Structural, Dielectric, Complex Impedance and Magnetoelectric Properties of the (1-x) KNbO 3 - xMgFe 2 O 4 Composites

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INTRODUCTION
Nowadays study on multiferroic materials that simultaneously display polarization, magnetization and ferroelastic ordering drastically increased not only for the underlying new physics but also for promising potential applications.The coexistence of both ferroelectric and ferromagnetic orders in the same material is predictable to create magneto-electric (ME) property due to the interaction between polarization and magnetization consequently, the coupling between electric and magnetic order occur.Magneto electric material with strong coupling between polarization and magnetization allows the ability to tune the electric property with the magnetic field and vice-versa (Raneesh et al., 2015).Magnetoelectric coupling effect has attracted significant interest due to its potential applications in new generation devices such as sensors, nonvolatile data storages, actuators, and transducers (Kim et al., 2014;Eerenstein et al., 2006;Fiebig, 2015).There are only very few naturally existing single-phase materials that exhibit room-temperature magneto-electric properties that show strong coupling Hill (2000).
Multiple-phase multiferroics that yield a giant extrinsic ME coupling effect at room temperature can be obtained by combining ferroelectric and ferromagnetic materials and coupling facilitated through piezoelectric and magnetostrictive elastic interactions (Ramesh et al, 2017;Spaladin et al 2005;Giorgio, 2013).
When an electric field is applied, to the piezoelectric phases its volume changes, and exerts a force against the ferromagnetic counterpart, latter the magnetostrictive with piezomagnetic phase also, in turn, develops magnetization as a result of mechanical strain.To improve coupling interaction through magnetic-piezoelectric phases, different connectivity patterns have been studied (Zhai et al., 2008;Ramesh et al, 2007;Nan et al., 2008).
ME response of a single-phase material is limited because, in most such materials, the origin of ferroelectricity and ferromagnetic order are largely unrelated, for example; BiFeO3, YMnO3, and BaTiO3 to obtain such multiferroic single-phase compound with large and strong ferromagnetic and ferroelectric ordering is a challenge (Wang et al., 2009).But, this situation in the composite multiferroic magnetoelectric system is different, since the origin of magnetoelectric coupling lies at the interface between the magnetic and ferroelectric phases (Liu et al., 2013;Vaz et al, 2010).Except for problems of high leakage current that arises due to magnetic phases, thermal expansion difference, and grain boundary between the two phases ME composites yields extrinsic ME effect.
In this study, KNbO3 selected as ferroelectric material and MgFe2O4 as ferromagnetic material.KNbO3 nanocrystalline powder synthesized using solid-state reaction and MgFe2O4 nanocrystalline powder using sol-gel method and the composites of (1-x)KNbO3 -xMgFe2O4 (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) were prepared mixing KNbO3 and MgFe2O4 nanocrystalline powders using solid-state reaction method with different molar fractions.The structural, dielectric, magnetic and magneto electric coupling of the synthesized composites were investigated.
Then the precursor heated to 90 0 C and calcined at 850 0 C for 4 hrs finally MgFe2O4 nanoparticles obtained.KNbO3 obtained by mixing KHCO3 and Nb2O5 with Agate Mortar and Pestle on wet-ground in molar proportion for 4 hrs and then at 850 0 C for 3 hrs.Composite ceramics samples prepared using conventional solid-state reaction method with weight fractions 10 %, 30 %, 50 %, 70 %, and 90 % of MgFe2O4 powder to 90 %, 70 %, 50 %, 30 %, and 10 % of KNbO3 powder, sintered at a temperature of 850 0 C for 5 hrs, and finally painted by silver paste for magnetoelectric coupling and dielectric spectroscopy measurements.

Measurnments
All the samples were analyzed by various techniques.Room temperature crystal structure of the samples was examined by using an X-ray diffractometer (PANalytical, X'Pert PRO) with a mono-chromatized Cu-Kα radiation (λ=1.54060Å).The surface morphology, size and the local crystallographic structure were investigated by high-resolution transmission electron microscope (HRTEM, JEOL-JEM 2100) operating at 200kV.Raman measurements were performed at room temperature using the Jobin-Yvon T64000 triple spectrometer system, equipped with a confocal microscope and a nitrogen-cooled CCD detector.Dielectric measurements as a function of frequency between 100 Hz to 2 MHz were performed using an impedance analyzer (Agilent, E4980A).Magnetic measurements were were conducted using vibrating sample magnetometer VSM (Lake-shore 7404) at applied field strength of ±3 kOe.The direct ME coupling coefficients of (1-x)KNbO3 -xMgFe2O4 (where x = 0.1, 0.3, 0.5, 0.7, 0.9) is measured using magneto-electric coupling set up.
Sample code and omposition of KNbO3 and MgFe2O4 in each sample shown in table 1.     (1) Where, c is capacitance, d thickness of the pellet, A cross-sectional area, and  0 (permittivity of free space),  0 = 8.85 × 10 −12 /.
Imaginary dielectric constant (permittivity) was calculated using the relation AC conductivity (  ) was calculated from the obtained dielectric data using the relation Where,  is the angular frequency which is 2  The applied electric field induces ferrite phase to accumulate space charge at the interface of the two phases due to the presence of different permittivity and conductivity (Pascua-Gonzalez et al., 2016).At low frequency excitation of bound electrons, lattice vibration, dipole orientation and interfacial polarization (Rani et al., 2014) cause for dielectric constant to be maximum.This can be explained in terms of Maxwell-Wagner polarization and Koop's phenomenological theory of dielectrics (Pandya et al., 2015;Wagner, 1993) induced by a different dielectric constant at the interface as a result, overall polarization enhanced causing the rise of dielectric constant.At the higher frequency dielectric constant becomes very small because of the inability of electric dipoles to match the applied alternating electric field.
Consequently, hopping of electrons between Fe 2+ -Fe 3+ in electronic polarization can't follow the applied electric field Koops (1951).It must be noted that the highest dielectric constant observed is for sample KN -MF1 sample which is a percolation limit due to the major   mixed composites.Variation of dielectric loss () of the composites is proportional to  ′ and  ′′ which is the same trend (Ryu et al., 2001;Huang et al., 2016) high dielectric loss value at low frequency and low dielectric loss at high frequency with similar dispersion and vice versa as shown in figure 6(a).Dielectric loss variation with frequency attributed to domain wall resonance in low frequency region and inhibition of domain wall motion at high frequency region (Chen et al., 2018).The magnitude of the peak value of loss tangent decreases as ferrite content decreases in the composite.Loss tangent value measures electricl energy loss at different frequency.All the composites have very low loss tangent value at higher frequency region from all composites sample KN -MF1 has the lowest value.Loss tangent value of the composites show abnormal value due to changes of ferrite content in the composites which is collective behavior of charge carriers (Rahamann et al., 2014).A dielectric material with low loss tangent value at higher frequency means the material has a potential application in magnetically tunable filters, microwave devices and oscillators (Rezluscu et al., 1974).
Variation of AC conductivity (  ) as a function of frequency is shown in figure 6(b).As it clearly seen the plots are nearly linear for famaller value of frequency indicating small dependence of AC conductivity.AC Conduction mechanisim of ferrite -ferroelectric composite is explained by polaron hopping (Adler et al., 1970) among localized states.The incorporation of ferrite material to ferroelectric material results an increase of   due to generation of charge carriers.At lower applied field frequency region resistive grain boundaries becomes more active consequently polaron hopping process becomes negligible.On the other hand, at higher applied field frequency region conductive phase becomes more active because hopping of charge carriers.Hopping of charges is restricted to the nearest site/inside the grain due to mismatch of the high frequency applied field and response time (Chen et al., 2018).This applied field frequency dependent AC conductivity attributed to small polaron type mechanism (Raveendran et al., 2019).
AC conductivity increases with an increase of ferrite content and the smallest value of the loss tangent is for the sample that has 50 % of KNbO3 and 50% of MgFe2O4 compositions.
Similarly this sample has largest dielectric constant.
In general, desirable properties of a dielectric material which include low dielectric loss tangent at high frequency and high permitivity that originate at interface were observed from all composite samples.The value for real permittivity, imaginary permitivity, loss tangent and AC   As seen in figure 7(b) all composite samples plots show the same variation with frequency attaining different maximum values at different frequency and merging at a higher frequency.Imaginary dielectric constant  ′′ peak frequency variation is due to the relaxation phenomenon (Devan et al., 2006).This merging of plots at a higher frequency attribute to the increase of conductivity at higher frequency.In both cases the merge of impedance value at at a higher frequency occurred this is due to the release of space charge carriers at the interface (Verma et al., 2012).

Complex Impedance Spectra Analysis
Figure 8 shows real impedance versus imaginary impedance (1-x)KNbO3 -xMgFe2O4 mixed composites.As seen in figure 8 all plots of the composites are composed of different semicircles.These semicircle represent a distinct process with a different range of frequencies that demonstrate the existence of a good homogeneity of dielectric and conductive properties.
Semicircle pattern change is with change of ferrite-ferroelectric composition which is an indication of modification of resistancereactance amount in the composite.The observed semicircles diameter variation resembles to the variation ferrite ferroelectric content (Chourashiya et al., 2008) which is dependent on the conduction mechanism of ferrite-ferroelectric content.
Semi-circles at lower and higher frequency represent bulk condition process and electron transfer process and the intermediate frequencies represent the grain boundary condition of the respective samples (Ya et al., 2008).

Magnetic Property Study
To study magnetic properties of the composites, room temperature VSM (Vibrating Spin Magnetometer) measurements were carriedout with maximum applied field of ±3 kOe as shown in figure 9.All the samples exhibit a magnetic hysteresis loop, which indicate the presence of orderd magnetic structure in the composites.and this confirm the presence of odered magnetic structure.Saturation magnetization of the composites increases with an increase of ferrite content from 0.04518 emu/g for KN-MF1 to 0.62976 emu/g for KN-MF5 (Mudinepalli et al., 2017).
The steepest parts of magnetic hysteresis loops correspond to to the process of rotation in the spontaneous magnetization area (Kanamadi et al., 2009).

ME Coupling Meaurements
Room-temperature magnetoelectric coupling measurments of (1-x)KNbO3 -xMgFe2O4 (x = 0.1, 0.3, 0.5, 0.7, 0. 9) mixed composites have been investigated using a lock-in amplifier technique by varying applied AC magnetic field.The output ME voltage is measured at a fixed frequency of 850 Hz with a constant DC bias field of 2000 Oe collinearly with sweeping AC magnetic field varying from 0 to 90 Oe.Magnetoelectric coupling voltage calculated using the slope of the developed electric field across the samples versus the applied AC magnetic field.
Magnetoelectric coupling response resulted from the appearance of electric polarization upon applying a magnetic field which increases with the an icrease of the applied AC magnetic field show linear dependence.Linear magnetoelectric coupling coefficient   of the composite samples calculated from the slope of magnetoelectric coupling voltage versus applied AC magnetic field using the following relation (Verma et al., 2017): Where,   is the output voltage,  sample/pellet thickness and   the amplitude of the applied AC magnetic field.
Figure 10 shows the magnetoelectric coupling response of (1-x)KNbO3 -xMgFe2O4 (x = 0.1, 0.3, 0.5, 0.7, 0. 9) mixed composites with varying AC magnetic field at room temperature that shows dependence of ME coupling voltage with AC magnetic field.The calculated linear magnetoelectric coupling coefficient result reveals all composite samples exhibit ME coupling that increases with the increase of the applied magnetic field.ME response is found to be increasing in the composite with the increase of ferrite (Komalavalli et al., 2019) content this can be explained by the increase in the magnetostriction due to the substantial increase of the magnetostrictive phase (Mudinepalli et al., 2017).Magnetoelectrric effect of the composite is associated with the movement of magnetic domains in ferrite which are spontaneously deformed to the direction of magnetization that contributes to the increase of magnetostriction (Mudinepalli et al., 2015).In these composite materials, the coupling of ferrite and ferroelectricity is expected to originate at the interface between ferrite-ferroelectric phases (Nan et al., 2001;Thankachan et al., 2018).

CONCLUSION
Multiferroic mixed composites were prepared from MgFe2O4 (MF) and potassium niobate (KN) with different molar ratio (1-x)KNbO3 -xMgFe2O4 (x = 0.1, 0.3, 0.5, 0.7, 0. 9) using conventional solid state reaction method and the existence of the two ferrite/ferroelectric pure phases verified in all samples using XRD analysis.Morphology and microstructure of composite samples were studied using Images from TEM and HRTEM reveal the existence of two phases.
Dielectric properties of the composite samples depend on ferrite content and dielectric dispersion at lower frequency is observed due to interfacial polarization.Low dielectric loss tangent at the high frequency of applied electric field and high dielectric constant that originate at the interface was observed.Maximum dielectric loss between 0.1 MHz and 1 MHz which is attributed to equal hopping frequency of electrons between different ionic states.The AC conductivity is related to the hopping of electrons and hopping conduction increases with frequency.Cole-Cole plots result demonstrate the formation of interfacial layer between KNbO3 and MgFe2O4 grains and the reduction of the bulk resistance of the ferroelectric material with the increase of ferrite contents in mixed composites.Magnetic properties improved with addition of ferrite/ MgFe2O4 material .Increament of ME coupling coefficient with an increase of ferrite content.This composite material can be highly useful for the design of ultra-modern devices based on magnetoelectric multiferroics and microwaves.

ACKNOWLEDGEMENTS
The author (TkW) is thankful to Department of Science and Technology, New Delhi under the Nano Mission, PURSE, FIST Programs, International Inter-University Center For Nanoscience and Nanotechnology, MGU, Kottayam, Kerala, India.

Figure 3 .
Figure 3. (a) TEM image and (b) HRTEM lattice pattern and the inset pattern SAED of KN-MF3 sample.

Figure 5
Figure 5(a) shows room temperature frequency dependence of real permittivity ( ′ ) and figure 5(b) room temperature frequency dependence of imaginary dielectric constant ( ′′ ) of (1x)KNbO3 -xMgFe2O4 mixed composites.All samples show a strong frequency dependence, at lower frequency have higher dielectric constant that decreases abruptly at lower frequency and have lower value/nearly constant at higher frequency as seen in figure 5(a) which is a common contribution of interfacial polarization.A similar fashion was observed for imaginary dielectric constant versus frequency as seen in figure 5(b) except decreasing very fast with increasing frequency from 2 MHz to 3.5 MHz whereas real dielectric constant decreases relatively slowly from 2 MHz to 5 MHz.

Figure 6 Tesfakiros
Figure 6(a) shows room temperature frequency dependence of loss () and figure (b) room temperature frequency dependence of AC conductivity (  ) of (1-x)KNbO3 -xMgFe2O4

Figure 7
Figure 7(a) show room temperature frequency dependence of real impedance ( ′ ) and figure 7(b)room temperature frequency dependence of imaginary impedance ( ′′ ) spectra of (1-x)KNbO3 -xMgFe2O4 mixed composites.As seen in figure7(a) real impedance ( ′ ) of all the composites samples plot show a low-frequency dispersion and all merging at higher frequency with low value.This is because as applied field frequency increases conductivity becomes more active this may be related to charge ordering matter and mismatch of the response of electrons to applied field frequency.

Table 1 .
Sample code and percentage of KNbO3 and MgFe2O4 in each sample.

Table 2 .
Elemental composition and atomic concentration of KN-MF3 sample.
MHz applied electric field frequency of the composites are shown in table 3.As seen in the table AC conductivity of the ccomposite increases with the increase of ferrite content and the highest value of real permittivity and lowest value of loss tangent value is for sample KN-MF3 good for dielectric application.

Table 4 .
Magnetoelectric voltage coefficient, ferrite and ferroelectric contents of each sample.Linear magnetoelectric coupling coefficient,   value of all mixed composite samples are tabulated in table 4. Sample KN-MF4 has the highest magnetoelectric coupling value this makes it choosable for memory device application.