**5. Results and discussions**

**Figure 1(a)** [25] Compares the polarization of undoped, hard and soft PZT. The max polarizations observed were 14.61, 23.54 and 31.65 μC/cm<sup>2</sup> respectively for these three groups. It was also recorded that elastic compliance (S11) of these three compositions are 1.74 x 10<sup>11</sup> (soft), 1.37 x 10<sup>11</sup> (hard) and 1.11 x 10<sup>11</sup> (undoped) m2 /N respectively. Due to the presence of metal vacancies, soft PZT's have higher polarizations per unit field applied, resulting in enhancement in both the dielectric and the piezoelectric susceptibilities.

An increase in piezoelectric constant (d33) (from 75 to 105 pC/N) and increase in dielectric constant (from 642 to 914) was observed when an undoped PZT was doped with PZN to make it soft. These increase in dielectric and piezoelectric properties clearly reflected in Magnetoelectric coefficient of PZT (soft) – 20 NZF composite *<sup>δ</sup><sup>E</sup> <sup>δ</sup><sup>H</sup>* = 186.5 mV/cm.Oe. as observed in **Figure 1(b)**. Due to increase in electromechanical coupling, smaller coercive field, and superior dielectric and piezoelectric properties Soft PZTs show larger ME voltage coefficient compared to hard (154 mV/cm.Oe) or undoped PZT (128 mV/cm.Oe).

Microstructure of PZT (undoped) – 20NZF and PZT (soft) – 20NZF composites were dense with the measured densities of ≥95% [25]. And the average grain size of the PZT – 20NZF composite was about 800 - 850 nm, whereas that of PZT (soft) – 20 NZF was smaller and the average ranges between 650 and 700 nm. Doping of PZN in PZT reduces the grain size as both the composite were sintered at 1150°C for 2 hours. In one of our previous studies it was shown that above 600 nm, the ME coefficient does not change much with increase in grain size [26]. So, both PZT (undoped) – 20NZF and PZT (soft) – 20NZF composites have optimum grainsize in terms of ME coefficient. Besides the grainsize reduction, the resistivity of PZT increases upon doping with PZN, as lower leakage currents were observed after poling (**Figure 2**).

**Figure 3** [25] shows the saturation magnetization (Ms), coercive field (Hc) and magnetic Curie Temperature of the PZT – 20NZF composite as a function of Zn doping in NZF. It is clearly observed that the coercive field starts to drop as we increase the Zn doping in NZF. On the other hand, saturation magnetization becomes optimum (0.72 emu/gm) at around 30% Zn doping in NZF and then starts

#### **Figure 1.**

*(a) Polarization vs. electric field loop and (b) ME coefficient vs. DC bias of different compositions of PZT – 20 NZF composites [25].*

**Figure 2.**

*Microstructure of (a) PZT – 20 NZF and (b) (0.85PZT –0.15 PZN) – 20 NZF [25].*

**Figure 3.**

*Magnetic properties as a function of Zn doping. (a) Ms and Hc vs. Zn concentration and (b) magnetization vs. temperature [25].*

to drop off with increase in Zn doping. In terms of Ferromagnetic Curie Temperature, it started to drop from 850 K to 549 K as the Zn concentration was increased from 0 to 50 mole %.

**Figure 4(a)** [25] Shows the hkl = (400) diffraction peak for NZF composites as a function of Zn concentration. The (400) peak started to shift towards lower Bragg angles as Zn concentration was increased which indicated an enlargement of lattice parameters. From Bragg's law, we determine a 0.9% lattice expansion (8.32 Å for NiFe2O4 and 8.394 Å for Ni0.5Zn0.5Fe2O4) with this change in crystal chemistry. **Figure 4(b)** shows the ME voltage coefficient as a function Zn concentration in PZT – 20 NZF. 30 at% Zn concentration in NZF showed the maximum value of *<sup>δ</sup><sup>E</sup> <sup>δ</sup><sup>H</sup>* = 138 mV/cm.Oe, whereas 0 at% Zn concentration in NZF (pure Ni ferrite) has a

**Figure 4.** *Effect of Zn concentration on (a) peak shift of 400 peaks and (b) magnetoelectric coefficient of PZT – 20 NZF [25].*

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

maximum value of 112 mV/cm.Oe. With Zn concentration above 30%, the ME coefficient dropped notably, reaching a value of 60 mV/cm.Oe for 50 at% Zn. Saturation magnetization and magnetoelectric responses as a function of Zn concentration in NZF was found to have similar effect because permeability μ is directly related to magnetization M, via μ =1+4π*<sup>M</sup> <sup>H</sup>*. Furthermore, the changes in the ME coefficient with Zn concentration depends on the change in effective piezomagnetic coefficient (d33,m = μ33.s33.λ33) which is directly related to the permeability.

**Figure 5 (a-c)**show the density, dielectric constant and piezoelectric constant as a function of mole percent ferrite in the composite for two different compositions. It is clear that as the ferrite concentration increases, density, dielectric constant and piezoelectric constant decreases. All the compositions showed more than 98% of the theoretical density and the microstructural analysis confirmed this measurement. For PZT – CFO, there is a slight increase in density from 3–5% concentration which can be attributed to better sintering as the.

ferrite becomes more homogenized in the matrix. The density increase was also observed when the composition changed from 10–15% for CFO in PZT – CFO composite. This can be explained by grain coarsening of PZT. As the CFO content increases from 10–15% there is slight increase in dielectric constant and then with further increase in ferrite concentration, dielectric constant starts to drop. It is wellknown that grain coarsening has direct effect on dielectric properties, whereby, dielectric constant increases with larger grain size. There was no significant difference in piezoelectric data between CFO and CZF based ferrite composites with ferrite concentration.

**Figure 5.** *(a), (b) and (c) The density, dielectric constant and piezoelectric constant as a function of mole percent ferrite.*

A comparison between PZT – CFO and PZT – CZF in terms of room temperature magnetic properties is presented in **Figure 6(a)** and **(b)** for 3% and 5% ferrite concentration. In both the cases, a considerable difference in coercivity between CFO and CZF particles was observed. For 3% ferrite concentration the saturation magnetization of CFO was slightly higher than CZF but the coercivity was much lower (33 Oe compared to 263 Oe). For 5% CFO and CZF concentration, coercive fields of 53 and 288 Oe were measured and the saturation magnetization of CFO particle (76.6 memu) was slightly lower than the CZF particle (88.54 memu). **Figure 6(c)** and **(d)** show the magnetization of PZT – 5 CFO and PZT – 5 CZF composites from 5 K to 300 K and from 310 K to 1000 K respectively. It is quite interesting to observe that the magnetization for both the composites start to drop from 5 K to 300 K. The drop for PZT – 5CFO is linear - 0.04 emu to 0.035 emu and for PZT – 5CZF is non –linear 0.075 to 0.065 emu respectively for 70 mg of sample weight. There is a slope change in the PZT – CZF magnetization curve at around 150 K – which is close to the curie temperature of ZnFe2O4. From 150 K to 5 K – an increased slope was observed. Below the curie temperature, Zn ferrite also contributed to the magnetization curve. Besides the increase in magnetization in subzero temperature for both the composites can also be explained by the atomic vibration of the crystal lattice. As the temperature starts to drop below the room temperature and approaches the absolute zero temperature, the atomic vibration is seized, resulting in much stable crystal lattice, which gives us accurate measurement of the magnetization. At high temperature (from 310 to 1000 K) the magnetization of PZT – CFO starts to increase and then decrease to zero at Curie temperature, which is almost 750 K. The PZT – CZF Curie temperature was recorded at 450 K. This drop

**Figure 6.**

*(a) and (b). Hysteresis loop for PZT – CFO and PZT – CZF. (c) and (d): the magnetization of PZT – 5 CFO and PZT – 5 CZF composites.*

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

in Curie temperature from 750 to 450 K is due to the substitution of Zn on the cobalt site. The substitution of Zn2+ for Fe3+ reduces the Curie temperature of the ferrite [6, 7, 9]. On the other hand, increasing the zinc content of cobalt-zinc ferrites increases their lattice parameter while decreasing the saturation magnetization above 50% Zn due to augmented B-B interaction followed by reduced A-B interaction. Also, the presence of Co2+ ion in the cobalt-zinc ferrite hastens the Co2+ + Fe3+⇔ Co3+ + Fe2+ exchange reaction in octahedral sites, while tetrahedral sites are preferentially occupied by zinc cations. Tetrahedral sites in the spinel structure are suitable for cationic radii in the range of 0.58 Å to 0.67 Å, while octahedral sites can accept cations with radii in the range of 0.70 Å to 0.75 Å [22]. Therefore, in the unit cell structure, Co2+ (0.72 Å) and Fe2+(0.75 Å) may replace Zn2+(0.74 Å), while Co3+(0.63 Å) can exchange sites with Fe3+(0.64 Å). This exchange in Co-Zn ferrite system, the substitution of non-magnetic zinc in place of ferromagnetic cobalt leads to a decrease in Curie temperature owing to diminishing A-B super exchange interaction.

**Figure 7** shows the XRD pattern for CFO and CZF particles. Inset shows magnified 311 peaks for CFO and CZF particles respectively. The shift in peaks to lower angle for CZF particles is clearly noticeable which results in a larger unit cell size for CZF particles. The increase in unit cell can induce strain. Strain can be revealed as strain fields or cleavage or other defects inside the microstructure. **Figure 8** shows the ME coefficient as a function.

of ferrite concentration. For PZT – CFO the maximum ME coefficient of 25 mV/ cm.Oe was recorded at 15% ferrite concentration, which drops again for 20% ferrite. The measured ME coefficient is quite low compared to the PZT – Nickel ferrite composites. Cobalt ferrite has a very high coercive field compared to the nickel ferrite composition. Thus, a high DC bias field is necessary to obtain the peak ME coefficient. Another contributing factor is the initial permeability. Cobalt ferrite has lower initial permeability than the nickel ferrite which contributes towards lower ME coefficient.

**Figure 9(a)** and **(b)** show the SEM images of the microstructure of the PZT-20CFO and PZT – 20CZF samples sintered at 1125°C respectively. Dense microstructures for both compositions were obtained and the sintered samples had grain size of 1 to 1.5 μm. Elemental analysis using the EDX showed that CFO and CZF

**Figure 7.** *XRD patterns for CFO and CZF particles.*

**Figure 8.** *ME coefficient as a function of ferrite concentration.*

**Figure 9.**

*(a) and (b): SEM images of the microstructure of the PZT-20CFO and PZT – 20CZF.*

grains are distributed in the piezoelectric matrix. **Figure 10(a)** and **(b)** shows the bright field TEM images of the sintered.

PZT – 20 CFO and PZT – 20 CZF samples respectively. Compared to PZT – CFO, PZT - CZF sintered samples were found to consist of twin boundaries, cleavage, and strain fields at the interface of PZT and CZF grains. These defects develop to accommodate the mismatch in the PZT and CZF lattices, as ferrite (CFO/CZF) lattice parameters are more than double the lattice parameter of the PZT lattice. The lattice parameter increases by 0.2% for 40% Zn doped sample as observed in the XRD patterns. In the inset, diffraction pattern of a PZT grain is shown. No superlattice diffraction spots were observed near the first order diffraction spots, which indicate less intense diffusion level. From the SAED diffraction pattern the lattice parameter a and c are calculated as 4.05 and 4.132 Å, hence the c/a ratio is 1.02. Larger width domain patterns were also observed near the interface, which is characteristic of 90<sup>o</sup> domains. Besides that, intergranular heterogeneity in domain width is observed all over the structure especially near the interface. The observed defects in PZT - 5% CZF are in line with the SEM images. A finer scale domain structure, which usually has striation like morphology and periodically spaced was

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

**Figure 10.** *(a) and (b): bright field TEM images of the sintered PZT – 20 CFO and PZT – 20 CZF.*

observed in this structure away from the interface. These finer domains appear when the stress is relieved from the structure. It can be inferred that near the ferroelectric – ferromagnetic interface, the stress is higher and defects are observed due to strain mismatch, whereas the area away from the interface has lower stress.
