**1. Introduction**

Magnetoelectric materials contains combination of the ferroic properties such as polarization, and magnetization [1–4]. Due to the interrelationship between ferroelectricity and magnetism, it is possible to control of ferroelectric properties using magnetic field and vice versa. Cr2O3, BiFeO3, YMnO3 etc. which are single phase magnetoelectric (ME) materials do not provide much benefits due to very weak magnetoelectric effect [5–7]. The solution to this issue is composites utilizing the product property of the two materials and have much better ME voltage coefficient [8–10]. Magnetoelectric (ME) particulate sintered composite can be fabricated in combination of magnetostrictive and piezoelectric phases [11–15]. Among the many advantages of sintered composites, simplicity in synthesis, cost-effective materials and fabrication process, and better control of desired geometry are the

most important ones. However, the ME effect in particulate sintered composite is still low and it is of the order of 100 mV/cm.Oe. Laminated composites which are fabricated by bonding colossus piezoelectric and magnetostrictive materials are very popular because they display excellent ME properties [16–19]. Dong et al. have shown a large response of 22 V/cm.Oe at 1 Hz using piezo fiber laminated between the high-permeability magnetostrictive FeBSiC alloy with epoxy [20]. The inherent materials property is the ferroelasticity that complemented both the ferroelectricity and ferromagnetism in same materials. This is a three-step mechanism: (a) ferroelectric ion movement needs structural building blocks, (b) super exchange type magnetic-interaction conduits and (c) the symmetry condition is satisfied [21]. One simple inference from this is that it is possible to synthesize the ferroelectric ferromagnets by replacing diamagnetic ions by paramagnetic ones on the B-site of oxyoctahedral ferroelectric perovskites.

Among the piezoelectric materials, Lead Zirconate Titanate (PZT) is the most popular for its high piezoelectric property and most importantly it shows magnetoelectric effect when used in a composite as reported by others. PZT when mixed with some magnetostrictive particles (such as ferrites) have been found to exhibit an extrinsic magnetoelectric effect resulting from a coupling interaction. The precondition to it is increased resistivity, almost zero to no interdiffusion, no chemical reaction and reduced interfacial such as microcracks and porosities [22]. Both the magnitude of magnetostriction and the slope of the magnetostriction curve with respect to applied magnetic field should have a large value in order to achieve high values of pseudo-piezomagnetic coefficients in the magnetostrictive phase [22–24]. In this regard, Ni ferrite doped with Mn (NiFe1.9Mn0.1O4) can be promising candidate due to its increased resistivity, superior magnetization and small coercive field in order to switch the domains.

Co-firing of piezoelectric and magnetostrictive phases with mismatch in coefficient of thermal expansion and lattice at high temperature induces strain in the sintered composite. Thus, a post sintering heat treatment is necessary to homogenize the matrix grain structure, reduce the strain and remove the chemical or stress gradients at the interface. It has been shown that piezoelectric and dielectric properties of sintered ceramics improve after annealing. Annealing also enhances the magnetostrictive properties of some common ferrites such as CoFe2O4. The annealing and aging technique for this reason has been used in fabricating materials with strong permanent magnetism [25, 26]. In this case, precipitates with soft magnetic nature are dispersed in a hard magnetic matrix, resulting in one of the best hard magnets.

In this chapter, the percentage of the Ni ferrite (doped with Mn) was varied from 3 to 10% (by mole). Two different sintering temperature (1100° and 1125°C) was investigated to see the sintering behavior and the effect of ferrite percentage and sintering temperature on physical, piezoelectric and magnetoelectric properties of the particulate composite. This chapter investigates a new post sintering treatment such as the annealing and aging technique for synthesizing ME composites with the objective of achieving a strong coupling between ferroelectric and magnetic order parameters.

## **2. Experimental**

#### **2.1 Processing**

Powders of PbO, ZrO2 and TiO2 (Alfa Aesar, Co. MA. USA) were mixed with alcohol and grinding media of YTZ (ϕ 5 mm, Tosoh Co. Tokyo, Japan) in a polyethylene jar for 24 hours. Similarly, NiO2, Fe2O3, MnCO3 was mixed and ball milled *Enhancement of the Magnetoelectric Effect in PZT-Ni Ferrite Composites Using Post Sintering… DOI: http://dx.doi.org/10.5772/intechopen.99870*

in same the fashion for 24 hours. After ball milling the powders were dried in an oven at 80°C. Then the powders were calcined (PZT at 750°C for 2 hours and NF at 1000°C for 5 hours). After calcinations, the powders were crushed and were examined by XRD to confirm the perovskite (for PZT) and spinel (for NF) phase. Then the powders were further crushed and sieved (US mesh # 270) very fine. Then the PZT and the NF powders were mixed stoichiometrically (for 3%, 5% and 10% NF by mole) and ball milled with alcohol and grinding media for 24 hours. After ball milling the powders were dried at 80°C, crushed in a mortar and sieved in a stainless-steel sieve of #170 US mesh. Then the powders were pressed to pellets of ϕ12.7x 1.5 mm in a hardened steel die using a hydraulic press under a pressure of 15 MPa. Pressureless sintering of composites was performed with Lindberg BlueM furnace with platinum foil (0.003 in. thick) at the bottom. The sintering temperatures were 1100° and 1125°C for 2 hrs. After sintering, samples were annealed at 800°C for 10 hours followed by air cooling and then aged at 400°C for 5 hours.

## **2.2 Characterization**

The density of the samples was measured by Archimedes principle. XRD was perfromed in a Siemens Krystalloflex 810 D500 x-ray diffractometer on samples after calcination, sintering and post sintering treatment. For the measurement of % Spinel phase, the area under the curve for the perovskite (101) peak and spinel (311) peak was measured. Then the % Spinel was found taking the percentage of the area under spinel (311) peak among the total are under these two peaks. Specimens were polished with 0.3 μm powder, thermally etched and examined under Scanning Electron Microscope (Zeiss Leo Smart SEM). The annealed and aged specimens were chemically etched in a solution of 95% H2O, 4% HCL and 1% HF. The average grain size of the composite was determined from SEM micrograph by linear intercept. X-ray mapping was done using the same Scanning Electron Microscope. TEM of sintered samples was done by JEOL – 1200 EX Scanning Transmission Electron Microscope.

#### **2.3 Property measurement**

An Ag-Pd electrode is painted on both surfaces of the pellets and fired at 825°C for 1 hr. The electroded specimens were polled by applying a D.C. field of 2.5 kV/ mm for 20 minutes in a silicone oil bath at 120°C. The piezoelectric and dielectric properties were measured by APC YE 2730A d33 meter and an impedance analyzer (HP 4192, Hewlett Packard Co. USA). The Curie temperature was measured from Capacitance vs. Temperature graph with the help of Multi frequency LCR meter (HP 4274 A Hewlett Packard Co. USA). The magnetoelectric property was measured in terms of variation of dE/dH coefficient with an A.C. Magnetic field and no D.C. magnetic bias. The coefficient was measured directly as response of the sample to an A.C. magnetic input signal at 3 Hz and 60 Oe amplitude. The voltage generated from the composite was measured using a charge amplifier. The output signal from the amplifier was measured with an Oscilloscope (54601A, HP Co. USA). The output voltage is converted into dielectric displacement (D = CV/A), which can also be expressed in electric field (E = D/εoε) and then the electric field divided by A.C. magnetic field gives the magnetoelectric voltage coefficient for PZT-NF composite.
