**3. Results and discussion**

**Figure 1** shows the density variation of the piezoelectric/magnetostrictive composites sintered at 1100 and 1125°C. Both shows similar pattern. The density

**Figure 1.** *Effect of % NFM on density at two different temperatures.*

**Figure 2.** *Piezoelectric properties at different % NFM of sintered as well as annealed and aged samples.*

increases with increasing percentage of NF and starts to plateau after 5 moles% of NFM. The samples sintered at 1125°C, showed higher densities than that of 1100°C. This is due to the better sintering behavior at higher (1125°C) temperature. **Figure 2(a)** shows the variation of the piezoelectric constant with NFM mole%. As the perovskite phase decreases with increasing NFM concentration, d33 starts to decrease. From **Figure 2(b)** an increase in d33 after annealing and aging was observed for both the sintering conditions. For the PZT-5NFM, 14% and 6% increase in d33 were recorded for samples sintered at 1100°C and 1125°C respectively. The difference in the magnitude of d33 before and after annealing and aging for other two composite samples (3% and 10% NFM) sintered at 1125°C was small. This can be associated to concurrent rise in the internal strain due to structural dissimilarity and domain size. Increase in stress decreases d33 while increase in domain size increases d33. **Figure 2(c)** shows the variation of the dielectric constant with NFM concentration. The dielectric constant magnitude for the samples sintered at 1100°C showed a decrease of about 10% after thermal treatment. Based on the behavior of the piezoelectric and dielectric constant, it can be seen that the total strain magnitude remains of the same order for the samples before and after the thermal treatment.

*Enhancement of the Magnetoelectric Effect in PZT-Ni Ferrite Composites Using Post Sintering… DOI: http://dx.doi.org/10.5772/intechopen.99870*

**Figure 3.**

*Variation of dielectric constant and dielectric loss with temperature of different composition and at different sintering temperature [27–30].*

**Figure 3(a)** and **(b)** shows the dielectric constant and loss factor as a function of temperature. The dielectric properties were measured at 1 kHz under 1 V excitation. The Curie temperature (Tmax) measured for all the samples compositions were in the vicinity of 375°C with slight decrease in Tmax with increasing % NFM. The loss factor (tanδ) measured was ~2% below 80°C. Beyond 200°C it increases sharply. The space charge effect and low resistivity of the PZT-NFM samples at elevated

**Figure 4.** *Dielectric constant as a function of the temperature at 1 kHz for PZT-5 NFM at three different conditions [28, 29].*

#### **Figure 5.**

*Magnetic field H vs magnetic moment curve of different composite.*

temperature caused the rapid rise of dielectric loss around curie temperature. The Tmax did not change much regardless of the thermal treatment. The dielectric constant vs. temperature for PZT-5 at % NFM composites before and after different *Enhancement of the Magnetoelectric Effect in PZT-Ni Ferrite Composites Using Post Sintering… DOI: http://dx.doi.org/10.5772/intechopen.99870*


#### **Table 1.**

*Magnetic properties of different PZT-NF powders [28].*

thermal conditions were shown in **Figure 4**. The ferroelectric Curie temperature only decreased slightly (~8°C) by annealing and aging. This drop in Tmax can be attributed to the stress relaxation which can be precisely understand by diffraction studies. During annealing as the composites were soaked in 800°C for 10 hours, grain grown occurs which can result into increased dielectric constant.

**Figure 5(a)** to **(f )** shows the variation of magnetic properties as a function of thermal treatment for the different composites. A substantial enrichment in remanent (Mr) and saturation (Ms) magnetization was observed after the thermal (annealing and aging) treatment. **Table 1** shows the magnetic data for the calcined powder, sintered samples and thermally treated samples. It is clearly observed that the magnetization values increased sharply after thermal treatment. No difference in coercive fields were observed between the sintered and thermally treated samples whereas remarkable differences were observed in magnetic resonance which decreased significantly after thermal treatment. These results can be explained if it is assumed that the size of spinel phase increases with the thermal treatment. As there are basic resemblance in the oxygen synchronization chemistry between the perovskite and spinel structure, it leads to the lattice dimensions that are compatible with the spinel building blocks (considering growth is along the c-axis).

**Figure 6(a)** shows the X-Ray diffraction patterns of composite sintered at 1125°C for 2 hrs. A pure perovskite phase was obtained with small fraction of 311 peak of spinel phase only observed for PZT-10 NFM. No other phase was detected. PZT composition of Zr: Ti = 52:48 was selected as it is closer to the morphotropic phase boundary (MPB) providing high piezoelectric property. On modification with NFM the perovskite phase was found to exhibit rhombohedral symmetry as shown by reduced splitting of 200/002 peaks. As expected, a higher sintering temperature resulted in higher content of spinel phase. **Figure 6(b)** shows the XRD pattern for same composition of samples after post sintering thermal treatment. In all three diffraction patterns, an increase in the fraction of spinel phase were clearly observed. The fraction of the spinel phase present was computed using the expression:

**Figure 6.**

*XRD pattern of different compositions: a. after sintering, b. after annealing and aging.*

PZT-10 NFM sintered at 1125°C showed an increase of % spinel of 1.03% (from 6.82% after sintering to 7.85% after annealing and aging). This is a significant increase considering the dissimilarity in the lattices of two phases. Comparing this result with the annealing and aging treatment, it can be theorized that there is a likelihood of homogenization in the PZT-NFM system.

**Figure 7(a)** and **(b)** shows the Magnetoelectric coefficient for 3, 5 and 10 mole% of NFM. Data are given in each figure for the three different thermal histories. For PZT-10NFM, the ME coefficient increased from 60 mV/cm-Oe after sintering to 88 mV/cm-Oe after annealing and aging which is nearly 50% increase. This is due to the reduction in misfit strain between perovskite and spinel phase, decrease in interface micro such as porosities and cracks after annealing. Reducing the interface defects, would increase the ability of piezoelectric domains to elastically react to strains induced on it by bordering magnetostrictive phases, or vice versa. To achieve high ME properties, the boundary conditions between phases needs to be as mechanically free as possible. Enhancements in the ME coefficient may also come from increased magnetization from Mn+3 to Mn+2 conversion, as an enhanced magnetic permeability has been reported to increase the effective piezomagnetic coefficient (dλ/dH).

*Enhancement of the Magnetoelectric Effect in PZT-Ni Ferrite Composites Using Post Sintering… DOI: http://dx.doi.org/10.5772/intechopen.99870*

**Figure 7.** *Variation of magnetoelectric coefficient as a function of % NFM [27–29].*

**Figure 8.** *Bright field TEM images PZT-5 NFM composites after (a) sintering, and (b) annealing [30].*

**Figure 8(a)** shows Transmission Electron Microscopy image of as-sintered sample. This image consists of facet phases (bright contrast) NFM particles embedded in the PZT matrix. The NFM particles vary from 300 nm to 1500 nm. **Figure 8(b)** shows TEM images of annealed and aged samples. Few distinctions of microstructural characteristics are observed from those shown in **Figure 8(a)**. The density of the NFM particles in this annealed sample is much less than that in the sintered sample. In addition, the NFM particles in the annealed sample have a typical size of 500 nm, much smaller than that in the sintered sample.

High magnification bright field TEM images of the sintered, annealed and aged samples were shown in **Figure 9(a)**–**(c)**. Misfit strain fields close to the

**Figure 9.**

*Bright field TEM images PZT-5 NFM composites after (a) sintering, (b) annealing and (c) aging [30].*

PZT/NFM were observed. These strain fields developed at the interface to acclimatize the mismatch in the PZT and NFM lattice. The domain patterns had larger width, which is the characteristic of 90° domains and there is intergranular discrepancy in domain width [31]. The image after annealing shows reduced misfit strain near the PZT/NFM interface. The remnant strain fields after annealing consist of additional constituent due to quenching process after annealing. Diffused grain boundaries were observed in low magnification images. The image after the aging in **Figure 9(c)** shows noticeably reduced strains. There are stripe like morphologies and they are extended from grain boundary to grain boundary. A finer scale domain structure is also observed to exist within larger domain patterns. This finer domain pattern has striation like morphology and is periodically spaced [32, 33].

*Enhancement of the Magnetoelectric Effect in PZT-Ni Ferrite Composites Using Post Sintering… DOI: http://dx.doi.org/10.5772/intechopen.99870*
