48] M-H loops measured at 15, 150, 250 and 300 K for 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 composite is shown in Fig. 5d. The remnant magnetization and coercivity values of 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 composites at different temperatures are shown in table I. It is found that the remnant magnetization and coercivity values increase from 2.51 to 3.36 emu/g and 47.7 to 62.3 Oe, respectively, with decreasing the temperature from 300 to 15 K. The observed room temperature coercivity is slightly greater than that of same composite synthesized by sol-gel method [38]. Increase in coercivity and remnance is due to the enhanced growth of magnetic anisotropy at lower temperatures. Increase in coercivity with decreasing temperature can be understood by considering the effects of thermal fluctuations of the blocked moment, across the anisotropy barrier. In the case of bulk ferromagnetic materials, the temperature dependent saturation magnetization (M S ) is expressed by considering the simple model of thermal activation of particle moments over the anisotropy barriers as using Bloch's law [49]. The Bloch's law is framed by considering the excitations (magnon) are of long wavelength spin-waves at low temperature. For the bulk materials, Bloch's law is valid at low temperatures (i.e., T is less than T C /2) and is expressed as: b Fig. 4 The X-ray Photoelectron spectroscopy of the 0. Temperature variation of saturation magnetization and the curve fit using Bloch's law (e). Zero-field cooled and field-cooled magnetization curve of 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 composite measured at 100 Oe in the temperature interval 15 to 300 K (f) where a is called as Bloch's exponent and its value for bulk material is 3/2. M (0) is the magnetization at T = 0 K, which can be estimated by extrapolating the M S (T) versus T curve towards the Y-axis. T 0 is the temperature at which the saturation magnetization becomes zero, which can be estimated by extrapolating the M S curve towards temperature axis. The best fit of the data to the model is shown in Fig. 5e, with estimated M (0) and T 0 values of 28.6 emu/g and 650 K respectively. Further, the law of approach to saturation (LAS) is employed to investigate the magnetic anisotropy constant values of 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 composite. According to the law of approach to saturation, the field dependence of magnetization close to saturation can be expressed as [50,51]: where, b= 8K 1 2 105l O M S 2 and rH is the forced magnetization coefficient. The value of rH is prominent at high fields and can be neglected at room temperature. The conditions for the above equation are H [ [ H C , and T \ T C /2. The magnetization M should be taken near to saturation, l o is permeability of free space (1.257 9 10 -6 kg.m/s 2 A 2 ), and K 1 is the first order cubic magnetocrystalline anisotropy constant. Substitution of above values in Eq. (2) results to.
The magnetocrystalline anisotropy constants (K 1 ) for the composite at different temperatures are calculated using Eq. (3) and are tabulated in Table 1. It is observed that the magnetocrystalline anisotropy increases from 0.23 9 10 4 to 0.34 9 10 4 erg/cc when the temperature is decreased from 300 to 15 K. The observed magnetocrystalline anisotropy values are lesser than the reported anisotropy values for NiFe 2 O 4 and nanoparticles of NFO-NBTO composite at room temperature [48,52,53]. In the present case, 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 particle size is seen to be in the order of lm with wide distribution. Therefore, the effect of particle size plays an important role in determining the anisotropy value.
Temperature dependent zero field cooled (ZFC) and field cool (FC) magnetization measurements are carried out to explore the magnetic interactions present in 0.6 NiFe 2 O 4 -0.4 (Na 0.5 Bi 0.5 )TiO 3 composite at low temperatures. Figure 5f shows the ZFC and FC magnetization recorded at 100 Oe field in the temperature range 15-300 K. It is observed that the ZFC magnetic moment continuously decreases with decreasing temperature. FC magnetization is seen to increase slowly with decreasing temperature, indicating the weakly coupled magnetic system. At very low temperatures, FC magnetization is seen to decrease with decreasing the temperature indicating the possibilities of existence of competing magnetic interactions. The ZFC and FC magnetization data is not to seen merge (irreversibility) up to the room temperature, indicating that the blocking temperature (T B ) is well above the RT. Irreversibility between ZFC and FC at RT indicates that the anisotropy energy is greater than the thermal energy (k B T) below RT. In addition to that, the irreversibility can be attributed to the reduced antiferromagnetic inter-spin cluster interactions due to the orientation of few spin clusters along the field direction. Further, the strong super-exchange interaction between Fe ions at tetrahedral and octahedral sites leads to ferromagnetic Curie temperature (T C ) well above the room temperature. Small hump in ZFC magnetization is seen to be present at 259.5 K, noted as T m , indicating the possibility of existance of short range ferromagnetic clusters which freezes into different metastable energy states. These short-range ferromagnetic clusters could couple with long range ferromagnetic phase. The decrease in ZFC magnetization below this hump is due to the random freezing of spin clusters into different metastable magnetic states [54][55][56].

Conclusions
0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 composite is obtained by mixing inverse spinel NiFe 2 O 4 and perovskite Na 0.5 Bi 0.5 TiO 3 bulk materials in appropriate ratio. Micron size particles in all the materials are confirmed from the scanning electron microscopy images. Oxidation states of all the elements are confirmed from XPS studies. Saturation magnetization, coercivity, remnant magnetization and magnetocrystalline anisotropy values of 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 are seen to increase with decreasing temperature from 300 to 15 K. ZFC and FC magnetization measurement carried out from 300 to 15 K reveals the existence of competing magnetic interaction in 0.6NiFe 2 O 4 -0.4Na 0.5 Bi 0.5 TiO 3 composite.