Nano size spinel ferrite CoFe2O4 (CFO), ferroelectric BaTiO3 (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 verifiedviaX-ray diffraction 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 (102 to 106 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 confirmed that the sample CFO@BTO exhibit maximum energy storage efficiency.
Research Article
Structural, Magnetic, Dielectric and Energy Storage Analysis of CoFe2O4@BaTiO3 and BaTiO3@CoFe2O4 Core-Shell Nano- Composites
https://doi.org/10.21203/rs.3.rs-33291/v1
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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 field using an magnetic field, and vice versa [3]. This is the consequence of magnetoelectric phenomenon. Scientific 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 single-phase multiferroic materials like BiFeO3, BiMnO3, and YMnO3, 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) coefficients having large values of magnetization and polarization than single phase multiferroic materials [7–9]. 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 modified 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]. Although, various research have been reported on the core/shell type nanostructures CoFe2O4/BaTiO3, CoFe2O4/BiFeO3 and NiFe2O4/ BiFeO3[11–13], 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]. BaTiO3/CoFe2O4 composites possess better magnetoelectric properties as a result of individual phases contribution. Perovskite BaTiO3 (BTO) is an excellent efficient 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]. BaTiO3 mainly exhibit ferroelectric property but it can be multiferroic if it synthesized at nanoscale. So in present work we explore multiferroic properties of BaTiO3 by synthesizing it via sol gel method to develop nanoparticles of BaTiO3. Spinel ferrite CoFe2O4 (CFO) possesses outstanding electrical, magnetic and optical properties, especially on a nanometre scale. Thus we explore the multifunctional properties of CFO@BTO and BTO@CFO core-shell nanocomposite by study their structural, magnetic, dielectric and energy storage properties.
2.1 Synthesis of BTO nanoparticles and CFO@BTO nanocomposite
BTO nanoparticles were synthesized via sol-gel technique. Firstly barium carbonate (1.9 gm) was dissolved in 10 ml ethanol with continuous stirring for half an hour at room temperature. In parallel, titanium tetra isopropxide (2.8 ml) was dissolved in 10 ml ethanol with continuous heating (at 80 oC) and 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 oC 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 oC for two hour and then it was annealed at 600 oC and 800 oC for 2 hour in programmable muffle furnace to get barium titanate powder. This powder was crushed in mortar pestle to get fine 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 oC.
2.2 Synthesis of CFO and BTO@CFO nanocomposite
CFO nanoparticles were prepared by chemical co-precipitation method. Cobalt nitrate Co(NO3)2.6H2O (6.39 gm) and iron nitrate Fe(NO3)3.9H2O (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 oC. After that the precipitating agent liquid NH3 was added drop-wise under constant stirring so as to attain the pH of solution equal to 9. The precipitate was then finally dried at 100 oC on hot plate and crushed by an agate mortar to get nanoparticles. Further, the produced CFO nanoparticles were annealed at 800 oC 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 oC.
2.3 Instrumentation Specifications
Rigaku Ultima IV powder X-ray diffractometer with CuKα radiation was used for structural and phase analysis of developed samples. HRTEM (Tecnai G2 20, S-Twin (FEI)) employ the identification of core-shell nanostructures. Vibrating sample magnetometer (Lakeshore Model 7410) of magnetic field range ± 2 T was used for room temperature and temperature dependent magnetisation measurements. For temperature dependent measurements constant magnetic field 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.
3.1XRD analysis
X-ray diffraction pattern of synthesized BTO at different annealing temperature are shown in figure1. BTO1 represent the raw sample without annealing and BTO2, BTO3 represent the samples annealedat 600 and 800 oC 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 oC the BTO phases appeared with some extra peak of residue material BaCO3.The causes of presence of residue BaCO3 peaks is the incomplete reaction between the precursors to form BTO.
These BaCO3phases of orthorhombic structureare identified by ICSD file number 91888. Finally the pure phase of BTO having fine crystallinity was obtained after annealing at temperature 800 oC. At this temperature the intensity of BTO phase is enhanced and all the extra peaks of residue materialBaCO3 disappeared.The peaks of perovskite BTO phase with tetragonal structure are identified as ICSD file 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 figure2, which is analogues to thepeak location and intensities recorded in ICSD data file 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).
|
Sample
|
Lattice parameters (Å) |
Crystallite size D (nm)
|
|||
|
CFO a |
BTO a = b c |
CFO |
BTO |
||
|
CFO |
8.382 |
- |
- |
29 |
- |
|
BTO@CFO |
8.349 |
3.993 |
3.991 |
32 |
38 |
|
CFO@BTO |
8.333 |
3.995 |
3.998 |
34 |
37 |
|
BTO |
- |
3.999 |
4.010 |
- |
39 |
3.2Microscopic analysis
TheHRTEM micrograph of BTO nanoparticles and CFO@BTO nanocomposite are shown in figure 3(a) and (b) respectively. The magnified view of CFO@BTO nanocomposite are depicted in figure 3 (c). Figure 3(a) clearly shows tetragonal BTO nanoparticles with particle size ~ 49 nm which is accordance with the XRD results. Figure 3(b) & (c) represent clearly the formation ofcore (dark) -shell (bright) nanostructures of CFO@BTO nanocomposite as a result of difference in transmission intensities of BTO and CFO phases. The core identified as CFO and shellas BTO because of measured value of lattice spacing d = 0.251 nm in dark region which is corresponding to CFO phase and d = 0.283 nm in bright region that correspond to BTO phase. Further, from figure 3 (b) the measured value of core diameter is ~ 50 nm and having shell thickness ~ 17.5 nm. Magnetic CFO nanoparticles form agglomerates in BTO matrix.
Figure4(a) shows HRTEM micrograph of BTO @CFO nanocomposite with the presence of some agglomerates and arbitrary mixture of BTO and CFO phases. Figure4 (b) shows the magnified view of this nanocomposite that revealed the interplanar distances d = 0.252 nm in bright part and d = 0.282 nm in dark part which corresponds to the BTO and CFO phases respectively.In figure4 (a) the magnetic CFO nanoparticles form agglomerates around the BTO core therefore it is difficult to measure thickness of shell and core. However, average particle size of BTO@CFO nanocomposite is 56 nm
3.3Magneticanalysis
M-H curve of BTO nanoparticles exhibit superposition of diamagnetic and weak ferromagnetic contributions as revealed by figure5 (a). Change in magnetization M in the vicinity of ± Hc (Hc ~ 830 gauss is coercive field strength) represent weak ferromagnetic behaviour. The M-H plot without diamagnetic contribution is shown figure5 (b). The observed value of spontaneous magnetization (0.003 ) 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 Ti3+- O in the interstitial position and Ti3+-Vo at the surface where, Vo is void due to oxygen deficiency[20]. These couples ferromagnetically interact as shown by mechanism in figure 6. Thus weak ferromagnetism arises in BTO nanoparticles due to its surface defects.
In figure7 the M-H loop of CFO, CFO@BTO and BTO@CFO exhibit hysteresis and show ferromagnetic behaviour. The saturation magnetization (Ms), remnant magnetization (Mr), coercivity (Hc), anisotropy constant (K) and squareness ratio (S) values are tabulated in table2. Both the nanocomposites exhibit reduced value of magnetizations than CFO nanoparticles. The magnetic moments in the nanocomposites arises mostly due to CFO nanoparticles, as BTO exhibit very little magnetization. CFO@BTO having slightly high value of magnetization than BTO@CFO. As in core-shell composites the core and shell material preserve their properties individually therefore magnetic core CFO exhibit large magnetization.Two type of magnetic interaction are occurred in the composites samples. One is super-exchange interaction withinCFO nanoparticles and another isexchange interaction atinterface of BTO and CFO phases. Thus thin layer of 17.5 nm of BTO in CFO@BTO sample favored the magnetization due to surface exchange interaction at the interface [21-22].However, surface spin canting occur due to thin layer of CFO nanoparticles in BTO@CFO nanocomposite that reduced magnetization.
To establish the domain structure of the CFO phase and composites the squareness ratio (S) were estimated as shown in table2. The observed values of S indicates that both CFO phase and nanocomposites are multidomain structured (S<0.5).To investigate the magnetic transition temperature(TC) of CFO, CFO@BTO and BTO@CFO nanoparticles, M-T plot having applied magnetic field of 500 gauss are revealed in figure8(a), (b)&(c) respectively. Figure8(a) shows that at low temperature there is small variation in magnetization and drastic variation occur at high temperature.This sharp decrease in magnetization at high temperature can be withdrawal of ferrimagnetic behavior at magnetic transition temperature Tc (= 734K)for pure CFO nanoparticles. Figure8 (b) and (c) shows this magnetic transition temperature at Tc (= 653K) and Tc (= 610K) for CFO@BTO and BTO@CFO nanocomposites 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].
|
Samples
|
Ms (emu/g)
|
Mr (emu/g)
|
Hc (gauss) |
S = Mr / Ms |
Tc(K)
|
|
|
BTO |
CFO |
|||||
|
CFO |
62.25 |
26.72 |
1687 |
0.43 |
- |
734 |
|
CFO@BTO |
32.61 |
13.74 |
1402 |
0.42 |
362 |
653 |
|
BTO@CFO |
31.33 |
14.04 |
1351 |
0.44 |
349 |
610 |
Lyubutin et al. [24]investigated the antiferromagnetic arrangement of CFO at low-temperature (non-collinear order of the magnetic moments of Fe and Co) and recognized a canted magnetic structure. When heating is applied with an applied magnetic field, 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 Tc values in nanocomposites are slightly reduced compared to pure CFO. This decrement in the Tc 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 ofincrease 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 ofTc of nanocomposites. In figure8(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 ferroelectricTC ~390 K of BTO. Also, increase in magnetization nearTc 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.
3.4Dielectric analysis
Figure9 (a) depicted change in dielectric constant (ε¢) and9(b) tangent loss (tanδ) with respect to frequency range 102 to 106 Hz for all the samples. The dielectric constant of all samples exhibit strong dependence on frequency, having larger value at low frequencies and decreases on growing frequency and attained the continual behaviour like a typical dielectrics. Largestdielectric constant value at small frequency region may be demonstrated on ground of polarization of charge by virtue of non-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 [27-31]. 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 field. 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 field frequency but in large frequency region these dipoles unable to pursue the fast changing in applied field. 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 field 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 first 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 first 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 Tc 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 difficulties 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 difficulty in the orientation of domains in path of functional electric field [33]. The curie temperature of BTO is ~390 K. But here we observed decrease in curie temperature (TC) 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 figure 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
Figure12displays P-E hysteresis loopat particular frequency 50 Hzfor the samples. BTOand nanocomposites exhibit typical ferroelectric behaviour. The value of maximum polarization (Pm), remnant polarization(Pr) and coercivity (Ec) are tabulated in table3. CFO@BTO sample exhibit highest value ofmaximum polarization(Pm) at applied electric field of 15 kV/cm which can be ascribed tolarge number of space polarization occur across thelong chain of ordered ferrite/ferroelectric phases.
BTO exhibit small Pm 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 efficiency of allsamples. Energy storage density(WU) and efficiency(η) can be calculated using the relations [15,37 ]:
where E and P representapplied electric field and polarization respectively,WUand WLare 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 efficiency than other samples as depicted in table3. Therefore these composite samples can be used for storage capacitor devices [38-39].
|
Sample
|
Pm (µC/cm2) |
Pr (µC/cm2) |
Ec (kV/cm) |
WU (mJ/cm3) |
(%) |
|
BTO |
5.96 |
2.39 |
11.168 |
102.31 |
62.10 |
|
CFO@BTO |
6.65 |
2.19 |
4.54 |
36.97 |
64.06 |
|
BTO@CFO |
7.01 |
1.66 |
2.33 |
19.49 |
48.01 |
BTO, CFO, their core-shell nanocomposite CFO@BTO and BTO@CFO were synthesized by sol-gel and co- precipitation methods.XRD analysis confirmed that allrespective phases are present in bare and nanocomposites sample.HRTEM confirm 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 Tc 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 efficiency 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
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