Abstract:
Multiferroic materials with the coexistence of ferroelectric and magnetic orders have
been stimulating significant interest both from the basic science and application point of
view. The magnetoelectric (ME) effect in multiferroic composites with piezoelectric and
magnetostrictive phases results from the cross interaction between the two phases in the
composite. In the present research four series of multiferroic composites i) (1-y)
BiFeO3(BFO)-yNi0.5Zn0.5Fe2O4(NZFO), ii) (1-y)BiFeO3(BFO)-yNi0.50Cu0.05Zn0.45Fe2O4
(NCZFO), iii) (1-y)Bi0.7La0.3FeO3(BLFO)-yNi0.5Zn0.5Fe2O4(NZFO) and iv) (1-y)
Bi0.8Dy0.2FeO3(BDFO)-yNi0.5Zn0.5Fe2O4(NZFO) (where y = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and
1.0) have been prepared by standard solid state reaction method. The structural, magnetic,
electrical and ME properties of these composites have been studied in details. Pellet- and
toroid-shaped samples are prepared from each composite and sintered at different
temperatures for 4 hours in air. Structural and morphological analyses are carried out by Xray
diffraction (XRD) and field emission scanning electron microscope, respectively. The
complex initial permeability, dielectric constant and ac conductivity have been measured
using a Wayne Kerr Impedance Analyzer. The XRD analysis reveals the coexistence of
both the structures of ferrite and ferroelectric phases in the composites with no other phase
thereby indicating that there is no chemical reaction between the two phases in the
composites. The NZFO and NCZFO show spinel structure whereas BFO, BLFO and BDFO
show distorted rhombohedral, tetragonal and orthorhombic perovskite structures,
respectively. Impurity phase has been suppressed significantly in (1-y)BLFO-yNZFO and
(1-y)BDFO-yNZFO composites due to the La and Dy substitution in BFO, respectively.
There is a slight change in the lattice parameter of both the ferrite and ferroelectric phases in
the composites which may be due to the stress exerted on each other by the two phases. The
X-ray density decreases almost linearly obeying the rule of mixture for all the series of
composites. The microstructural study shows that both the sintering temperature and the ferrite content in the composites have significant influence on the average grain size. The
average grain size of all the composites increases with increasing sintering temperature. The
average grain size initially decreases with the ferrite content up to certain concentration and
then increases with further increase of ferrite content in the (1-y)BFO-yNZFO and (1-
y)BDFO-yNZFO composites. On the other hand, the average grain size decreases for (1-
y)BFO-yNCZFO and increases for (1-y)BLFO-yNZFO with the ferrite content in the
composites. A significant enhancement in the initial permeability and relative quality factor
has been observed with the increase of ferrite content in the composites. The optimum
initial permeability is obtained for (1-y)BFO-yNZFO composites. The saturation
magnetization, remanent magnetization and coercivity have been calculated from the M-H
loop at room temperature. The saturation magnetization increases with the increase in ferrite
content in the composites and it fairly follows the sum rule. The dielectric dispersion at
lower frequency is due to the Maxwell-Wagner type interfacial polarization. The highest
dielectric constant is obtained for (1-y)BDFO-yNZFO composites. The complex impedance
spectroscopy is used to distinguish between the grain and grain boundary contribution to the
total resistance. The grain resistance is extremely low as compared to the grain boundary
resistance indicating the conducting nature of the grain. The ME voltage coefficient ���
is measured as a function of applied dc magnetic field. The highest ��� (~66 mV/cm Oe) is
obtained for 0.6BDFO-0.4NZFO composite which is attributed to the enhanced mechanical
coupling between the two phases. This value of ��� is found to be larger than the values of
some reported bulk composites and thus the (1-y)BDFO-yNZFO composites can be better
alternatives to single phase ME materials.