Structural, ferroelectric, dielectric, impedance and magnetic properties of Gd and Nb doped barium titanate-lithium ferrite solid solutions

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

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Research articles

Structural, ferroelectric, dielectric, impedance and magnetic properties of Gd and Nb doped barium titanate-lithium ferrite solid solutions

T

⁎

Ganapathi Rao Gajulaa, , Lakshmi Rekha Buddigab a b

Department of Physics, BS&H, Sree Vidyanikethan Engineering College, Tirupati 517102, AP, India Department of Chemistry, Andhra University, Visakhapatnam 530 003, AP, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Resistivity Dielectric constant Ferroelectric properties Magnetic moment Coercive magnetic field Gadolinium Impedance

The Gd, Nb substituted barium titanate-lithium ferrite composite are synthesized using the normal solid state technique. The structural, morphological, ferroelectric, electric and magnetic properties of doped and undoped composites are investigated using X-ray diffraction, FESEM, P-E loop tracer, dielectric spectrometer and vibrating sample magnetometer respectively. The diffraction peaks in XRD confirm the formation of tetragonal and the ferrite peaks of composites reduce with increases in concentration of Nb and Gd in BL. The FESEM reveals the formation of dense microstructure with low pores and the average grain size of composites increase first and later it decreases when increase concentration of Gd and Nb in BL composites. The unsaturated hysteresis loops of BTG-1 and BTG-2 are representing poor ferroelectricity in the samples. The dielectric constant (ε′) of all composite exhibits high at low frequency which is decreases steeply with increasing frequency upto certain frequency beyond this it becomes constant. The impedance (Z′) of the BL, BTG-1 and BTG-2 composites shows dispersion and also impedance (Z′) of the BL, BTG-1 and BTG-2 composites are decreasing with temperature at low frequency region. The capacitance (Cp') of all composites is decreasing with decreasing temperature. The resistivity (ρ) of BL increases when Gd and Nb doped in it. The magnetic properties of BL are changing as and when Gd and Nb doped in BL composites and these properties of all composites are obtained from magnetic hysteresis loop.

1. Introduction The material which exhibits more than one ferroic order such as ferroelectric, ferroelectricity, ferroelastic, ferroelasticity, ferromagnetic, ferrotoroidic, antiferromagnetism and ferrimagnetism are considered as multiferroics. The combined ferroelectric and magnetic materials provide interfaces between the phases are striking and confirmed way to attain multiferroicity [1]. A multiferroic material possessing large polarization and large magnetization exhibits strong coupling between the two phases at room temperature and because of this strong coupling between the constituent phases, they are used in memory devices, actuators, new types of electronic memory devices, switches, spintronics, sensors, magnetic field sensors [2,3]. The synthesis of composite materials exhibiting both ferromagnetic and ferroelectric phases paves way to a class of new materials called magneto-electric materials having novel properties[4–6]. Many ferroelectric materials have been synthesized using the solid state technique. One such material is barium titanate (BaTiO3) which has a tetragonal pervoskite ABO3 structure at room temperature.

⁎

Doping suitable materials at A and B site modifies the electrical, dielectric and magnetic properties of BaTiO3 ceramics [7]. The BaTiO3 has a huge range of application in different fields like electronic industry to manufacture infrared detectors, electrostriction [8], pulse generating devices, transducers and tunable microwave devices, power transmission devices, etc. [9–12] due to its excellent dielectric properties like high dielectric constant, good thermal shock resistance, good ferroelectric, piezoelectric nature and dielectric reliability. The BaTiO3 ceramics synthesized by the conventional solid state technique have coarseness nature and agglomeration of the particles starts at high calcination temperature [13]. The lithium ferrite (Li0.5Fe2.5O4) has an inverse spinel structure. The physical properties of Li0.5Fe2.5O4 have changes exceptionally like the squareness of the magnetic hysteresis curve have observed at higher temperature [14]. The lithium ferrite has been used both as a dopant as well as in pure form to study both fundamental and extensive technical properties [15,16]. Li0.5Fe2.5O4 is a low mobility semiconductor and attributed the conduction mechanism to unused Fe2+ ions in Li0.5Fe2.5O4 and conductivity is assigned to Polaron hopping [17].

Corresponding author. E-mail addresses: [email protected] (G.R. Gajula), [email protected] (L.R. Buddiga).

https://doi.org/10.1016/j.jmmm.2019.165822 Received 21 June 2019; Received in revised form 4 September 2019; Accepted 9 September 2019 Available online 10 September 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

5.30 g/cm3 and 5.552 g/cm3 respectively.

Lithium ferrite has a wide range of applications in memory core devices, microwave devices due to its excellent square loop properties, low dielectric loss, high value of saturation magnetization and high transition temperature [18–20]. The variation in the electrical properties such as low dielectric loss, low eddy current losses and magnetic properties like magnetic loss tangents, saturation magnetization, magnetic anisotropy and coercivety of lithium ferrite depend on different factors like chemical compositions, different method of preparations, sintering temperature, grain size and cation distribution [21–23]. Many researchers have focused on BaTiO3 doped with rare earth elements to obtain good electrical properties. The rare earth elements (lanthanides) consists of elements Nb, Gd, Sm, Yb, Y, La, Nd and Dy etc. [24–28] and the dopent site depends on the ionic radius of material. The ionic radii of these elements are comparable to that of Ti and hence when these elements are doped in BL, these ions occupy the position of Ti. The heterovalent elements Nd3+, Gd3+, Yb3+, Sm3+, Er3+, Ho3+, Sb3+, Sc3+, La3+, Nb5+ etc. substituted in place of Ba2+ or Ti4+causes polarity of charge and vacancy creation at A sites or B sites or creation of holes to keep electrical charge neutrality [29–32]. The Gd, a rare earth element, has many applications like making magnets, electronic components and data storage disks. The Gd3+ ions enhances the magnetic properties of BaTiO3 [33]. The Nb ions may be incorporated into the BaTiO3 lattice in a narrow temperature and the amount of Nb incorporated into BaTiO3 depends substantially on oxygen partial pressure [34]. The incorporation of Nb into BaTiO3 involves the titanium vacancy creation as an ionic indemnity process [35]. In this paper, BaTiO3 – Li0.5Fe2.5O4 composite doped with Gd and Nb have been prepared using the normal solid state method and systematic study of structural, morphological, ferroelectric, dielectric, impedance and magnetic properties of (0.9) BaTi (1-2x) NbxGdxO3+ (0.1) Li0.5Fe2.5O4 (where x = 0, 0.05 and 0.1) composites is presented. The ionic radius of Gd (1.05 Å) and Nb (0.64 Å) are considerable to Ti (0.61 Å) of BaTiO3. Moreover Gd and Nb substituted in composites form a solid solution in which minor elements are uniformly distributed in the crystal lattice of major BaTiO3. When Gd and Nb are substituted in BaTiO3 – Li0.5Fe2.5O4 composite, the electrical and magnetic properties of BaTiO3 – Li0.5Fe2.5O4 composite will definitely change and hence the doped composite will retained the features of the multiferroic material. The samples BaTiO3 as BT, Li0.5Fe2.5O4 as LF, (0.9) BaTiO3+ (0.1) Li0.5Fe2.5O4 as BL, (0.9) BaTi0. 9Nb0.05Gd0.05O3+ (0.1) Li0.5Fe2.5O4 as BTG-1 and BaTi0. 8Nb0.1Gd0.1O3+ (0.1) Li0.5Fe2.5O4 as BTG-2 are represented throughout the manuscript.

3. Characterization of samples The prepared samples are characterized using different techniques. The X-Ray diffraction studies were measured with the wavelength (λ) of radiation is 1.5406 Å in the 2θ range 10–80° with step size 0.02° with Bruker D8 Advance X-ray diffractometer. The morphological studies and elemental analysis are characterized by Carlzeiss ultra-55 FESEM/ EDAX. The ferroelectric properties were measured over the frequency 50 Hz by P–E loop tracer, TF analyzer 2000. The dielectric, impedance, capacitance and resistivity measurements were measured using an LCR meter manufactured by Novocontrol Technologies, Germany, Concept 80 by applying AC Volt [Vrms] = 1.000e + 00 V at different temperatures over the frequence beween 1 Hz and 10 MHz. The magnetic properties are estimated from magnetization versus magnetic field (MH) loop are measured using vibrating sample magnetometer (VSM) by Quantum Design PPMS, Model 6000. 4. Results and discussions 4.1. X-ray diffraction The X-ray diffraction pattern of BL, BTG-1 and BTG-2 composites are shown in Fig. 1(a). Fig. 1(a), clearly seen that all the diffraction peaks of BaTiO3 in all composites are indexed by JCPDS card no 792263 [36]. The diffraction peaks of BaTiO3 in BL, BTG-1 and BTG-2 composites confirm the formation of tetragonal pervoskite crystal structure without phase change [36]. Also the diffraction peaks of lithium ferrite in all composite are indexed using JCPDS no. 89-7832 & 88-6711. The lithium ferrite peaks in composites are represented by ‘#” shown in Fig. 1(a). The intensity of ferrite peaks in BL composite reduces with increases in concentration of Nb and Gd in BL. The diffraction peaks of BTG-1 and BTG-2 shifted towards higher diffraction angle due to substitution of Gd and Nb in BL. The shifted diffraction peaks strongly confirms the incorporation of Gd and Nb in BL. The width of diffracted peaks of BL composite decreases when Gd and Nb doped in BL composite and further decrease small with increase concentration of Gd and Nb in BL due to difference of ionic radius between Ti4+ and Gd3+, Nb5+ as a result crystallite size of BTG-1 increases abruptly and crystallite size of BTG-2 increases small. No extra peaks related impurities are obtained in the all the composites confirm the crystalline nature. The lattice parameters of all composites are shown in

2. Preparation method The Gd, Nb doped BaTiO3 – Li0.5Fe2.5O4 composites are synthesized using the normal solid state technique. The chemicals compositions BaCO3, TiO2, Li2CO3, Fe2O3, Nb2O5 and Gd2O3 are used to form required composites. The chemical purity of the samples and raw material supplied is BaCO3 (98%) (Merck), TiO2 (99.5%), Li2CO3 (99%), Fe2O3 (98%)(Loba Chemie), Nb2O5 (99.9%)(Titan biotech Ltd.), Gd2O3 (99.9%) (Himedia) and all compounds are AR grade. All samples are weighed according to stoichiometrically. All precursors of each sample are mixed together using agate mortar and pestle. The grinding of each sample is grinded for 10 h to become uniform fine powder. The powder samples are calcinated at 900 °C for 3 h using the furnace. This calcinations process gives heat to the samples then uniform mixing of precursors are decomposes. The calcinated powder samples are grinded for 3 h to become a fine powder. A 5% of polyvinyl alcohol acts as a binder which is added to calcinated powder and mixed together to obtain a fine powder. These powders are placed in to die set which having diameter 10 mm and pressed into pellets using a hydraulic press. These pellets of all the samples are sintered at temperature 1150 °C kept for 3 h. Finally, these sintered pellets and sintered powder samples are used for measuring different characterizations. The density of the samples BT, BL, BTG-1 and BTG-2 are found to be 5.677 g/cm3, 5.0048 g/cm3,

Fig. 1a. X-ray diffraction of BT, BL, BTG-1 and BTG-2 composites ceramics. 2

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Table 2 Lattice parameters, constituent phases of the BL, BTG-1 and BTG-2 composite ceramics. Samples

BL BTG-1 BTG-2

Lattice parameters (Å)

Percentage of Phases

Ferrite phase

Ferroelectric phase

a

a

c

c/a

Ferrite

Ferroelectric

8.504 8.286 8.206

4.002 3.988 3.994

4.007 3.980 4.007

1.001 0.998 1.003

0.343 2.508 1.068

99.65 97.49 98.93

We see from Table 2, the ferrite phase of BL composite increases initially with an increase in concentration of Gd and Nb, then the ferrite phase of BL decreases with further increase in the concentration of Gd and Nb in BL composite due to Gd and Nb suppress ferrite phase in BL composite. 4.2. Morphology studies

Fig. 1b. Rietveld refined XRD pattern of typical BL composite.

The micrographs of BL, BTG-1 and BTG-2 are shown in Fig. 2. Fig. 2 sees that all the composites exhibit clear grains and grain boundaries. All the composites perform dense microstructure with low pores. The surfaces of grains appeared as smoothness and larger grains are distributed among smaller grains in BL composite. The BTG-1 and BTG-2 micrographs exhibits coarseness in the sample and larger grains are distributed among smaller grains. The sizes of the grains are large which is formed due to chemical reaction, an agglomeration of grains between the ferroelectric and ferrite phases. The shape of the grains arranged in BTG-1 seems to be sediments at the seashore. The shape of the grains of BTG-2 seems to mushroom. The average grain size of BL, BTG-1 and BTG-2 are 1.163 µm, 1.6243 µm and 1.0477 µm respectively. The average grain size of BL, BTG-1 and BTG-2 is obtained at micro level and the average grain size of BL increases first as Gd, Nb doped in it and is decreasing with increase Gd, Nb in BL composite might be due to strain induced which caused by difference of ionic radii between the doped and undoped elements. The grain size and grain boundaries are playing key role in electrical properties of the composites. The conduction action of the samples is sensitive to the grain size effect [39] and the smaller grain size results exhibits in a low electrical conductivity. The EDAX of BL, BTG-1 and BTG-2 composites perform the elements which are analogous to the stoichiometric ratio of powders used in preparation of samples are shown in Table 3. The elements observed in EDAX spectra of the BL, BTG-1 and BTG-2 composite are Ba, Ti, O, Fe, Gd and Nb shown in Fig. 3. No other elements are obtained in all composites of EDAX spectra. Hence, no impurities present in all BL composites. Due to the low atomic number, the Li element is not observed in all composites.

Table 2, which have been calculated in ferroelectric, ferrite phase using the following relations respectively [37].

dhkl =

dhkl =

1 h2 + k2 a2

+

l2 c2

(1)

a h2 + k 2 + l 2

(2)

From Table 2, the lattice parameter of BL composites in the ferrite phase decreases slightly with increase Gd, Nb in BL and the further lattice parameters of BL composites in the ferroelectric phase nearly same represents no changes have been observed in its structure of BL when doped with Nb and Gd in Ti of BL composite. The lattice parameters of typical BL composites have been obtained from Rietveld refinement (PROGRAM FullProf.2 k (Version 5.60)) shown in Fig. 1(b). The lattice paramaters in ferroelectric phase and ferrite are tabulated in Table 1. The unit cell volume in ferroelectric phase is 63.9919 Å3 and unit cell volume in ferrite phase is 614.03830 Å3. The lattice parameters obtained from Rietveld refinement which are almost same as obtained from Eqs. (1) & (2) shown in table 2. The space group of BT (ferroelectric phase) is P 4 mm, crystal system is tetragonal, general multiciplicty 8 source. The space group of LF (ferrite phase) is F d −3 m, crystal system is cubic, general multiplicity 192 and R-Factors: 5.88 10.6 Chi2: 8.18 and Conventional Rietveld Rp, Rwp, Re and Chi2: 50.2 43.7 15.3 8.179 respectively which are obtained from Rietveld refinement. The phase percentage in composites are calculated using corresponding intensity peaks of ferroelectric and ferrite in the XRD pattern [38]. The amounts of constituent phases are approximately in Table 2. The phase percentages of composites are calculated using the following equation

Phase percentage =

Iferroelectric/ferrite × 100 Iferroelectric + Iferrite + Iunknown

4.3. Ferro electric studies The ferroelectric properties BTG-1 and BTG-2 are obtained over the frequency 50 Hz at room temperature using polarization versus electric field loops are shown in Fig. 4 and the comparison of P-E loops for BT,

(3)

Table 1 Rietveld refinement values for Lattice parameters, atomic positions of BT and LF in BL composite. Composite

BL

Lattice parameters (Å)

Atomic positions

ferroelectric phase

ferrite phase

a=b

c

a=b=c

4.0001

4.0065

8.4997

Ba

Ti

O1

O2

Fe

Li

Li

Fe

O

1a

1b

1b

2c

8b

8b

16c

16c

32e

3

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Fig. 2. FESEM images of the BL, BTG-1 and BTG-2 composite ceramics.

higher than BTG-1 and BTG-2 composites. The values of Ps of BL decrease with increase in concentration of Gd and Nb in it. Moreover the values of Pr, Ec of BL composite decreases initially and later increase with increase Gd, Nb in BL composite, suggesting the decrease in leakage current in BL composites [43]. The remnant polarization (Pr) and coercive field (Ec) of BTG-1 is becoming smaller as increases Gd and Nb in BL composite representing the deviations from the ferroelectric behavior of BL composite might be due to the grain size effect, oxygen vacancy [40].

Table 3 Elemental analysis of the BL, BTG-1 and BTG-2 composite ceramics. (a) Element Weight % Atomic %

OK 29.72 69.65

Ti K 15.92 12.46

Fe K 7.66 5.14

Ba L 46.70 12.75

Totals 100.00

(b) Element Weight % Atomic %

OK 30.57 71.59

Ti K 14.04 10.98

Fe K 5.42 3.63

Nb L 1.79 0.72

Ba L 46.04 12.56

Gd L 2.15 0.51

Totals 100.0

OK 23.23 64.42

Ti K 12.80 11.86

Fe K 5.72 4.54

Nb L 3.55 1.70

Ba L 50.43 16.29

Gd L 4.27 1.20

Totals 100.0

4.4. Dielectric properties

(c) Element Weight% Atomic%

4.4.1. Dielectric constant (ε′) Fig. 5 shows that the frequency dependence of dielectric constant (ε′) of BL, BTG-1 and BTG-2 ceramics at different temperatures (30 °C, 50 °C, 150 °C, 250 °C, 350 °C, 450 °C and 550 °C). From Fig. 5, in the low frequency region, at high temperature the dielectric constants (ε′) of composites are maximized. The value of dielectric constant (ε′) of BL composite is high at 550 °C in the low frequency region, which is decreases monotonically with increase frequency up to 1.2 kHz frequency after this frequency, the dielectric constant (ε′) becomes frequency independent. At low frequency region, the dielectric constant (ε′) of the BL decreases with decreasing temperature and beyond certain frequencies, the dielectric constant (ε′) becomes frequency independent at all temperatures. The dielectric constant (ε′) of BL exhibits dispersion between temperatures at low frequency. The nature of dielectric constant (ε′) of BTG-1 and BTG-2 composites is same as BL composites when the concentration of Gd, Nb increases in BL composite. The values of dielectric constant (ε′) of all composites at 1 Hz frequency at different temperatures are shown in table 5. As and when concentration of Gd and Nb in BL increases, the dielectric constant (ε′) of BL increases in low frequency region at all temperatures shown in Fig. 6(a), which might be due to free dipoles oriented in alternating field which can describe using the following

BL, BTG-1 and BTG-2 are shown in the inset on Fig. 4. The saturation polarization (Ps) of BL composite is smaller than pure BT. The unsaturated polarization is observed in the BTG-1 and BTG-2 composites which are clearly shown in Fig. 4. The unsaturated hysteresis loops of BTG-1 and BTG-2 are representing poor ferroelectricity in the samples [40]. The unsaturated P-E loops of BTG-1 and BTG-2 representing the contribution of leakage current are minimum [41]. The leakage current of BL decreases with increase concentration of Gd, Nb in BL composite. The ferroelectric properties like saturation polarization (Ps), remnant polarization (Pr) and coercive field (Ec) of the BL, BTG-1 and BTG-2 composites are measured from P-E hysteresis loop (Fig. 4). The value of saturation polarization (Ps), remnant polarization (Pr) and coercive field (Ec) of the BL, BTG-1 and BTG-2 composites are presented in Table 4. It describes that the BL composites exhibits softening in nature due to low values of saturation polarization (Ps), remnant polarization (Pr) and coercive field (Ec) [42]. Also area of the loop becomes small as and when Gd, Nb doped in BL. The ferroelectric properties of BL are 4

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Fig. 3. Energy dispersive X-ray spectra (EDAX) spectrum of the BL, BTG-1 and BTG-2 composite ceramics. 1

1

equation. At very low frequency f < < 2πτ the dipoles oriented along that field, then (ε′) ≈ (εs) where εs represent dielectric constant at quasi static field, τ is relaxation time. The dipole lags behind the field as 1 frequency increases like f < 2πτ then dielectric constant decreases slightly. The dielectric constant drops to zero when frequency reaches 1 characteristic frequency i.e. f = 2πτ . The dipoles cannot follow the field

at very high frequency, i.e. f > > 2πτ [44,45]. At low frequency, the dispersion in the dielectric constant of all composites could be explained using a Maxwell conduction mechanism [46]. Moreover, at low frequency region different types of polarizations like electronic, interfacial, atomic and ionic polarization are contributing the dielectric constant and also the dielectric constant increase with increase

Fig. 4. Polarization versus electric field loops of BTG-1 and BTG-2 composite ceramics. Inset shows a comparison of P-E loops for BT, BL, BTG-1 and BTG-2 composite ceramics. 5

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Table 4 The values of ferroelectric properties of composites.

Table 5 The values of dielectric constant(ε′) of composites.

Sample

(Ps) µC/cm2

(Pr) µC/cm2

(Ec) kv/cm

BL BTG-1 BTG-2

2.39 0.575 0.396

1.32 0.0622 0.0733

3.52 1.243 2.29

Concentration of Nb and Gd

0 0.05 0.1

temperature due to space charge polarization [47,48]. The dielectric constant of all composites becomes frequency independent at high frequency might be due to absence of interfacial polarization in which dipoles are could not follow the applied field [49] and also due to some factors like grain boundaries, dipolar interactions [50].

Dielectric constant (ε′) at 1 Hz 550 °C

450 °C

350 °C

250 °C

150 °C

50 °C

30 °C

194,464 114,758

91,203 43,267 318,208

27,077 4996 273,414

5291 2174 173,104

2116 1987 28,345

1746 900 2302

2301 32 975

4.4.2. Dielectric loss (tan δ) The variation of dielectric loss (tan δ) with frequency of BL, BTG-1 and BTG-2 composites at different temperatures (30 °C, 50 °C, 150 °C, 250 °C, 350 °C, 450 °C and 550 °C) are presented in Fig. 5. It is observed that the dielectric loss (tan δ) trend follows dielectric constant. At low frequency region, the dielectric loss (tan δ) of all composites is high at

Fig. 5. Frequency dependence of dielectric constant (ε′) and dielectric loss (tan δ) of BL, BTG-1, BTG-2 composite ceramics at different temperatures. 6

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Fig. 6. (a) Dielectric constant (ε′) (b) dielectric loss (tan δ) with concentration of Niobium and Gadolinium at different temperatures.

550 °C) are shown in Fig. 7. It is shown that the capacitance (Cp’) behavior of all composites are same as the dielectric constant behavior of the composites shown in Fig. 5. Fig. 7 clearly seen that the capacitance (Cp’) is high in the low frequency region at temperature 550 °C. The capacitance (Cp’) of all composites decreases steeply with increasing frequency up to frequency 1 kHz after this frequency the capacitance (Cp’) becomes constant with increasing frequency at all temperatures. The capacitance (Cp’) of all composites decreases with decrease temperature at low frequency region. The capacitance (Cp’) of BL composite are increased with increasing concentration Gd and Nb in BL. The frequency dependence of resistivity (ρ) of the BL and BTG-1 composites at different temperatures (30 °C, 50 °C, 150 °C, 250 °C, 350 °C, 450 °C and 550 °C) is shown in Fig. 8. It is shown that the resistivity (ρ) of all composites, high at low temperature in the low frequency region and A.C resistivity (ρ) of all composites decreases with increasing frequency till 1 kHz after this frequency the resistivity (ρ) becomes frequency independent with increasing frequency at all temperatures. The resistivity (ρ) of all composites decreases with increasing temperature at low frequency region. The resistivity (ρ) of BL increases when Gd, Nb doped in it. The resistivity (ρ) of BL composites decreases with increasing frequency due to charge carriers are amplifying between the confined states [52,56].

Table 6 The values of dielectric loss (tan δ) of composites. Concentration of Nb and Gd

0 0.05 0.1

Dielectric loss (tan δ) at 1 Hz 550 °C

450 °C

350 °C

250 °C

150 °C

50 °C

30 °C

3265 2084

958 1976 1108

204 1024 132

35 167 14.46

0.99 6.8 6.78

0.34 0.84 0.7

0.85 0.52 0.4

all temperatures and decreases with increase frequency up with some frequency after this frequency the dielectric loss (tan δ) becomes frequency independent [51]. As and when Gd, Nb doped in BL composite, the dielectric loss (tan δ) of BL increase initially, then decrease with increase in concentration of Gd, Nb in BL shown in Fig. 6(b) and the values of dielectric loss (tan δ) of different composites at 1 Hz frequency at different temperatures are shown in Table 6. The dielectric loss (tan δ) of BTG-2 is lower than BL, BTG-1. The dielectric loss of all composites decrease and reaches a constant at high frequency region might be due to inability dipole polarization [52]. 4.5. Impedance, A.C resisitivity and capacitance with frequency

4.6. Magnetic properties

The impedance (Z′) of the BL, BTG-1 and BTG-2 with frequency at different temperatures (30 °C, 50 °C, 150 °C, 250 °C, 350 °C, 450 °C and 550 °C) are presented in Fig. 7. It is observed that the impedance (Z′) of BL composite is maximum in the low frequency region at low temperature and impedance (Z′) of BL decrease monotonically with increase frequency up to 100 Hz after this frequency, the impedance (Z′) reaches constant with increasing frequency at all temperatures. The impedance (Z′) decreases with increase temperature at low frequency region, and impedance of composite shows dispersion. The impedance (Z′) trend of BTG-1, BTG-2 composites are followed as same as BL composite. The impedance (Z′) of BTG-1 is higher than BL when Gd, Nb doped in BL and the impedance (Z′) of BTG-2 is lower than BTG-1 and higher than BL when an increase in concentration of Gd, Nb in BL. At low frequency region, the value of Z′ is high in all composites due to liberate space charge polarization in the sample as a result, reduces the height of the potential barrier of the material [53,54]. Moreover, at low frequency, the impedance of BL, BTG-1 and BTG-2 composites decreases with an increase in temperature due to increase conductivity in the samples [55]. The variation of capacitance (Cp’) of the BL, BTG-1 and BTG-2 at different temperatures (30 °C, 50 °C, 150 °C, 250 °C, 350 °C, 450 °C and

The magnetic properties of all the composites are obtained by computing magnetization Vs magnetic field loops. The magnetization of BL, BTG-1 and BTG-2 composite ceramics with the magnetic field at temperature 303 K are prese6nted in Fig. 9. The large view of M-H loops of BL, BTG-1 and BTG-2 composite between magnetic field −2.5 kOe − 2.5 kOe shown in the inset of Fig. 9. Fig. 9 clearly see that the saturation magnetization (Ms) of the BL decreases with the substitution of Gd, Nb in BL composite. The magnetic properties like saturation magnetization (Ms) coercive magnetic field (Hc), remnant magnetization (Mr) of the BL, BTG-1 and BTG-2 composites are depicted in Table 7. It is observed that the value of Mr of BL decreases with increasing concentration of Gd, Nb in BL composite. The value of coercive magnetic field (Hc) of BL increases as and when substitution of Gd, Nb in BL composite and after decrease with an increase in concentration of Gd, Nb in BL composites are clearly depicted in Fig. 10. The decrease in saturation magnetization of BL is due to the existence of non magnetic phase between BT and LF by substitution of Gd, Nb in place of Ti of BL composite [57]. The remnant magnetization (Mr) of BL composites is decreasing due to magnetic interaction between magnetic moments of BT phase and LF 7

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Fig. 7. Frequency dependent of impedance (Z′) and capacitance (Cp’) of composite ceramics.

5. Conclusions

phase doped with Gd, Nb in Ti of BL composite. Then ionic radius of Fe, Ti, Gd, Nb is mismatched with valence states of Nb+5/Fe3+ or Gd3+/ Fe3+ or Ti 4+/Fe3+ [58]. The coercive magnetic field (Hc) of the BL composite increase initially when doped with Gd, Nb in it due to domain wall motion which is turned out [59]. The coercive magnetic field (Hc) is changing with grain size and surface morphology [60] shown in Fig. 2 and the values of grain size of composites shown Section 4.2. The Bohr magnetron of BL, BTG-1 and BTG-2 composites are calculated using the below equation [61]

μB =

Ms xM 5585

The Gd, Nb doped BL composites successively prepared using a normal solid state technique. The structural, morphological, ferroelectric, dielectric, impedance, capacitance, resistivity and magnetic properties of all the composites have been investigated. The XRD confirms high crystalline nature and diffraction peaks of BTG-1 and BTG-2 shifted towards higher diffraction angle due to substitution of Gd and Nb in BL. The BTG-1 and BTG-2 FESEM images exhibit the coarseness in the sample and larger grains are distributed among smaller grains. The remnant polarization (Pr) and coercive field (Ec) of BL composite decreases initially when doped with Gd and Nb in BL and then increases with increasing concentration of Gd, Nb in BL composite, suggesting decreasing in leakage current in BL composites. At low frequency, the dispersion in the dielectric constant of all composites could be explained using Maxwell conduction mechanism. The dielectric loss (tan δ) of all composites decreases with increase frequency and becomes frequency independent in the high frequency region might be due to inability dipole polarization. Impedance (Z′) of all composites is high at low frequency due to liberate space charge polarization. The capacitance (Cp’) of all composites decreases with decrease temperature at low frequency region. The coercive magnetic field (Hc) of the BL composite increase initially when doped with Gd, Nb in it due to domain wall motion which is turned out. The magnetic moment of BTG-1 is greater than BL and BTG-2 composites. The squareness of all the composites are less than 0.5 represents anisotropic nature in the composites. Thus, all the composites exhibit both electric and magnetic nature in the same

(4)

where μ B represents Bohr’s magnetron, Ms is saturation magnetization, M is the molecular weight and 5585 is the magnetic factor. The values of Bohr’s magnetron of composites are presented in Table 7. It is clearly shown that the magnetic moment in terms of Bohr’s magnetron ( μ B ) of composites increases with the substitution of Gd, Nb in BL and later decrease with increasing concentration of Gd, Nb. The squareness of BL composites decreases with increase in concentration of Gd, Nb in BL is shown in Table 7. The squareness of BL, BTG-1 and BTG2 composites are estimated using the formulae Mr . The squareness of all Ms the composites are less than 0.5 represents small domains which are randomly oriented assemble of spherical particles confirms anisotropic nature in the samples. The area under the curve of BL composite decreases with increases concentration of Gd, Nb in it, which reveal the soft magnetic nature in all the composites. 8

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Fig. 8. Variation of A. C resistivity (ρ) of composite ceramics with frequency at different temperatures. Inset in BTG-1 shows A. C resistivity (ρ) of BTG-1 at different temperatures (from 50 °C to 550 °C).

phase considers as multiferroics used many fields like memory devices, actuators, switches, spintronics, sensors, magnetic field sensors.

Acknowledgements We wish to thank Dr. P.D Babu for providing VSM (M-H) facility at UGC-DAE Consortium for Scientific Research, Mumbai center, R5-shed, BARC, Mumbai – 400 085, India, Dr. V. Raghavendra Reddy and Dr.

Fig. 9. Magnetic hysteresis loop of the BL, BTG-1 and BTG-2 composite ceramics. Inset shows enlarged view of Magnetic hysteresis loops. 9

Journal of Magnetism and Magnetic Materials 494 (2020) 165822

G.R. Gajula and L.R. Buddiga

Table 7 The values of ferro magnetic parameters, squareness and Magnetic Moment (µB) of BL, BTG-1 and BTG-2 composite ceramics. Composite

BL BTG-1 BTG-2

Ferro Magnetic Parameters Ms values (emu/g)

Mr values (emu/g)

Hc values (kOe)

4.30 4.20 2.92

1.62 1.42 0.320

0.65 0.945 0.271

Squareness

Magnetic moment (µB)

0.3767 0.3380 0.1095

0.1775 0.1785 0.1277

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