**1. Introduction**

Magnetoelectric [ME] particulate composites combine the magnetostrictive and the piezoelectric properties of materials, through product tensor properties [1].

Multiferroic magnetoelectric materials possess two or more ferroic properties such as ferroelectricity, ferromagnetism and ferroelasticity [2–4]. The spin lattice structure in a magnetoelectric composite can be directly related to (i) linear or non-linear shape change in magnetostrictive phase under alternative magnetic field, (ii) polarization change in piezoelectric phase through field induced alternating stress–strain and finally (iii) charge developed in the piezoelectric phase due to this alternating stress [1, 5, 6]. The interrelationship between ferroelectricity and magnetism allows magnetic control of ferroelectric properties and vice-versa. Single phase magnetoelectrics such as Cr2O3, BiFeO3, YMnO3 etc. exhibit poor combination of electric and magnetic properties at room temperature [7–9]. On the other hand, two-phase magnetoelectric (ME) materials provide large coupling and may play important role in future magnetoelectric devices [10]. Another important issue that can be very influential not only in nanostructures or multilayer structure but also in bulk ceramic composite is the interface chemistry. Migration of mobile atoms (from ferroelectric and magnetic phases) through the interface causes ferroelectric and magnetic instability and alters the interface chemistry, which affects the interface magnetoelectric properties [10, 11]. There are lot of advantages that sintered particulate offers, compared to in-situ composites (i.e. unidirectionally solidified of BaTiO3 – CoFe2O4), such as they are cost effective to produce, fabrication is easy and finally and most importantly the process parameters can be controlled much better. In terms of ME responses, laminate magnetoelectric composites gained a lot of popularity and can be fabricated by attaching piezoelectric layer between two layers of magnetostrictive discs or plates. Sintered particulate composites exhibits low resistivity, defects, diffusions at the interface and incompatibility of elastic compliances and mismatch in coefficient of thermal expansion. As a result, sintered composites show inferior ME responses compared to laminated composites. Therefore, it is essential to augment the composition, grain size, grain orientation, and sintering conditions in order to enhance the Magnetoelectric properties of the sintered composites.

The composites exploit the product property of the materials [12–14] where the ME effect can be realized by mixing individual piezomagnetic and piezoelectric phases or individual magnetostrictive and piezoelectric phases. In early 70s, researchers at Philips Laboratories demonstrated ME composites [15–18] by unidirectional solidification of eutectic composition of BaTiO3 – CoFe2O4. The results showed a high ME voltage coefficient dE/dH of 50 mV/cm•Oe with 1.5 wt % of excess of TiO2 [15]. Later an even higher ME coefficient of 130 mV/cm•Oe was obtained in eutectic composition of BaTiO3-CoFe2O4 by unidirectional solidification [17]. Currently, various particulate composites consisting of piezoelectric and magnetostrictive materials with different connectivity schemes including "3-0" and "2-0" have been reported, using LiFe5O8, NiFe2O4, (Ni,Zn)Fe2O4, CoFe2O4, CuFe2O4 as magnetostrictive materials and BaTiO3, Pb(Zr,Ti)O3 as piezoelectric phase [15–24].

The figure of merit for a ferromagnetic-ferroelectric composite is large magnetoelectric coefficient (i.e., susceptibility) given as:

$$\text{Figure or } \texttt{merit} = \sqrt{\mu\epsilon};\tag{1}$$

Here μ = ferromagnetic permeability and ε is the dielectric permittivity. It is eminent from Eq. (1) that a high dielectric constant piezoelectric phase and a high permeability magnetic phase would produce a composite with optimum ME response if we can keep the high resistivity, low interface defects and lower rate of interface diffusion. Literatures and experimental review showed that the Nickel

*Doping Effect on Piezoelectric, Magnetic and Magnetoelectric Properties of Perovskite… DOI: http://dx.doi.org/10.5772/intechopen.95604*

Ferrites are stable in PZT up to 1250°C and offers higher permeabilities. Also in terms of resistivity and loss, Ni based ferrite are preferable over Mn based ferrite, because Ni based Ferrites have higher electric resistivity and lower dielectric loss.
