2.1.Sample

preparation

Multiferroic composite with the formula x Li0.1Ni0.2Mn0.6Fe2.1O4

+ (1–x) BiFeO3 (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by mixing stoichiometric

proportions of pure phase BiFeO3 and Li0.1Ni0.2Mn0.6Fe2.1O4

powder which were prepared by the standard solid state reaction technique. For the preparation of BFO

powders, high purity (99.9%) raw materials of Bi2O3, and

Fe2O3 were mixed thoroughly according to stoichiometric formula

in a mortar with pestle for 5–6 h using acetone as mixing medium. The mixed

powder of BFO was calcined at 1073 K for 4 h and pre-sintered at 1123 K for 4 h.

Appropriate amount of high purity (99.9%) raw materials of Li2CO3,

NiO, MnCO3 and Fe2O3 were mixed to synthesize LNMFO

by the same procedure as BFO. The mixed powder was then dried and pre-sintered

at 1473 K for 4h. The pre-sintered powders were again ground thoroughly in an

agate mortar. For preparing the composites, we have mixed LNMFO and BDFO powders

together according to the stoichiometric ratio and ground in acetone media for

3 – 4 h to allow good mechanical mixing. The composite powders were mixed with

polyvinyl alcohol (PVA) as a binder for granulation. From these powders disk-

and toroid-shaped samples were prepared by applying a uniaxial pressure of 55

MPa. Finally the samples were sintered at 1148K(x=0.0), 1173K(x=0.1 and 0.2)

and 1198K(x=0.3, 0.4 and 0.5) for 4 h.

2.2. Characterizations

The structural analysis of the sintered samples were carried out

with the help of X-ray diffractometer with CuK? radiation (?= 1.5418×10-10

m) at room temperature. The lattice parameters were calculated from X-ray

diffraction (XRD) data. In order to analyze the distribution of grain on the

surface of sintered samples, Field Emission Scanning Electron Microscopy

(FESEM) images were taken with the help of JEOL JSM 7600F electron microscope.

The average grain size was calculated using linear intercept technique. The

bulk density (?B) of each composite was calculated using the formula:

where r is the radius, m is the mass and t is the thickness of the

sample. The X-ray density of the composites was measured by the formula,

, where

is (1-x) times molecular weight of BFO and

is x times molecular weight of LNMFO,

(ferroelectric)

and

(ferrite), x is the

weight fraction of LNMFO in the samples 26. The ?x (ferroelectric

and ferrite) are calculated by the general formula,

, where n is the number of atoms in a unit cell, M is the molar mass of the sample, NA is Avogadro’s number and V is the volume of the unit cell. The

porosity of the compositions was determined using the relation

. The dielectric and magnetic properties were studied at room

temperature as a function of frequency by using a WAYNE KERR 6500B Impedance

Analyzer. To determine dielectric properties, the samples were painted with

conducting silver paste on both sides of the samples to ensure good electrical

contacts. The dielectric constant (??) was measured from the capacitance using

the formula:

, where

is the capacitance of the pellet,

is the cross-sectional area of the electrode

and

(= 8.85×10-12 F/m) is the permittivity

in free space. The ac

conductivity (?ac) of the compositions was calculated using room

temperature dielectric data from the relation:

, where ? is the angular

frequency and tan? is the dielectric

loss.

The real part

of the

complex initial permeability and loss

tangent (

) were calculated using the formula:

and

, where Ls is the self-inductance of the sample core,

is the

inductance of the winding of the coil without the sample, and

is the imaginary part of complex initial

permeability.

is derived from the geometrical relation,

where,

is the

permeability in vacuum, N is the number of turns of the coil (N = 4), S is the

area of cross section ,

is the mean diameter of the toroidal sample, where

d1 and d2 are the inner and outer diameter of the

toroidal sample, respectively 28. The magnetic hysteresis (M-H) loops were determined using a

vibration sample magnetometer (VSM, model Micro Sense, EV9). The number of Bohr

magneton, n(?B), was measured using the relation:

, where

is the molecular weight of the composition,

is the saturation magnetization and NA is Avogadro’s number and ?B

is 9.27×10-21 emu. The ME effect was obtained by applying an ac

magnetic field superimposed on a dc magnetic field on the sample, and then

measuring the output signal with applied dc magnetic field. An electromagnet

was used to supply a dc magnetic field of up to 0.77 T. A signal generator was

used to drive the Helmholtz coil to generate an ac magnetic field of 0.0008T. The

output voltage generated from the composite was determined using a Keithley

multimeter (Model 2000) with of dc magnetic field. ME voltage coefficient (?ME) was measured using relation

29

, where

is the ME voltage across the

sample surface and h0 is

the amplitude of the ac magnetic field.