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Microstructure, dielectric, ferroelectric and magnetoelectric coupling of a novel multiferroic of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 nanocomposite
Materials Chemistry and Physics 240 (2020) 122242
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Microstructure, dielectric, ferroelectric and magnetoelectric coupling of a novel multiferroic of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 nanocomposite A. Mitra, A. Shaw, P.K. Chakrabarti * Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan, 713104, India
H I G H L I G H T S
� [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 exhibits room temperature multiferroicity. � Presence of TiO2 matrix helps to introduce RT ferroelectric polarization. � Dielectric behaviour of the nanocomposite is drastically improved. � Magnetoelectric coupling of the nanocomposite is observed at RT. A R T I C L E I N F O
A B S T R A C T
Keywords: XRD HRTEM Dielectrics Ferroelectrics Magnetoelectric coupling
A new multiferroic is derived from GdMnO3 (GMO), CoFe2O4 (CFO) and TiO2 in the nanocomposite phase. Here the good dielectric properties of GMO and TiO2 including the multiferroicity of GMO below ~13 K along with the magnetic ordering of CFO are exploited to develop the desired nanocomposite system. The nanocomposite of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 (GMO-CFO@TO) was obtained from chemical preparation route. Detailed investigations of structural and morphology are carried out by analyzing the X-ray diffractograms and pictures recorded during high resolution transmission electron microscopy. Dielectric behavior like dielectric strength, loss tangent etc. of the nanocomposite system are drastically improved compared to each component. The amalgamation of three phases destroys the center of inversion symmetry, which is not applicable in case of an individual component. The improved dielectric behavior, introduction of magnetic ordering by CFO, the lack of center of inversion symmetry together with the high strain in the lattice helps to achieve the nanocomposite multiferroic at room temperature.
1. Introduction In magnetoelectric multiferroics ferromagnetism and ferroelectricity are present in a single phase, and the possibility of spin-phonon and spin-polar couplings in these systems offer the opportunity to design a range of unconventional devices, such as multiple-state memory ele ments and electric-field controlled magnetic sensors [1–5]. The type-II multiferroics are the subject of intense research for the past few years as in these materials the ferroelectricity is caused by an exotic magnetic order rather than by a non-centrosymmetric crystal structure. However, their ultimate uses depend upon the nature and strength of the magne toelectric coupling around room temperature (RT). From the viewpoint of material constituents, the magnetoelectric multiferroics are divided
into two categories: single-phase and composite system. It has been observed that the single phase compound possess very low Curie tem perature and the intrinsic magnetoelectric coupling amplitude is rela tively low at RT which restrict them from the practical applications [6, 7]. Since none of the existing single phase multiferroics offers robust ferromagnetic and ferroelectric ordering at RT, multiferroics combining ferromagnetic and ferroelectric phases together are becoming a great alternative as there is flexibility in material design [8]. In multiferroic composites neither of the phases retains magnetoelectric coupling, however, the cross-interaction between the multiple phases can produce notable magnetoelectric effect [8]. The magnetoelectric effect in com posite materials is recognized as a product tensor property was proposed by van Suchtelen [9]. Many bulk multiferroic composites exhibit
* Corresponding author. E-mail addresses: [email protected] (A. Mitra), [email protected] (A. Shaw), [email protected] (P.K. Chakrabarti). https://doi.org/10.1016/j.matchemphys.2019.122242 Received 27 March 2019; Received in revised form 24 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Theoretically generated pattern of GMO-CFO@TO obtained from Rietveld analysis (The (hkl) planes of GMO are assigned by first bracket whereas the planes of CFO and TiO2 are assigned by a single asterisk and double asterisk notation). Inset of Fig. 1 (a) shows that the XRD peak at 2θ near 25.3� of TiO2 is not properly fitted using I41 /amd space group but fitted properly using I41md. (b) Representative crystal structure of a unit cell with bond length and bond angle of GMO in GMO-CFO@TO. (c) Represen tative of TiO6 octahedra of centrosymmetric and non-centrosymmetric TiO2 crystal structure.
strain-mediated magnetoelectric effects [10,11]. Recently, it has been shown that nanostructure multiferroic composite [12,13] provides a novel way to enhance magnetoelectric coupling by maximizing the interfacial area between the two phases of the nanocomposite. The exact origin of the magnetoelectric coupling is difficult to identify due to the complexities of multiferroic nanocomposites, as the ferroelectric and ferromagnetic domain structure of the constituent materials may be altered in the nanoscale region [14] and also the magnetoelectric coupling is dependent on the domain level process. Here in the present paper, the motivation is to develop a multiferroic nanocomposite using three constituents like GdMnO3 (GMO), CoFe2O4.(CFO) and TiO2. Each component in the composite has a high potential for different properties. The combined phases of GMO and CFO in the nanocomposite state have some limitations as multiferroic [15] owing to high dielectric loss, though magnetic ordering is found at room temperature. TiO2 matrix hosts the GMO and CFO where the dielectric loss has been remarkably lowered and the dielectric and ferroelectric ordering (below ~13 K) of GMO along with the magnetic phase of CFO are utilized to develop the multiferroic. The strength of the multi ferroicity is also improved owing to mismatch of crystal structure that destroys the center of inversion symmetry in the high strained lattice.
the dried powder was annealed at 500 C for 5 h to get the desired nanocomposite of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 (GMO-C FO@TO). X-ray observation of the nanocomposite was done in BRUKER D8 Advance diffractometer with da Vinchi using Cu Kα radiation and the Rietveld analysis of the recorded data was carried out by MAUD pro gram (version 2.33). The measurements in HRTEM were carried out by using a JEOL JEM 2100 HRTEM operating at 200 kV. Dielectric constant (ε’), the loss tangent (tanδ) and the ac conductivity of GMO-CFO@TO were measured as functions of temperature and frequency by HIOKI 3532-50 LCR HiTESTER. Polarization vs. electric field (P-E) loops of the nanocomposite sample were recorded at RT by a P-E loop tracer, Mul tiferroic Precission Premier - II ferroelectric test system, supplied by Radiant Technologies Inc. USA. The variation of magnetocapacitance co-efficient (MC) as a function of magnetic field (H) was calculated from the dielectric constant of the nanocomposite sample measured in the presence and absence of the magnetic field. �
3. Results and discussions 3.1. XRD analysis X-ray data of GMO-CFO@TO recorded at RT is shown in Fig. 1 (a). It is observed from JCPDs file (JCPDs file no. 22–1086) that CFO belongs to the cubic structure with space group Fd3m and GMO are in the family of the orthorhombic structure with space group Pbnm (25–0337). The host matrix of TiO2 crystallizes in tetragonal structure with a space group of I41 /amd (JCPDs file no. 75–1537). No other peak except the desired components is detected in Fig. 1 (a) which confirms the absence of any impurity phase in the nanocomposite. Rigorous Rietveld analysis is performed to know the formation of the crystallographic phase of the
2. Materials and methods The details of the preparation of (GdMnO3)0.7(CoFe2O4)0.3 (GMOCFO) were reported in the previous report [15]. For the preparation of GMO-CFO@TO, in the first phase, the aqueous solution of the desired amount of titanium (IV) butoxide (C16H36O4Ti) kept in a beaker. The particles of GMO-CFO are uniformly dispersed in the solution by soni � cation for a duration of ~1 h. The mixture was dried at 60 C and this as 2
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Table 1 Different values of pristine GMO and GMO-CFO@TO obtained from Rietveld analysis of XRD pattern. Samples
Lattice parameters (Å)
Cell volume (Å3)
Rwp(%)
Rexp (%)
χ2 (%)
Mn–O
Gd–O
GMO in GMO-CFO
a ¼ 5.311(4) b ¼ 5.782(3) c ¼ 7.447(3) a ¼ 5.311(2) b ¼ 5.732(2) c ¼ 7.412(4)
228.72
2.74
2.05
1.33
1.94(2) 1.59(1)
2.19(2) 2.07(4)
158.7(1)
225.67
4.78
2.57
1.85
1.93(3) 1.58(7)
2.18(5) 2.03(1)
160.1(3)
GMO in GMO-CFO@TO
Interatomic distances (Å)
Mn–O–Mn Ø (deg)
Fig. 2. HRTEM observation of GMO-CFO@TO (a) Micrographs, (b) particle size distribution including lognormal fitting, (c) and (d) fringe pattern, (e) EDS, and (f) SAED pattern.
nanocomposite system of GMO-CFO@TO. Different structural and pro file fitting parameters are determined from the analysis, which are shown in Table 1. The values of lattice parameters and unit cell volume of GMO in the nanocomposite is lowered with respect to that of GMO causes an increase of lattice strain due to the wrapping of GMO-CFO by TiO2 matrix. One of the representative unit cells of GMO in the
nanocomposite is shown in Fig. 1 (b) which exhibits that the bond length of Mn–O (~1.93(3), ~1.58(7)) and Gd–O (~2.18(5), 2.03(1)) is shorter compared to that of bond length of Mn–O (~1.94(2), ~1.59(1)) and Gd–O (~2.19(2), 2.07(4)) of GMO in the nanocomposite sample of GMO-CFO which are also substantiated from the Rietveld analysis. Whereas the Mn–O–Mn bond angle (~160.1� ) of GMO in GMO3
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Fig. 3. Frequency dependence of real part of dielectric permittivity ε0 and dielectric loss factor tanδ at different temperatures for (a) and (b) GMO-CFO, (c) and (d) GMO-CFO@TO.
CFO@TO increases compared to that of Mn–O–Mn bond angle (~158.7� ) of GMO in GMO-CFO. The decrease of bond length and the increase of bond angle of GMO in GMO-CFO@TO compared to that of GMO-CFO may be due to the increase of lattice strain arises from the presence of TiO2 matrix. Actually, the lattice strain is originated from the mismatch of the unit cell of adjacent constituents. Rietveld analysis shows that the XRD pattern of GMO-CFO@TO has been successfully fitted with the space group I41md (non-centrosymmetric) of TiO2 (ob tained from symmetry according to symmetry analysis by ISOTROPY Software Suite [16]) rather than the tetragonal centrosymmetric struc ture of TiO2 with space group symmetry I41/amd (shown in the inset of Fig. 1 (a)). The crystal structure of TiO2 in GMO-CFO@TO nano composite corresponds to the lower symmetry space group (I41md) (non-centrosymmetric). However, the space group symmetry of GMO (Pbnm) and CFO (Fd3m) was not necessary to alter for good fitting of XRD spectra of GMO-CFO@TO. The structural change of TiO2 from centrosymmetric I41/amd space group to non-centrosymmetric I41md space group in the nanocomposite state of GMO-CFO@TO is mainly originated from lattice strain as mentioned earlier. One representative unit cell of the tetragonal centrosymmetric structure of TiO2 along with TiO6 octahedra [17] and non-centrosymmetric structure of TiO2 along with TiO6 octahedra obtained from said Rietveld analysis of the XRD pattern of GMOCFO@TO nanocomposite, is shown in Fig. 1 (c). The nanocrystallite size of GMO, CFO, and TiO2 in GMO-CFO@TO is esti mated from the highest peak in the XRD pattern using Scherer’s equa tion [15]. The calculated crystallite size of GMO, CFO and TiO2 are ~55.8, ~12.5, and ~9.1 nm, respectively. In order to estimate the lat tice microstrain of GMO in the nanocomposite sample of GMO-CFO@TO and GMO-CFO, the Hall–Williamson (H–W) method [18] has been considered and the value of lattice strain of GMO in GMO-CFO@TO is 20 � 10 4 which is increased compared to the lattice strain ~11 � 10 4 of GMO in GMO-CFO nanocomposite state.
diffraction (SAED) pattern are recorded during HRTEM observations which are shown in Fig. 2. One representative micrograph is shown in Fig. 2 (a) which exhibits that most of the particles are spherical in shape and well-dispersed i.e., the particles are not yet agglomerated. In order to estimate the average particle size, diameters of about 300 nano particles are measured from different micrographs and the corre sponding particle size distribution (PSD) is shown in Fig. 2 (b). The PSD is tried to fit using log-normal function and the corresponding fitting is shown in Fig. 2 (b). Fig. 2 (b) shows that the PSD is fitted well in lognormal function and most of the particles are distributed in the range of 20–45 nm that is the particles are concentrated in the narrow region which arises due to the application of sonication during the course of preparation [19]. The estimated average particle size of GMO-CFO@TO is ~44.7 nm which is very close to the average crystallite size (~42.5 nm) obtained from XRD. This fact indicates that most of the particles are nanocrystallite in nature. In order to confirm the nano crystalline nature of the nanoparticles, different fringe patterns are recorded and two representative fringe patterns are shown in Fig. 2 (c) and (d). The fringe width estimated from the separation of two adjacent dark fringes is ~0.14, ~0.10 and ~0.13 nm (Fig. 2 (c)) which corre sponds to (224), (800) and (220) crystallographic plane of GMO, CFO, and TiO2 and the separation of two adjacent bright fringes are ~0.11, ~0.10 and ~0.12 nm (Fig. 2 (d)) which corresponds to the crystallo graphic plane (731), (800) and (224) of CFO and TiO2, respectively. The presence of all the cations (Gd, Mn, Co, Fe, and Ti) is also detected and confirmed in the EDS spectrum as shown in Fig. 2 (e). Fig. 2 (f) shows the SAED pattern of GMO-CFO@TO with rings of different diameters which correspond to different lattice planes of the individual component of GMO, CFO, and TiO2 in the nanocomposite state of GMO-CFO@TO. The planes of GMO are assigned by parenthesis whereas the planes of CFO and TiO2 are assigned by a single asterisk and double asterisk notation with parenthesis which matches well with those obtained in the XRD pattern.
3.2. TEM analysis
3.3. Dielectric and ferroelectric properties
To investigate the morphology of GMO-CFO@TO, different micro graphs, nanocrystallite fringe patterns, and selected area electron
Fig. 3 (a), (b) and (c), (d) show the variation of dielectric permittivity 4
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Fig. 4. (a) Cole-Cole fit of dielectric spectra at RT (b) Frequency dependent dielectric loss factor at RT for GMO-CFO and GMO-CFO@TO.
Fig. 5. Thermal variation of dielectric permittivity recorded at 100 kHz, 1 MHz, and 4 MHz for (a) GMO-CFO and (b) GMO-CFO@TO.
(ε’) and loss tangent (tanδ) with frequency measured at different tem peratures in the frequency range of 42 Hz-5 MHz of GMO-CFO and GMOCFO@TO respectively. The value of ε’ is quite high in the low frequency region for both the samples due to the Maxwell-Wagner polarization and it decreases rapidly with the increase of frequency for GMO-CFO whereas the value of ε’ for GMO-CFO@TO decreases up to ~12 kHz and above 12 kHz the value of ε’ becomes almost independent with frequency. The variation of ε’ with frequency indicates the presence of non-Debye like relaxation in the samples. The dielectric permittivity does not approach a constant value at the highest frequency in our experiment and also electrode polarization dominates at low fre quencies. Therefore, in order to determine the values of ε(0) and ε(∞) we have fitted real part of the dielectric spectra in the intermediate to
high frequency range using the Cole-Cole relaxation equation [20], h � . �i 0 1 0 1 Δε 1 þ ðωτcc Þ1 α sin απ 2 0 ε @ωA ¼ ε@∞A þ (1) �2ð1 αÞ � . � � 1 þ 2ðωτcc Þ1 α sin απ 2 þ ωτcc where τcc is the relaxation time, Δε ¼ εð0Þ εð∞Þ is the dielectric strength and α is the shape parameter. One of the representative ColeCole fittings in the real part of the dielectric spectra of GMO-CFO and GMO-CFO@TO at RT is shown in Fig. 4 (a). The value of dielectric strength for GMO-CFO@TO increases considerably to that of GMO-CFO. The frequency variation of tanδ (shown in Fig. 4 (b)) exhibit that dielectric loss of GMO-CFO@TO at RT is reduced ~3 times compared to
Fig. 6. (a) Polarization vs. electric field loop for GMO-CFO and GMO-CFO@TO recorded at RT. (b) Variation of MC with magnetic field of GMO-CFO@TO recorded at 100 kHz, 1 MHz and 4 MHz. 5
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Materials Chemistry and Physics 240 (2020) 122242
Fig. 7. (a) Ac conductivity spectra at RT. The lines represent the power law fittings. (b) Complex impedance plots at RT. (c) Scaling of conductivity spectra according to Summerfield scaling law. (d) Scaling of time dependent. The inset represent the time variation of for GMO-CFO.
that of GMO-CFO which is suitable for the applications in microwave devices. Thermal variation of ε’ of GMO-CFO and GMO-CFO@TO (Fig. 5 (a) and (b)) exhibit that the value of ε’ of GMO-CFO increases with tem perature whereas the value of ε’ in case of GMO-CFO@TO gradually increases with the rise in temperature and attains a maximum value, then decreases with further rise in temperature. This type of variation of ε’ is basically observed in case of ferroelectric materials [21,22] and the temperature at which the value of ε’ is maximum, called transition temperature (Tc) which is ~385 K above which GMO-CFO@TO exhibits a ferroelectric to a paraelectric phase transition. In order to investigate the presence of ferroelectric property of the nanocomposite samples of GMO-CFO and GMO-CFO@TO, polarization vs. electric field (P-E) loops of these samples are recorded at RT at an applied voltage of 800 V at a frequency of 50 Hz which are shown in Fig. 6 (a). Fig. 6 (a) shows that GMO-CFO exhibits very weak ferro electric response due to the presence of high leakage current problem whereas GMO-CFO@TO shows good ferroelectric polarization at RT compared to that of GMO-CFO. Different parameters such as maximum polarization (Pmax), remnant polarization (Pr) and coercive field (Hc) are extracted from the P-E loop of GMO-CFO@TO and the corresponding value of these parameters are ~0.2 μC/cm2, 0.1 μC/cm2 and 1.6 kV/cm, respectively. Ferroelectric polarization of GMO-CFO@TO nano composite arises mainly due to the non-centrosymmetric symmetry (I41md) of the TiO2 crystal structure as GMO and CFO crystallize into centrosymmetric space group. Due to the presence of small distortion, TiO2 crystal structure deviates from the centrosymmetric to noncentrosymmetric position and correspondingly, the charge centers of the negative and positive ions get separated giving rise to a permanent electric dipole moment. When a voltage is applied across the sample, the dipoles will align along the direction of the external electric field which induces a local electric field, results in a ferroelectric response of GMOCFO@TO. The coupling between magnetic and ferroelectric ordering present in GMO-CFO@TO is confirmed by measuring dielectric constant (ε’) in presence and absence of the applied magnetic field and the
corresponding value of magnetocapacitance (MC) is calculated using the following equation, 0
MC ¼
0
½ε ðH; TÞ ε ð0; TÞ� ε0 ð0; TÞ
(2)
where, ε’ ðH; TÞ and ε’ð0; TÞ represent the value of dielectric constant measured in presence and absence of magnetic field, respectively [23, 24]. This is one of the important ways to investigate the coupling effect, as used by many authors [23–25]. The corresponding variation of MC with the applied magnetic field, recorded at 100 kHz, 1 MHz and 4 MHz, respectively, is shown in Fig. 6 (b) which indicates that the value of MC increases nonlinearly with the application of an external magnetic field. This kind of nonlinear variation of MC indicates the presence of positive magnetoelectric coupling in GMO-CFO@TO at RT. The value of MC of GMO-CFO@TO decreases with the increase of frequency. No such coupling is observed in case of GMO-CFO nanocomposite. Due to the presence of coupling between magnetic and electric ordering in the nanocomposite sample of GMO-CFO@TO, when placed in an external magnetic field a strain is induced in it. This strain induces a stress in the nanocomposite sample which produces an electric field in the sample. Thus the ferroelectric domains in GMO-CFO@TO are oriented by this induced electric field results in an increase of the polarization value. The corresponding strain increases with the increase of the magnetic field causing the polarization value to change further. 3.4. Ac conductivity analysis The conductivity spectra (real) at RT for GMO-CFO and GMOCFO@TO are shown in Fig. 7 (a). It is observed that at low-frequency region the ac conductivity is almost independent of frequency and is recognized to the diffusive motion of the mobile charge carriers. How ever, a crossover from the frequency independent dc conductivity region to frequency dependent dispersive region has been observed indicating the onset of the relaxation processes. The ac conductivity spectra can be well described by the power law model given by, [26].
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Table 2 The values of dc conductivity, crossover frequency, power law exponent, dielectric strength, √ and √ at RT for GMO-CFO and GMO-CFO@TO. Samples
σdc ðΩ 1 cm 1 Þ
GMO-CFO GMO-CFO@TO
3.27 � 10 1.49 � 10
6 7
ωc ðrad s 1 Þ
n
Δε
√ (Ǻ) (�0.05 Å)
√ (Ǻ) (�0.05 Å)
361159.5 1629.8
0.39 0.49
182.5 306.1
4.79 4.57
3.12 1.27
(3)
σ(ω) ¼ σdc[1þ(ω/ ωc)n]
n is the power law exponent. At longer time scale the charge carriers have enough time for forwarding hopping and are categorized by the diffusive motion of mobile charge carriers: ~ t. The charac teristic means square displacement of the center of charge of mobile charge carriers () has been defined as the average distance the mobile charge carriers travel at the transition region of the sub-diffusive and the diffusive motion. The characteristic mean square displacement of the center of charge of mobile charge carriers implies the average distance the mobile charge carriers have to cover to overcome the nearest highest barrier on the conduction path. The Summerfield scaling law [Eq. (4)] can be converted into the scaling law for mean square displacement of charge carriers, given by, ¼ F(tσdcT) [30]. Fig. 7 (d) shows the Summerfield scaling for GMO-CFO and GMO-CFO@TO. It is observed that do not overlap, which also signifies that the characteristic mean square displacement of the center of charge of the charge carriers for GMO-CFO and GMO-CFO@TO are different. The charge carrier dynamics has been further investigated in the longer scale by determining the spatial extent of the localized motion qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 < R ð∞Þ >. The long time limiting value of is given by Ref. [32],
where ωc is the characteristic frequency indicating crossover from the dc to the dispersive conductivity and n (0 < n < 1) is a power law exponent. The power law exponent n depends on the dimensionality of the con duction space [27]. The ac conductivity spectra at RT for GMO-CFO and GMO-CFO@TO has been fitted to Eq. (3). The value of these parameters, obtained from the power law fitting is listed in Table 2. It is observed that the value of dc conductivity decreases significantly for GMO-CFO@TO compared to that of GMO-CFO, which is analogous to the ferroelectric response observed for GMO-CFO@TO. In order to accurately measure the dc conductivity, the values were determined using a Nyquist plot as shown in Fig. 7 (b). The contribution of electrode polarization is clearly noticed (arc converted to spike) at the low-frequency region. We have fitted the impedance data excluding the electrode interface region by a simple R–C circuit, as shown in Fig. 7 (b). The value of dc conductivity, obtained from the fitting of complex impedance plot is in well agreement with those obtained from the power law fitting of the ac conductivity spectra. In order to better understand of the conduction mechanism, we have applied the Summerfield scaling formalism [28] on the RT ac conduc tivity spectra for GMO-CFO and GMO-CFO@TO given by, � � σ ðωÞ ω ¼F (4) σdc :T σ dc
02
< R ð∞Þ >¼
The conductivity master-curve obtained using Summerfield scaling formalism is shown in Fig. 7 (c). It is noted indeed that the scaled con ductivity spectra do not map onto the same curve. The reason behind this result might be due to the different characteristic length scales of charge carriers are associated with these two materials. The conductivity spectra can be related to the evolution of the timedependent mean square displacement of charge carriers () in thermal equilibrium using the linear response theory [29]. The fre quency dependent conductivity is given by Ref. [30], 0 1 Z ∞ � � N q 2 ω2 σ@ωA ¼ c < r2 t > sin ωt dt (5) 6kВ THR 0
12kВ THR Nc q2 π
Zt
Z∞ dt
0
0
σðωÞ sinðωt’Þdω ω
0
(7)
where Δε is the dielectric strength of the sample, obtained from the Colepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Cole fit of the dielectric spectra. The values of < R2 ðtp Þ > and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 < R ð∞Þ > for GMO-CFO and GMO-CFO@TO are listed in Table 2. It pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi is noticed that the value of < R2 ðtp Þ > decreases for GMO-CFO@TO by a small fraction than that of GMO-CFO. Whereas the value of qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 < R ð∞Þ > decreases significantly for GMO-CFO@TO. Thus, the decrease of dc conductivity for GMO-CFO@TO may be directly related pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi to the decrease of these two characteristic length scales < R2 ðtp Þ > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 and < R ð∞Þ >. 4. Conclusion
where q is the charge of mobile charge carriers, Nc is the number density of the mobile charge carriers and HR is the Haven ratio. Haven’s ratio signifies the degree of correlation between successive hops. Conversely, the mean square displacement of charge carriers, can be calculated from the conductivity spectra from the Fourier trans formation of the above equation given by Ref. [30], < r2 ðtÞ >¼
6 k B T ε0 Δε Nc q2
A novel multiferroic nanocomposite of GMO-CFO@TO is derived by using three potentially important components. The different unit cells of these components are clubbed together which generates a lattice strain and this fact leads to the departure from the center of inversion to the non-centrosymmetric structure of TiO2. These extracted results are duly supported by Rietveld analysis of X-ray diffractogram spectra. The lack of center of inversion symmetry, the presence of TiO2 matrix helps to introduce the ferroelectric polarization at RT in the nanocomposite. CFO which itself a good system for magnetic and dielectric properties and GMO, which is multiferroic below ~13 K incorporated in TiO2 matrix helps to achieve the multiferroic at RT.
(6)
2
¼< R ðtÞ > HR is the mean square displacement of the center of charge of the mobile charge carriers [31]. The mobile charge carriers are moving in three-dimensional potential landscapes and prefer to stay at the low energy sites. The height of the potential barriers varies from sites to sites. At high-frequency region or shorter time scale, the charge carriers have less time for the forward hop to the higher energy sites and the proba bility of backward jump is higher. Thus, in this time scale the charge carrier dynamics is characterized by the sub-diffusive motion of mobile charge carriers and follows the relation, ~ t1 n, where
Acknowledgment One of the authors, Mr. A. Mitra wishes to acknowledge the Department of Science and Technology for the fellowship in INSPIRE Program. One of the authors A. Shaw wishes to acknowledge SERB (PDF/2016/000643) for their financial support and fellowship in the NPDF program. The authors are thankful to UGC DAE CSR MUMBAI 7
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Center for financial support under CRS-M-265/2017/553 and CAS-II physics (File No. CAS F. 530/20/CAS-II/2018 (SAP-I)) to carry out this work.
[16] [17] [18]
References [1] M. Fiebig, T. Lottermoser, D. Meier, M. Trassin, The evolution of multiferroics, Nat. Rev. Mater. 1 (2016) 16046. [2] N.A. Spaldin, Multiferroics: from the cosmically large to the subatomically small, Nat. Rev. Mater. 2 (2017) 17017. [3] S. Ghara, E. Suard, F. Fauth, T.T. Tran, P.S. Halasyamani, A. Iyo, J. RodríguezCarvajal, A. Sundaresan, Ordered aeschynite-type polar magnets RFeWO6 (R¼Dy, Eu, Tb, and Y): a new family of type-II multiferroics, Phys. Rev. B 95 (2017) 224416. [4] B.R. McBride, J. Lieschke, A. Berlie, D.L. Cortie, H.Y. Playford, T. Lu, N. Narayanan, R.L. Withers, D. Yu, Y. Liu, Study of the B-site ion behaviour in the multiferroic perovskite bismuth iron chromium oxide, J. Appl. Phys. 123 (2018) 154104. [5] J. Liu, T.L. Sun, X.Q. Liu, H. Tian, T.T. Gao, X.M. Chen, A novel room-temperature multiferroic system of hexagonal Lu1 xInxFeO3, Adv. Funct. Mater. 28 (2018) 1706062. [6] S.K. Upadhyay, K.K. Iyer, S. Gohil, S. Ghosh, P.L. Paulose, E.V. Sampathkumaran, Pyrocurrent anomalies and intrinsic magnetodielectric behavior near room temperature in Li2Ni2Mo3O12, a compound with distorted honeycomb and spinchains, Sci. Rep. 7 (2017) 4449. [7] A. Maignan, C. Martin, Fe4Nb2O9: a magnetoelectric antiferromagnet, Phys. Rev. B 97 (2018) 161106. [8] J. Ma, J. Hu, Z. Li, C.-W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater. 23 (2011) 1062. [9] J. van, Philips Res. Rep. 27 (1972) 28. [10] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, Multiferroic magnetoelectric composites: historical perspective, status, and future directions, J. Appl. Phys. 103 (2008), 031101. [11] Y. Zhang, Z. Li, C.Y. Deng, J. Ma, Y.H. Lin, C.W. Nan, Demonstration of magnetoelectric read head of multiferroic heterostructures, Appl. Phys. Lett. 92 (2008) 152510. [12] H. Zheng, F. Straub, Q. Zhan, P.L. Yang, W.K. Hsieh, F. Zavaliche, Y.H. Chu, U. Dahmen, R. Ramesh, Self-assembled growth of BiFeO3–CoFe2O4 nanostructures, Adv. Mater. 18 (2006) 2747. [13] C. Schmitz-Antoniak, D. Schmitz, P. Borisov, F.M.F. de Groot, S. Stienen, A. Warland, B. Krumme, R. Feyerherm, E. Dudzik, W. Kleemann, H. Wende, Electric in-plane polarization in multiferroic CoFe2O4/BaTiO3 nanocomposite tuned by magnetic fields, Nat. Commun. 4 (2013) 2051. [14] X.Z. Yu, Y. Onose, N. Kanazawa, J.H. Park, J.H. Han, Y. Matsui, N. Nagaosa, Y. Tokura, Real-space observation of a two-dimensional skyrmion crystal, Nature 465 (2010) 901. [15] A. Mitra, A.S. Mahapatra, A. Mallick, P.K. Chakrabarti, Room temperature magnetic ordering, enhanced magnetization and exchange bias of GdMnO3
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[24]
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8
nanoparticles in (GdMnO3)0.70(CoFe2O4)0.30, J. Magn. Magn. Mater. 424 (2017) 388–393. ISOTROPY software suite. iso.byu.edu. C.J. Howard, T.M. Sabine, F. Dickson, Structural and thermal parameters for rutile and anatase, Acta Crystallogr. B 47 (1991) 462–468. E. Swatsitang, A. Karaphun, S. Phokha, S. Hunpratub, T. Putjuso, Investigation of structural, morphological, optical, and magnetic properties of Sm-doped LaFeO3 nanopowders prepared by sol–gel method, J. Sol. Gel Sci. Technol. 81 (2016) 483–492. K. Mukhopadhyay, S. Sutradhar, S. Modak, S.K. Roy, P.K. Chakrabarti, Enhanced magnetic behavior of chemically prepared multiferroic nanoparticles of GaFeO3 in (GaFeO3)0.50(Ni0.4Zn0.4Cu0.2Fe2O4)0.5 nanocomposite, J. Phys. Chem. C 116 (2012) 4948–4956. K.S. Cole, R.H. Cole, Dispersion and absorption in dielectrics I. Alternating current characteristics, J. Chem. Phys. 9 (1941) 341. H. Yang, F. Yan, Y. Lin, T. Wang, F. Wang, Y. Wang, L. Guo, W. Tai, H. Wei, Leadfree BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 relaxor ferroelectric ceramics for high energy storage, Eur. Ceram. Soc. 37 (2017) 3303–3311. P. Sharma, P. Kumar, R.S. Kundu, J.K. Juneja, N. Ahlawat, R. Punia, Structural and dielectric properties of substituted barium titanate ceramics for capacitor applications, Ceram. Int. 41 (2015) 13425–13432. K. Mukhopadhyay, A.S. Mahapatra, P.K. Chakrabarti, Modulated magnetodielectric property and exchange bias effect of BiFeO3 incorporated in (BiFeO3)0.50(Li0.30Zn0.35Fe2.35O4)0.50 nanocomposite, J. Magn. Magn. Mater. 385 (2015) 347–357. S. Acharya, S. Sutradhar, J. Mandal, K. Mukhopadhyay, A.K. Deb, P.K. Chakrabarti, Sol–gel derived nanocrystalline multiferroic BiFeO3 and R3þ (R¼Er and Tm) doped therein: magnetic phase transitions and enhancement of magnetic properties, J. Magn. Magn. Mater. 324 (2012) 4209–4218. T. Katsufuji, H. Takagi, Coupling between magnetism and dielectric properties in quantum paraelectricEuTiO3, Phys. Rev. B 64 (2001), 054415-1–;054415–4. A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric, London, 1996. D.L. Sidebottom, Dimensionality dependence of the conductivity dispersion in ionic materials, Phys. Rev. Lett. 83 (1999) 983. S. Summerfield, Philos. Mag. B 52 (1985) 9. R. Kubo, Statistical-mechanical theory of irreversible processes. I. General theory and simple applications to magnetic and conduction problems, J. Phys. Soc. Jap. 12 (1957) 570. B. Roling, C. Martiny, S. Brückner, Ion transport in glass: Influence of glassy structure on spatial extent of nonrandom ion hopping, Phys. Rev. B 63 (2001) 214203. S. Kabi, A. Ghosh, Silver ion dynamics in mixed network former glasses: evidence of correlation with characteristic lengths and network structure, EPL (Europhys. Lett.) 100 (2012) 26007. T. Paul, A. Ghosh, Transport of oxygen ions in Er doped La2Mo2O9 oxide ion conductors: correlation with microscopic length scales, J. Appl. Phys. 123 (2018), 045107.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructure, dielectric, ferroelectric and magnetoelectric coupling of a novel multiferroic of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 nanocomposite A. Mitra, A. Shaw, P.K. Chakrabarti * Solid State Research Laboratory, Department of Physics, Burdwan University, Burdwan, 713104, India
H I G H L I G H T S
� [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 exhibits room temperature multiferroicity. � Presence of TiO2 matrix helps to introduce RT ferroelectric polarization. � Dielectric behaviour of the nanocomposite is drastically improved. � Magnetoelectric coupling of the nanocomposite is observed at RT. A R T I C L E I N F O
A B S T R A C T
Keywords: XRD HRTEM Dielectrics Ferroelectrics Magnetoelectric coupling
A new multiferroic is derived from GdMnO3 (GMO), CoFe2O4 (CFO) and TiO2 in the nanocomposite phase. Here the good dielectric properties of GMO and TiO2 including the multiferroicity of GMO below ~13 K along with the magnetic ordering of CFO are exploited to develop the desired nanocomposite system. The nanocomposite of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 (GMO-CFO@TO) was obtained from chemical preparation route. Detailed investigations of structural and morphology are carried out by analyzing the X-ray diffractograms and pictures recorded during high resolution transmission electron microscopy. Dielectric behavior like dielectric strength, loss tangent etc. of the nanocomposite system are drastically improved compared to each component. The amalgamation of three phases destroys the center of inversion symmetry, which is not applicable in case of an individual component. The improved dielectric behavior, introduction of magnetic ordering by CFO, the lack of center of inversion symmetry together with the high strain in the lattice helps to achieve the nanocomposite multiferroic at room temperature.
1. Introduction In magnetoelectric multiferroics ferromagnetism and ferroelectricity are present in a single phase, and the possibility of spin-phonon and spin-polar couplings in these systems offer the opportunity to design a range of unconventional devices, such as multiple-state memory ele ments and electric-field controlled magnetic sensors [1–5]. The type-II multiferroics are the subject of intense research for the past few years as in these materials the ferroelectricity is caused by an exotic magnetic order rather than by a non-centrosymmetric crystal structure. However, their ultimate uses depend upon the nature and strength of the magne toelectric coupling around room temperature (RT). From the viewpoint of material constituents, the magnetoelectric multiferroics are divided
into two categories: single-phase and composite system. It has been observed that the single phase compound possess very low Curie tem perature and the intrinsic magnetoelectric coupling amplitude is rela tively low at RT which restrict them from the practical applications [6, 7]. Since none of the existing single phase multiferroics offers robust ferromagnetic and ferroelectric ordering at RT, multiferroics combining ferromagnetic and ferroelectric phases together are becoming a great alternative as there is flexibility in material design [8]. In multiferroic composites neither of the phases retains magnetoelectric coupling, however, the cross-interaction between the multiple phases can produce notable magnetoelectric effect [8]. The magnetoelectric effect in com posite materials is recognized as a product tensor property was proposed by van Suchtelen [9]. Many bulk multiferroic composites exhibit
* Corresponding author. E-mail addresses: [email protected] (A. Mitra), [email protected] (A. Shaw), [email protected] (P.K. Chakrabarti). https://doi.org/10.1016/j.matchemphys.2019.122242 Received 27 March 2019; Received in revised form 24 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Theoretically generated pattern of GMO-CFO@TO obtained from Rietveld analysis (The (hkl) planes of GMO are assigned by first bracket whereas the planes of CFO and TiO2 are assigned by a single asterisk and double asterisk notation). Inset of Fig. 1 (a) shows that the XRD peak at 2θ near 25.3� of TiO2 is not properly fitted using I41 /amd space group but fitted properly using I41md. (b) Representative crystal structure of a unit cell with bond length and bond angle of GMO in GMO-CFO@TO. (c) Represen tative of TiO6 octahedra of centrosymmetric and non-centrosymmetric TiO2 crystal structure.
strain-mediated magnetoelectric effects [10,11]. Recently, it has been shown that nanostructure multiferroic composite [12,13] provides a novel way to enhance magnetoelectric coupling by maximizing the interfacial area between the two phases of the nanocomposite. The exact origin of the magnetoelectric coupling is difficult to identify due to the complexities of multiferroic nanocomposites, as the ferroelectric and ferromagnetic domain structure of the constituent materials may be altered in the nanoscale region [14] and also the magnetoelectric coupling is dependent on the domain level process. Here in the present paper, the motivation is to develop a multiferroic nanocomposite using three constituents like GdMnO3 (GMO), CoFe2O4.(CFO) and TiO2. Each component in the composite has a high potential for different properties. The combined phases of GMO and CFO in the nanocomposite state have some limitations as multiferroic [15] owing to high dielectric loss, though magnetic ordering is found at room temperature. TiO2 matrix hosts the GMO and CFO where the dielectric loss has been remarkably lowered and the dielectric and ferroelectric ordering (below ~13 K) of GMO along with the magnetic phase of CFO are utilized to develop the multiferroic. The strength of the multi ferroicity is also improved owing to mismatch of crystal structure that destroys the center of inversion symmetry in the high strained lattice.
the dried powder was annealed at 500 C for 5 h to get the desired nanocomposite of [(GdMnO3)0.7(CoFe2O4)0.3]0.5[TiO2]0.5 (GMO-C FO@TO). X-ray observation of the nanocomposite was done in BRUKER D8 Advance diffractometer with da Vinchi using Cu Kα radiation and the Rietveld analysis of the recorded data was carried out by MAUD pro gram (version 2.33). The measurements in HRTEM were carried out by using a JEOL JEM 2100 HRTEM operating at 200 kV. Dielectric constant (ε’), the loss tangent (tanδ) and the ac conductivity of GMO-CFO@TO were measured as functions of temperature and frequency by HIOKI 3532-50 LCR HiTESTER. Polarization vs. electric field (P-E) loops of the nanocomposite sample were recorded at RT by a P-E loop tracer, Mul tiferroic Precission Premier - II ferroelectric test system, supplied by Radiant Technologies Inc. USA. The variation of magnetocapacitance co-efficient (MC) as a function of magnetic field (H) was calculated from the dielectric constant of the nanocomposite sample measured in the presence and absence of the magnetic field. �
3. Results and discussions 3.1. XRD analysis X-ray data of GMO-CFO@TO recorded at RT is shown in Fig. 1 (a). It is observed from JCPDs file (JCPDs file no. 22–1086) that CFO belongs to the cubic structure with space group Fd3m and GMO are in the family of the orthorhombic structure with space group Pbnm (25–0337). The host matrix of TiO2 crystallizes in tetragonal structure with a space group of I41 /amd (JCPDs file no. 75–1537). No other peak except the desired components is detected in Fig. 1 (a) which confirms the absence of any impurity phase in the nanocomposite. Rigorous Rietveld analysis is performed to know the formation of the crystallographic phase of the
2. Materials and methods The details of the preparation of (GdMnO3)0.7(CoFe2O4)0.3 (GMOCFO) were reported in the previous report [15]. For the preparation of GMO-CFO@TO, in the first phase, the aqueous solution of the desired amount of titanium (IV) butoxide (C16H36O4Ti) kept in a beaker. The particles of GMO-CFO are uniformly dispersed in the solution by soni � cation for a duration of ~1 h. The mixture was dried at 60 C and this as 2
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Table 1 Different values of pristine GMO and GMO-CFO@TO obtained from Rietveld analysis of XRD pattern. Samples
Lattice parameters (Å)
Cell volume (Å3)
Rwp(%)
Rexp (%)
χ2 (%)
Mn–O
Gd–O
GMO in GMO-CFO
a ¼ 5.311(4) b ¼ 5.782(3) c ¼ 7.447(3) a ¼ 5.311(2) b ¼ 5.732(2) c ¼ 7.412(4)
228.72
2.74
2.05
1.33
1.94(2) 1.59(1)
2.19(2) 2.07(4)
158.7(1)
225.67
4.78
2.57
1.85
1.93(3) 1.58(7)
2.18(5) 2.03(1)
160.1(3)
GMO in GMO-CFO@TO
Interatomic distances (Å)
Mn–O–Mn Ø (deg)
Fig. 2. HRTEM observation of GMO-CFO@TO (a) Micrographs, (b) particle size distribution including lognormal fitting, (c) and (d) fringe pattern, (e) EDS, and (f) SAED pattern.
nanocomposite system of GMO-CFO@TO. Different structural and pro file fitting parameters are determined from the analysis, which are shown in Table 1. The values of lattice parameters and unit cell volume of GMO in the nanocomposite is lowered with respect to that of GMO causes an increase of lattice strain due to the wrapping of GMO-CFO by TiO2 matrix. One of the representative unit cells of GMO in the
nanocomposite is shown in Fig. 1 (b) which exhibits that the bond length of Mn–O (~1.93(3), ~1.58(7)) and Gd–O (~2.18(5), 2.03(1)) is shorter compared to that of bond length of Mn–O (~1.94(2), ~1.59(1)) and Gd–O (~2.19(2), 2.07(4)) of GMO in the nanocomposite sample of GMO-CFO which are also substantiated from the Rietveld analysis. Whereas the Mn–O–Mn bond angle (~160.1� ) of GMO in GMO3
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Fig. 3. Frequency dependence of real part of dielectric permittivity ε0 and dielectric loss factor tanδ at different temperatures for (a) and (b) GMO-CFO, (c) and (d) GMO-CFO@TO.
CFO@TO increases compared to that of Mn–O–Mn bond angle (~158.7� ) of GMO in GMO-CFO. The decrease of bond length and the increase of bond angle of GMO in GMO-CFO@TO compared to that of GMO-CFO may be due to the increase of lattice strain arises from the presence of TiO2 matrix. Actually, the lattice strain is originated from the mismatch of the unit cell of adjacent constituents. Rietveld analysis shows that the XRD pattern of GMO-CFO@TO has been successfully fitted with the space group I41md (non-centrosymmetric) of TiO2 (ob tained from symmetry according to symmetry analysis by ISOTROPY Software Suite [16]) rather than the tetragonal centrosymmetric struc ture of TiO2 with space group symmetry I41/amd (shown in the inset of Fig. 1 (a)). The crystal structure of TiO2 in GMO-CFO@TO nano composite corresponds to the lower symmetry space group (I41md) (non-centrosymmetric). However, the space group symmetry of GMO (Pbnm) and CFO (Fd3m) was not necessary to alter for good fitting of XRD spectra of GMO-CFO@TO. The structural change of TiO2 from centrosymmetric I41/amd space group to non-centrosymmetric I41md space group in the nanocomposite state of GMO-CFO@TO is mainly originated from lattice strain as mentioned earlier. One representative unit cell of the tetragonal centrosymmetric structure of TiO2 along with TiO6 octahedra [17] and non-centrosymmetric structure of TiO2 along with TiO6 octahedra obtained from said Rietveld analysis of the XRD pattern of GMOCFO@TO nanocomposite, is shown in Fig. 1 (c). The nanocrystallite size of GMO, CFO, and TiO2 in GMO-CFO@TO is esti mated from the highest peak in the XRD pattern using Scherer’s equa tion [15]. The calculated crystallite size of GMO, CFO and TiO2 are ~55.8, ~12.5, and ~9.1 nm, respectively. In order to estimate the lat tice microstrain of GMO in the nanocomposite sample of GMO-CFO@TO and GMO-CFO, the Hall–Williamson (H–W) method [18] has been considered and the value of lattice strain of GMO in GMO-CFO@TO is 20 � 10 4 which is increased compared to the lattice strain ~11 � 10 4 of GMO in GMO-CFO nanocomposite state.
diffraction (SAED) pattern are recorded during HRTEM observations which are shown in Fig. 2. One representative micrograph is shown in Fig. 2 (a) which exhibits that most of the particles are spherical in shape and well-dispersed i.e., the particles are not yet agglomerated. In order to estimate the average particle size, diameters of about 300 nano particles are measured from different micrographs and the corre sponding particle size distribution (PSD) is shown in Fig. 2 (b). The PSD is tried to fit using log-normal function and the corresponding fitting is shown in Fig. 2 (b). Fig. 2 (b) shows that the PSD is fitted well in lognormal function and most of the particles are distributed in the range of 20–45 nm that is the particles are concentrated in the narrow region which arises due to the application of sonication during the course of preparation [19]. The estimated average particle size of GMO-CFO@TO is ~44.7 nm which is very close to the average crystallite size (~42.5 nm) obtained from XRD. This fact indicates that most of the particles are nanocrystallite in nature. In order to confirm the nano crystalline nature of the nanoparticles, different fringe patterns are recorded and two representative fringe patterns are shown in Fig. 2 (c) and (d). The fringe width estimated from the separation of two adjacent dark fringes is ~0.14, ~0.10 and ~0.13 nm (Fig. 2 (c)) which corre sponds to (224), (800) and (220) crystallographic plane of GMO, CFO, and TiO2 and the separation of two adjacent bright fringes are ~0.11, ~0.10 and ~0.12 nm (Fig. 2 (d)) which corresponds to the crystallo graphic plane (731), (800) and (224) of CFO and TiO2, respectively. The presence of all the cations (Gd, Mn, Co, Fe, and Ti) is also detected and confirmed in the EDS spectrum as shown in Fig. 2 (e). Fig. 2 (f) shows the SAED pattern of GMO-CFO@TO with rings of different diameters which correspond to different lattice planes of the individual component of GMO, CFO, and TiO2 in the nanocomposite state of GMO-CFO@TO. The planes of GMO are assigned by parenthesis whereas the planes of CFO and TiO2 are assigned by a single asterisk and double asterisk notation with parenthesis which matches well with those obtained in the XRD pattern.
3.2. TEM analysis
3.3. Dielectric and ferroelectric properties
To investigate the morphology of GMO-CFO@TO, different micro graphs, nanocrystallite fringe patterns, and selected area electron
Fig. 3 (a), (b) and (c), (d) show the variation of dielectric permittivity 4
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Fig. 4. (a) Cole-Cole fit of dielectric spectra at RT (b) Frequency dependent dielectric loss factor at RT for GMO-CFO and GMO-CFO@TO.
Fig. 5. Thermal variation of dielectric permittivity recorded at 100 kHz, 1 MHz, and 4 MHz for (a) GMO-CFO and (b) GMO-CFO@TO.
(ε’) and loss tangent (tanδ) with frequency measured at different tem peratures in the frequency range of 42 Hz-5 MHz of GMO-CFO and GMOCFO@TO respectively. The value of ε’ is quite high in the low frequency region for both the samples due to the Maxwell-Wagner polarization and it decreases rapidly with the increase of frequency for GMO-CFO whereas the value of ε’ for GMO-CFO@TO decreases up to ~12 kHz and above 12 kHz the value of ε’ becomes almost independent with frequency. The variation of ε’ with frequency indicates the presence of non-Debye like relaxation in the samples. The dielectric permittivity does not approach a constant value at the highest frequency in our experiment and also electrode polarization dominates at low fre quencies. Therefore, in order to determine the values of ε(0) and ε(∞) we have fitted real part of the dielectric spectra in the intermediate to
high frequency range using the Cole-Cole relaxation equation [20], h � . �i 0 1 0 1 Δε 1 þ ðωτcc Þ1 α sin απ 2 0 ε @ωA ¼ ε@∞A þ (1) �2ð1 αÞ � . � � 1 þ 2ðωτcc Þ1 α sin απ 2 þ ωτcc where τcc is the relaxation time, Δε ¼ εð0Þ εð∞Þ is the dielectric strength and α is the shape parameter. One of the representative ColeCole fittings in the real part of the dielectric spectra of GMO-CFO and GMO-CFO@TO at RT is shown in Fig. 4 (a). The value of dielectric strength for GMO-CFO@TO increases considerably to that of GMO-CFO. The frequency variation of tanδ (shown in Fig. 4 (b)) exhibit that dielectric loss of GMO-CFO@TO at RT is reduced ~3 times compared to
Fig. 6. (a) Polarization vs. electric field loop for GMO-CFO and GMO-CFO@TO recorded at RT. (b) Variation of MC with magnetic field of GMO-CFO@TO recorded at 100 kHz, 1 MHz and 4 MHz. 5
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Materials Chemistry and Physics 240 (2020) 122242
Fig. 7. (a) Ac conductivity spectra at RT. The lines represent the power law fittings. (b) Complex impedance plots at RT. (c) Scaling of conductivity spectra according to Summerfield scaling law. (d) Scaling of time dependent
that of GMO-CFO which is suitable for the applications in microwave devices. Thermal variation of ε’ of GMO-CFO and GMO-CFO@TO (Fig. 5 (a) and (b)) exhibit that the value of ε’ of GMO-CFO increases with tem perature whereas the value of ε’ in case of GMO-CFO@TO gradually increases with the rise in temperature and attains a maximum value, then decreases with further rise in temperature. This type of variation of ε’ is basically observed in case of ferroelectric materials [21,22] and the temperature at which the value of ε’ is maximum, called transition temperature (Tc) which is ~385 K above which GMO-CFO@TO exhibits a ferroelectric to a paraelectric phase transition. In order to investigate the presence of ferroelectric property of the nanocomposite samples of GMO-CFO and GMO-CFO@TO, polarization vs. electric field (P-E) loops of these samples are recorded at RT at an applied voltage of 800 V at a frequency of 50 Hz which are shown in Fig. 6 (a). Fig. 6 (a) shows that GMO-CFO exhibits very weak ferro electric response due to the presence of high leakage current problem whereas GMO-CFO@TO shows good ferroelectric polarization at RT compared to that of GMO-CFO. Different parameters such as maximum polarization (Pmax), remnant polarization (Pr) and coercive field (Hc) are extracted from the P-E loop of GMO-CFO@TO and the corresponding value of these parameters are ~0.2 μC/cm2, 0.1 μC/cm2 and 1.6 kV/cm, respectively. Ferroelectric polarization of GMO-CFO@TO nano composite arises mainly due to the non-centrosymmetric symmetry (I41md) of the TiO2 crystal structure as GMO and CFO crystallize into centrosymmetric space group. Due to the presence of small distortion, TiO2 crystal structure deviates from the centrosymmetric to noncentrosymmetric position and correspondingly, the charge centers of the negative and positive ions get separated giving rise to a permanent electric dipole moment. When a voltage is applied across the sample, the dipoles will align along the direction of the external electric field which induces a local electric field, results in a ferroelectric response of GMOCFO@TO. The coupling between magnetic and ferroelectric ordering present in GMO-CFO@TO is confirmed by measuring dielectric constant (ε’) in presence and absence of the applied magnetic field and the
corresponding value of magnetocapacitance (MC) is calculated using the following equation, 0
MC ¼
0
½ε ðH; TÞ ε ð0; TÞ� ε0 ð0; TÞ
(2)
where, ε’ ðH; TÞ and ε’ð0; TÞ represent the value of dielectric constant measured in presence and absence of magnetic field, respectively [23, 24]. This is one of the important ways to investigate the coupling effect, as used by many authors [23–25]. The corresponding variation of MC with the applied magnetic field, recorded at 100 kHz, 1 MHz and 4 MHz, respectively, is shown in Fig. 6 (b) which indicates that the value of MC increases nonlinearly with the application of an external magnetic field. This kind of nonlinear variation of MC indicates the presence of positive magnetoelectric coupling in GMO-CFO@TO at RT. The value of MC of GMO-CFO@TO decreases with the increase of frequency. No such coupling is observed in case of GMO-CFO nanocomposite. Due to the presence of coupling between magnetic and electric ordering in the nanocomposite sample of GMO-CFO@TO, when placed in an external magnetic field a strain is induced in it. This strain induces a stress in the nanocomposite sample which produces an electric field in the sample. Thus the ferroelectric domains in GMO-CFO@TO are oriented by this induced electric field results in an increase of the polarization value. The corresponding strain increases with the increase of the magnetic field causing the polarization value to change further. 3.4. Ac conductivity analysis The conductivity spectra (real) at RT for GMO-CFO and GMOCFO@TO are shown in Fig. 7 (a). It is observed that at low-frequency region the ac conductivity is almost independent of frequency and is recognized to the diffusive motion of the mobile charge carriers. How ever, a crossover from the frequency independent dc conductivity region to frequency dependent dispersive region has been observed indicating the onset of the relaxation processes. The ac conductivity spectra can be well described by the power law model given by, [26].
6
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Table 2 The values of dc conductivity, crossover frequency, power law exponent, dielectric strength, √
σdc ðΩ 1 cm 1 Þ
GMO-CFO GMO-CFO@TO
3.27 � 10 1.49 � 10
6 7
ωc ðrad s 1 Þ
n
Δε
√
√
361159.5 1629.8
0.39 0.49
182.5 306.1
4.79 4.57
3.12 1.27
(3)
σ(ω) ¼ σdc[1þ(ω/ ωc)n]
n is the power law exponent. At longer time scale the charge carriers have enough time for forwarding hopping and are categorized by the diffusive motion of mobile charge carriers:
where ωc is the characteristic frequency indicating crossover from the dc to the dispersive conductivity and n (0 < n < 1) is a power law exponent. The power law exponent n depends on the dimensionality of the con duction space [27]. The ac conductivity spectra at RT for GMO-CFO and GMO-CFO@TO has been fitted to Eq. (3). The value of these parameters, obtained from the power law fitting is listed in Table 2. It is observed that the value of dc conductivity decreases significantly for GMO-CFO@TO compared to that of GMO-CFO, which is analogous to the ferroelectric response observed for GMO-CFO@TO. In order to accurately measure the dc conductivity, the values were determined using a Nyquist plot as shown in Fig. 7 (b). The contribution of electrode polarization is clearly noticed (arc converted to spike) at the low-frequency region. We have fitted the impedance data excluding the electrode interface region by a simple R–C circuit, as shown in Fig. 7 (b). The value of dc conductivity, obtained from the fitting of complex impedance plot is in well agreement with those obtained from the power law fitting of the ac conductivity spectra. In order to better understand of the conduction mechanism, we have applied the Summerfield scaling formalism [28] on the RT ac conduc tivity spectra for GMO-CFO and GMO-CFO@TO given by, � � σ ðωÞ ω ¼F (4) σdc :T σ dc
02
< R ð∞Þ >¼
The conductivity master-curve obtained using Summerfield scaling formalism is shown in Fig. 7 (c). It is noted indeed that the scaled con ductivity spectra do not map onto the same curve. The reason behind this result might be due to the different characteristic length scales of charge carriers are associated with these two materials. The conductivity spectra can be related to the evolution of the timedependent mean square displacement of charge carriers (
12kВ THR Nc q2 π
Zt
Z∞ dt
0
0
σðωÞ sinðωt’Þdω ω
0
(7)
where Δε is the dielectric strength of the sample, obtained from the Colepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Cole fit of the dielectric spectra. The values of < R2 ðtp Þ > and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 < R ð∞Þ > for GMO-CFO and GMO-CFO@TO are listed in Table 2. It pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi is noticed that the value of < R2 ðtp Þ > decreases for GMO-CFO@TO by a small fraction than that of GMO-CFO. Whereas the value of qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 < R ð∞Þ > decreases significantly for GMO-CFO@TO. Thus, the decrease of dc conductivity for GMO-CFO@TO may be directly related pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi to the decrease of these two characteristic length scales < R2 ðtp Þ > qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 and < R ð∞Þ >. 4. Conclusion
where q is the charge of mobile charge carriers, Nc is the number density of the mobile charge carriers and HR is the Haven ratio. Haven’s ratio signifies the degree of correlation between successive hops. Conversely, the mean square displacement of charge carriers,
6 k B T ε0 Δε Nc q2
A novel multiferroic nanocomposite of GMO-CFO@TO is derived by using three potentially important components. The different unit cells of these components are clubbed together which generates a lattice strain and this fact leads to the departure from the center of inversion to the non-centrosymmetric structure of TiO2. These extracted results are duly supported by Rietveld analysis of X-ray diffractogram spectra. The lack of center of inversion symmetry, the presence of TiO2 matrix helps to introduce the ferroelectric polarization at RT in the nanocomposite. CFO which itself a good system for magnetic and dielectric properties and GMO, which is multiferroic below ~13 K incorporated in TiO2 matrix helps to achieve the multiferroic at RT.
(6)
2
¼< R ðtÞ > HR
Acknowledgment One of the authors, Mr. A. Mitra wishes to acknowledge the Department of Science and Technology for the fellowship in INSPIRE Program. One of the authors A. Shaw wishes to acknowledge SERB (PDF/2016/000643) for their financial support and fellowship in the NPDF program. The authors are thankful to UGC DAE CSR MUMBAI 7
A. Mitra et al.
Materials Chemistry and Physics 240 (2020) 122242
Center for financial support under CRS-M-265/2017/553 and CAS-II physics (File No. CAS F. 530/20/CAS-II/2018 (SAP-I)) to carry out this work.
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