Structural, Magnetic, Dielectric and Energy Storage Analysis of CoFe2O4@BaTiO3 and BaTiO3@CoFe2O4 Core-Shell Nano- Composites

Nano size spinel ferrite CoFe 2 O 4 (CFO), ferroelectric BaTiO 3 (BTO) and their core-shell nanocomposites BTO@CFO and CFO@BTO were synthesized using combination of chemical co-precipitation and sol-gel route respectively. The phase formation and crystallinity of bare CFO, BTO and their core-shell nanocomposites were veriedviaX-ray di ﬀ raction pattern (XRD). High resolution transmission electron microscopy(HRTEM) revealed the core-shell structure of the nanocomposites.Magnetization measurements exhibitferromagnetic behaviour of all the samples except BTO in which superposition of weak ferromagnetic and diamagnetic response occurred due to its nanostructure. Magnetization versus temperature (M-T plot) measurements show anomaly near ferroelectric to paraelectric phase transition of BTO. Also,dielectric constant(ε¢) and tangent loss (tanδ) variation with respect to frequency (10 2 to 10 6 Hz) and temperature (300-700 K) were presented. ε¢-T curve of nanocomposites exhibit anomaly at the same temperature as observed in M-T plot of nanocomposites that indicate the inherent magneto-electric coupling in nanocomposites. Energy storage properties of BTO and nanocomposites have been examined via P-E loop analysis and conrmed that the sample CFO@BTO exhibit maximum energy storage eciency.


Introduction
Multiferroic materials perform important role in development of multifunctional devices that simultaneously show ferroelectric, ferromagnetic, and piezo-elastic orders in the same phase [1][2].
Materials revealing spontaneous magnetization due to spontaneous polarization by a large coupling interaction could permit the capability to control electric eld using an magnetic eld, and vice versa [3]. This is the consequence of magnetoelectric phenomenon. Scienti c communities paying attention on these multiferroic materials because of its possible applications in several multifunction devices, like memory elements with multiple states, transducers, spintronics, sensors and terahertz radiation [1,4]. As a result of mutual exclusivity of ferroelectricity and ferromagnetic arrangement, the existence of singlephase multiferroic materials like BiFeO 3 , BiMnO 3 , and YMnO 3 , are very small [1]. Additionally, the magnetic response, dielectric constant and magnetic permeability of single-phase multiferroics are restricted in accordance with phenomenological theory [5]. Therefore, to produce single-phase multiferroic compounds having high magnetoelectric coupling to use it as multifunction devices is hard [6]. In the last two decades, most of the work have been accomplished to remove the drawbacks of single-phase multiferroic materials. Earlier studies show that composite of multiphase multiferroics possesses high magnetoelectric coupling (ME) coe cients having large values of magnetization and polarization than single phase multiferroic materials [7][8][9]. ME coupling in composites occurs at the boundary of magnetic and ferroelectric phases. Nanocomposites propose an interesting and effective technique for novelty in functioning materials. However, the synthesis mechanism of nanocomposites has a number of disadvantages, like differences in thermal expansion, grain boundaries, and discrepancies in between crystal parameters. Thus to buried these problems, core-shell nanostructures consisting of core as ferromagnetic materials and shell as ferroelectric materials or vice versa, are excellent candidates. Core-shell nanocomposite provide modi ed properties which rises either from the core or the shell or the joint effect of core and shell which make it highly functional materials [10] [11][12][13], but still there have been a lack of systematic reports on development of uniform core/shell nanostructures of multiferroic materials. Core-shell nanocomposite having very strong interactions are anticipated to be (a) a medium that transfer strain ideally, (b) Dielectric layer as a shell reduces the leakage problem by decrease the conductivity of the composites and (c) Enhancement in magnetoelectric coupling [14][15]. BaTiO 3 /CoFe 2 O 4 composites possess better magnetoelectric properties as a result of individual phases contribution. Perovskite BaTiO 3 (BTO) is an excellent e cient ceramic having interesting ferroelectric properties, and its optical properties, such as multilayer ceramic capacitors (MLCC), converters, actuators, random access ferroelectric memory devices with energy storage applications [16]. BaTiO  stirring on hot plate separately for half an hour. Later, these two solutions were mixed in a single beaker with continuous stirring for 20 minute. After that citric acid (4.20 gm) was added in the mixed solution followed by continuous stirring and heating at 80 o C for half an hour. Finally, a thick white paste was formed in form of gel. Further, this gel was dried inside the oven at 120 o C for two hour and then it was annealed at 600 o C and 800 o C for 2 hour in programmable mu e furnace to get barium titanate powder.
This powder was crushed in mortar pestle to get ne nanoparticles of BTO.
Further, CFO@ BTO nanocomposite of 1:1 weight ratio of CFO and BTO were synthesized by following the same synthesis steps of BTO except one additional step in which already synthesized CFO nanoparticles were added near the step of gel formation. The synthesis steps of CFO nanoparticles are given in Sect. 2.2. Finally, the obtained nanocomposites were annealed at 800 o C.

Synthesis of CFO and BTO@CFO nanocomposite
CFO nanoparticles were prepared by chemical co-precipitation method. Cobalt nitrate Co(NO 3 ) 2 .6H 2 O (6.39 gm) and iron nitrate Fe(NO 3 ) 3 .9H 2 O (17.57 gm) were dissolved in 60 ml of distilled water and stirring have to be done for 30 minute to get the homogeneous solution. Oleic acid as a surfactant (10 ml) was added to prevent the agglomeration of the particles, followed by stirring for about 30 minute at 80 o C. After that the precipitating agent liquid NH 3 was added drop-wise under constant stirring so as to attain the pH of solution equal to 9. The precipitate was then nally dried at 100 o C on hot plate and crushed by an agate mortar to get nanoparticles. Further, the produced CFO nanoparticles were annealed at 800 o C for 2 hour.
The synthesis steps of BTO@CFO nanocomposite of 1:1 weight ratio of BTO and CFO are same as of synthesis steps of CFO nanoparticles except one additional step of addition of BTO nanoparticles before precipitation step. Finally, the obtained nanocomposites were annealed at 800 o C.

Instrumentation Speci cations
Rigaku Ultima IV powder X-ray diffractometer with CuKα radiation was used for structural and phase analysis of developed samples. HRTEM (Tecnai G 2 20, S-Twin (FEI)) employ the identi cation of coreshell nanostructures. Vibrating sample magnetometer (Lakeshore Model 7410) of magnetic eld range ± 2 T was used for room temperature and temperature dependent magnetisation measurements. For temperature dependent measurements constant magnetic eld of 500 gauss and temperature range 300 to 800 K were used. Impedance analyser having frequency range 100 Hz to 1 MHz (Wayne Kerr-6500B) was used for dielectric measurements. Ferroelectric measurements were carried out by using P-E Loop Tracer (Marine India) at room temperature.

Results And Discussion
3.1XRD analysis X-ray diffraction pattern of synthesized BTO at different annealing temperature are shown in gure1.
BTO1 represent the raw sample without annealing and BTO2, BTO3 represent the samples annealedat 600 and 800 o C temperature of BTO powder. The XRD results shows that BTO1 have very small crystallinity and phases do not grow at this temperature and at annealing temperature 600 o C the BTO phases appeared with some extra peak of residue material BaCO 3 .The causes of presence of residue BaCO 3 peaks is the incomplete reaction between the precursors to form BTO.
These BaCO 3 phases of orthorhombic structureare identi ed by ICSD le number 91888. Finally the pure phase of BTO having ne crystallinity was obtained after annealing at temperature 800 o C. At this temperature the intensity of BTO phase is enhanced and all the extra peaks of residue materialBaCO 3 disappeared.The peaks of perovskite BTO phase with tetragonal structure are identi ed as ICSD le number 29148. Figure 2 represents XRD pattern of bare CFO, BTO and their Core-Shell nanocomposites CFO@BTO and BTO@CFO. XRD pattern of both the core shell nanocomposites revealed characteristics peaks offerrite (CFO) and ferroelectric (BTO) phases. All peaks of nanocomposites are indexed as shown in gure2, which is analogues to thepeak location and intensities recorded in ICSD data le for CFO and BTO phase. Also, no any other undesired extra impurity phases are detected in the XRD pattern that indicate in between these two phases chemical reaction is absent.
The lattice parameter for CFO are measured by the equation where h, k, l denotes miller indices to equivalent planes. For BTO, lattice parameters a and c are measured by equation The calculatedvalues of lattice parametersof BTO and CFO phases are tabulated in table1. These lattice parameters are nearly matched with the earlier results [17][18]. Further, average crystallite size (D) was calculated by the Debye-Scherrer equation, (where, k is symbol for shape factor with 0.89 value,λ represent wavelength of X-rays, peak position determined by θ and β stands for full width at half maxima for maximum intense peak).

3.2Microscopic analysis
TheHRTEM micrograph of BTO nanoparticles and CFO@BTO nanocomposite are shown in gure 3(a) and (b) respectively. The magni ed view of CFO@BTO nanocomposite are depicted in gure 3 (c). Figure   3(a) clearly shows tetragonal BTO nanoparticles with particle size ~ 49 nm which is accordance with the XRD results. Figure 3 ) is analogous to the previous reported results [19]. Such type of behaviour in BTO nanoparticles isattributedtopresence of defects as oxygen vacancies on the grains surfaces. These oxygen vacancies at surface generate couple of Ti 3+ -O in the interstitial position and Ti 3+ -V o at the surface where, V o is void due to oxygen de ciency [20]. These couples ferromagnetically interact as shown by mechanism in gure 6. Thus weak ferromagnetism arises in BTO nanoparticles due to its surface defects.
In respectively. It has been considered that the magnetic moment decreases with increasing temperature, causing a phase transition from ferrimagnetism to paramagnetism. This is due to increased thermal randomization of the magnetic moment atenhancing temperature [23]. Lyubutin et al. [24]investigated the antiferromagnetic arrangement of CFO at low-temperature (noncollinear order of the magnetic moments of Fe and Co) and recognized a canted magnetic structure.
When heating is applied with an applied magnetic eld, magnetic arrangement in CFO nanoparticles is constantly transformed from an canted state to a collinear one, which leads to an increase in magnetization. However, it has been shown that T c values in nanocomposites are slightly reduced compared to pure CFO. This decrement in the T c values for the nanocomposites are due to the diffusion of BTO domain into the spinel lattice that weaken the super exchange interaction [25]. This weakening of super exchange interaction is the result o ncrease in the separation of magnetic moments at A and Bposition of the spinel structure.Weak super-exchange interactions are more effected by the thermal motion, that results in reduction ofT c of nanocomposites. In gure8(b) and (c) M-T measurements shows the magnetic anomaly at 362 K and 349 K respectively. These magnetic anomaly in both the nanocomposite exhibit the ferroelectric phase transition of BTO because these anomalies lies near the standard ferroelectricT C ~390 K of BTO. Also, increase in magnetization nearT c of BTO in both the nanocomposites might be due to sharp increase in compressive strain [26].Thus these anomaly indicate the magnetoelectric coupling effect in both the nanocomposites. homogeneous dielectric structure and Maxwell-Wagner space charge polarization, that accordanceto Koops phenomenological theory. These models play major role in such type of multiphase composites [27][28][29][30][31]. In this model, the dielectric structure of ferrite and composites were supposed to be consist of good conducting layers of grain surrounded by weakly conducting grain boundaries. At small frequencies more polarization occur due to the active participation of grain boundary and fast response of it to the applied eld. Therefore, more charge accumulation take place at the grain boundary interface which result in high dielectric constant.In small frequency region, electric dipole is in phase with the applied eld frequency but in large frequency region these dipoles unable to pursue the fast changing in applied eld.

3.4Dielectric analysis
Thus CFO@BTO exhibit high value of dielectric constant than BTO@CFO due to its core-shell structure in which there is ordered arrangement of ferroelectric and ferrite layer. But in BTO@CFO due to agglomeration of CFO nanoparticles around BTO core large conducting channel formation take place that decrease the value of dielectric constant. The tanδ represent the tangent loss in the sample which measure the electrical energy loss due to applied electric eld at various frequencies.
The nanocomposites exhibit small value of tangent loss than BTO and CFO phase. Therefore these nanocomposites are useful for high frequency microwave devices.
Figure10 displays change in dielectric constant with respect to temperature of nanocomposites and BTO (in inset) at frequency 10 kHz. Thedielectric constant increases upto rst transition temperature of range 350 to 370 K then it reaches to a second maximum and then decrease again. The increment in dielectric constant with temperature areascribed to interfacial polarization atferrite / ferroelectric boundary and also due to the mechanism of hopping conduction, which is a process of thermal activation [28].The rst anomaly in the temperature range 350-370 K correspond to ferroelectric phase transition of BTO phase which suggest presence ferroelectric phase in these nanocomposites.The another peakin the temperature range 640-680 Kattributed to the transition in dielectric constant close to T c of CFO phase [8].
Near transition temperature, large dielectric constant are attributed to effect of temperature alteration on domain wall motion. At lower temperature the domain wall contributed to small dielectric constant because of di culties in movement of domains [32]. However, at transition temperature the high dielectric constant value is attributed to domain wall motion and beyond the transition temperature, it decreases due to di culty in the orientation of domains in path of functional electric eld [33]. The curie temperature of BTO is ~390 K. But here we observed decrease in curie temperature (T C ) for both BTO nanoparticles and nanocomposites which can be ascribed to the intrinsic size effect. As due to nanosized(< 100 nm) BTO induces stress inside the grain, thereby inhibiting the movement of domain wall. Thus the competition between the shrinkage of surface bonds and the pinning of domain walls affects the phasetransition of the BTO [34]. The M-T plot of nanocomposite exhibit the anomaly at the same temperature that indicate the intrinsic magnetoelectric coupling in both the nanocomposites. Figure 11 displays change in tangent loss with respect to temperature at particular frequency of 10 kHz and it exhibit same behaviour as observed for dielectric constant variation in gure 10. The increasing behaviour of tanδ with temperature may be attributed to the thermally activated conduction mechanism [35]. At high temperature the substantial rise of tanδ of composites attributed to interfacial polarization at CFO/BTO interface and enhancement of thermally stimulated dielectric relaxation [36].
3.5Energy storage analysis BTO exhibit small P m value because ferroelectric behaviour may reduce in nanosized BTO particles due to enhancement in the oxygen vacancies. The P-E loop is used to evaluate energy storage density and storage e ciency of allsamples. Energy storage density(W U ) and e ciency(η) can be calculated using the relations [15,37 ]: where E and P representapplied electric eld and polarization respectively,W U and W L are useful energy storage density andenergy loss density respectively. The grey region in P-E loops show the losses and orange region exhibit the recoverable useful energy. CFO@BTO exhibit high e ciency than other samples as depicted in table3. Therefore these composite samples can be used for storage capacitor devices [38][39]. BTO, CFO, their core-shell nanocomposite CFO@BTO and BTO@CFO were synthesized by sol-gel and coprecipitation methods.XRD analysis con rmed that allrespective phases are present in bare and nanocomposites sample.HRTEM con rm the core-shell structure and nano size of developed materials.
M-H hysteresis curve showed the typical ferromagnetic behaviour of CFO and core-shell nanocomposites and weak ferromagnetism was observed in BTOsample around ± Hc ~ 830 gauss ( excluding diamagnetic part) due to its nanosized effect. CFO@ BTO having large value of magnetization and curie temperature than BTO@CFO nanocomposite. Dielectric constant versus temperature curve explained the presence of magneto-electric coupling in composite samples on the basis of anomaly present in curve.
First anomaly intemperature range 350-370 K correspond to ferroelectric phase transition in BTO. Another peak in the temperature range 640-680 K was attributed to the transition in dielectric constant close to T c for CFO phase. The M-T plot of nanocomposite exhibited anomaly in the same temperature range and indicated the intrinsic magnetoelectric coupling in both the nanocomposites. Energy storage analysis of the developed samples show that CFO@BTO exhibit energy storage e ciency of 64% which was found higher than othermaterials present in this work. Thus, all these dielectric, magnetic and ferroelectric properties exhibit that these nanocomposites of ferrite (CFO)/ferroelectric(BTO) phase will be useful for the storage and multistate memory devices