Received 16 May 2016; accepted 1 August 2016; published 4 August 2016
Magnetoelectric (ME) laminates, the composites composed of magnetostrictive and piezoelectric layers show giant ME effects over that of natural multiferroics and particulate composites by up to several orders of magnitude  -  . The ME effect in laminate composites is realized by a stress mediated mechanical coupling between the magnetostrictive and piezoelectric layers which is often referred as a “product property”  . The ME effect in these laminates has drawn significant interest during the recent few years due to their extensive potential applications. To date, different laminate composites with various material compositions of piezoelectric lead zirconate titanate (PZT) or lead magnesium niobate-lead titanate (PMN-PT) and magnetostrictive Terfenol-D or metglas were reported  -   . Among these laminates, PZT based ME laminates show giant magnetoelectric voltage coefficients in the order of several V/cm∙Oe  -  . However, these laminates with PZT or other lead- based materials are indeed susceptible to pollution and toxicity because of Pb  . Hence, from the environmental point of perspective, it is highly demanding to explore the lead-free laminates and devises to replace the commonly used lead based laminates. The most commonly adopted lead free piezoelectric phase in the composites is BaTiO3 due to its superior piezoelectric properties. BaTiO3 has been widely used in the ME composites in the form of particulate composites or sintered ME laminates however, these composites show comparatively smaller values than the present lead free composites having 2-2 connectivity  -  . On the other hand towards the device miniaturization, there are several reports available on nanostructure composites in the shape of wires, pillars, and films  -  . However, the ME coupling of these nanostructures is much smaller to be used for device applications. It is well known that 2-2 type sandwiched ME laminates show very large ME coupling than the particulate or sintered ME composites. In recent years, metglas is widely used as a magnetostrictive material due to its very large piezomagnetic coefficient, q11 compared to that of other magnetostrictive materials  . Moreover, there is very less study on ME laminates composed of BTO and metglas as constituent phases. One can expect very large ME coupling if these two constituent phases can be combined. In this respect, herein we report symmetric ME composites made up of BaTiO3 (BTO) as a piezoelectric phase and NiFe2O4 (NFO) and metglas as magnetostrictive phases.
BaTiO3 and NiFe2O4 phases were prepared by conventional solid state sintering process. The phase formation of BaTiO3 and NiFe2O4 phases was investigated via powder X-ray diffraction (XRD) technique using (XRD-D8, Advance, Bruker-AXS). The morphological features of composites were investigated by field emission scanning electron microscopy (FESEM; Hitachi, S-4800 II, Japan). For further study, the samples in the pellet form were cut into square shape having dimensions of 5 (length) × 5 (width) × 0.3 (thickness) mm3. The ME laminates were prepared by sandwiching BaTiO3 layer between NiFe2O4 layers with thickness of 0.125 mm on the top and bottom surfaces using a silver epoxy. NFO/BTO/NFO ME laminates were prepared with different thickness ratios of tm/tp = 0.8, 1.0, and 1.2, where, tm is the thickness of the NFO, and tp is the thickness of the BTO, by keeping the same thickness (tp = 0.3 mm) of BTO layer. Similarlly, the metglas/BTO/metglas laminates were prepared by stacking metglas layers (2605SA1, Metglas Inc., USA) with thickness of 0.025 mm on the top and bottom surfaces of the BaTiO3 using a silver epoxy between magnetic and piezoelectric layers. A ME voltage across the sample, in response to a driven ac field Hac = 4 Oe at a frequency of 1 kHz, was measured by a lock-in amplifier as a function of dc magnetic field, Hdc. The output voltage measurements along the out-of-plane direction (3-axis) were performed for both in-plane (1-axis) and out-of-plane (3-axis) Hdc (Hac) orientations to estimate the transverse (αME31) and longitudinal (αME33) ME voltage coefficients, respectively. Magnetostriction, λij was measured by a strain gauge method. Here λij is the magnetostriction in i-axis for H along j-axis. 2.2. Maintaining the Integrity of the Specifications.
3. Results and Discussion
3.1. Structural Characterization
The XRD patterns of BaTiO3 and NiFe2O4 are shown in Figure 1. The XRD patterns show well defined peaks with no any identified peak. The peaks in the XRD patterns were identified to be characteristics peaks of both BaTiO3 and NiFe2O4 phases and indexed to be tetragonal pervoskite structure (JCPDS card No. 31-0174) and cubic spinel structure (JCPDS no. 44-1485), respectively. The lattice parameters for both the phases have been calculated. The lattice parameters for BaTiO3 and NiFe2O4 were found to be a = 3.97 Å and c = 4.03 Å (c/a = 1.01) and a = 8.35 Å, respectively. The morphology and microstructure of the sintered samples were revealed by scanning electron microscopy. Figure 2 shows the SEM images of BaTiO3 and NiFe2O4 samples. Figure 2(a) and Figure 2(b) show typical SEM images of the sintered BaTiO3 sample. A uniform densly packed grain size distribution ranging from 0.3 to 3 microns was observed, with well-defined crystal grain boundaries. The SEM
Figure 1. XRD patterns of (a) NiFe2O4 and (b) BaTiO3.
Figure 2. FESEM micrographs of BaTiO3 (a) (b) and NiFe2O4 (c) (d) samples.
images of NiFe2O4 indicate (Figure 2(c) and Figure 2(d)) that the sample consists of uniform, spherical grains. The corresponding high-magnified SEM image shows that the spherical grains are well separated, and their sizes are less than 500 nm.
3.2. Ferroelectric Properties
In order to examine the ferroelectric nature of the BaTiO3 (BTO) ceramic layer, ferroelectric P-E hysteresis behavior was measured at room temperature and shown in Figure 3. BTO shows the ferroelectric hysteresis behavior at room temperature. The polarization was not fully saturated possibility due to the occurrence of the electric breakdown at high electric fields. A well defined ferroelectric hysteresis loop was observed with a saturation polarization of Ps = 15.87 μC/cm2 and coercive electric field and very large remanent polarization of about 130 kV/cm and 12 μC/cm2, respectively. The observed values are consistent with that of previously reported values for BaTiO3 and BaTiO3 based ceramics   .
3.3. Magnetoelectric (ME) Measurement
Figure 4 shows the variation of αME31 and αME33 as a function of dc magnetic field, Hdc in response to an ac magnetic field of Hac = 4 Oe for NFO/BTO/NFO laminates having thickness ratio of tm/tp = 0.8. As shown in figure, αME shows typical Hdc dependent, initially increases with increasing Hdc, attends a maximum, and subsequently decreases as Hdc increases further. Moreover, αME31 and αME33 show different maximum magnitudes of 27.4 and 14.73 mV/cm∙Oe at different peak fields of 0.24 k∙Oe and 1.5 k∙Oe, respectively. The maximum value of αME31 is about 2 times larger than that of αME33. The most significant observation, however, is the remarkable remnant αME31 of 10 mV/cm∙Oe at zero bias field (Hdc = 0 Oe). This can be attributed to the remnant magnetostriction of NFO layers at zero field   . As αME31 shows maximum value than αME33, we herein only consider αME31 for further study.
Next, we focused on enhancing the αME31 of the laminates by optimizing the layer thickness ratio between the NFO and BTO layers. Figure 5 presents αME31 curves of the NFO/BTO/NFO laminates with different thickness ratios, tm/tp = 0.8, 1.0 and 1.2. With increasing thickness ratio, the peak value of αME31 first increases and reaches the maximum for tm/tp = 1.0 and further decreases for higher tm/tp. Moreover, the optimum peak field is found to be increased with increasing thickness ratio, this is because to produce the same amount of magnetostriction, the required static magnetic field has to increase as the thickness ratio increases  . The maximum value of αME31 = 37 mV/cm∙Oe is observed for tm/tp = 1.0, which indicates that the interface coupling between the two phases is optimal when their thickness (volume) ratios are same.
Figure 3. Ferroelectric (P vs E hysteresis loop) properties of BaTiO3 ceramic.
Figure 4. Hdc dependence of transverse (αME31) and longitudinal (αME33) ME voltage coefficients at a frequency f = 1 kHz for NFO/BTO/NFO ME laminates.
Although, we achieved large magnetoelectric coupling in the NFO/BTO/NFO laminates with tm/tp = 1.0, the values obtained for αME are still much smaller than that reported for other lead free laminates  . In order to further enhance the αME, we replaced NFO magnetostrictive layers by highly magnetostricitve metglas foils. Next we prepared metlgas/BTO/metglas laminates having different thickness ratio between metglas and BaTiO3 layers. We indeed found that the thickness ratio of tm/tp = 1.0 shows larger value than other thickness ratios consistent with that of NFO/BTO/NFO ME laminates. Therefore in the present study we have only shown the ME data of metglas/BTO/metglas laminates with tm/tp = 1.0 and compared with that of NFO/BTO/NFO. The metglas/BTO/metglas laminates with tm/tp = 1.0 were prepared by stacking six metglas layers (2605SA1, Metglas Inc., USA) with thickness of 0.025 mm on the top and bottom surfaces of the BaTiO3 using a silver epoxy. Figure 6 shows the comparative study of Hdc dependence of αME31 for NFO/BTO/NFO and metglas/BTO/metglas laminates. The αME31 (H) for metglas/BTO/metglas also shows a qualitatively similar behaviour to that of αME31 for NFO/BTO/NFO. However, the peak field for metglas/BTO/metglas is only 0.145 k∙Oe, which is lower than that of NFO/BTO/NFO. Moreover, the peak value of αME31 is 81 mV/cm∙Oe, about 2.3 times higher in magnitude, which can be attributed to the large piezomagnetic coefficient, q11 of metglas than that of NFO. One can expect even much higher values of αME31 for metglas based laminates as q11 of metglas is much larger than that of NFO. However, it is important to note that to achieve the optimized volume fraction, six metglas layers were stacked on the top and bottom surfaces of the BTO layer using a silver epoxy between each metglas layer. Therefore, under the applied magnetic field magnetostriction of the metglas is constrained by the epoxy layers in between them and as a result, q11 decreases and the peak field increases, which in turn reduce the overall αME31
Figure 5. Hdc dependence of αME31 for NFO/BTO/NFO ME laminates with different thickness ratios of tm/tp = 0.8, 1.0 and 1.2.
Figure 6. Hdc dependence of αME31 for NFO/BTO/NFO and metglas/BTO/metglas ME laminates.
Figure 7. Hdc dependence of longitudinal magnetostriction (αME31) for NFO and metglas layers. Inset shows the Hdc dependence of piezomagnetic coefficients, q11 for NFO and metglas.
 . The Hdc dependence of αME can be easily understood from the magnetic-field dependent piezomagnetic coefficient, qij, i.e., dλij/dHj, where λij is the magnetostriction in i-axis for H along j-axis. To confirm this, we measured the H-dependent magnetostriction λij of metglas and NiFe2O4 (Figure 7) and calculated corresponding qij = dλij/dHj (Inset of Figure 7). We indeed find that αME31 (H) shows a qualitatively similar behavior with that of q(H), having similar curvature and same peak fields. The αME31 observed for metglas/BTO/metglas laminate is indeed comparable to other reported lead free laminates  . One can further enhance the αME31 by optimizing the dimensions and/or total thickness of the laminates.
In conclusion, we have successfully synthesized and measured the ME properties of the Pb-free ME laminates. αME31 is found to be larger than αME33. The αME is found to be thickness fraction dependent showing maximum for optimized thickness ration of tm/tp = 1.0 for NFO/BTO/NFO laminates. The αME31 is further enhanced by replacing NFO layers by metglas. The maximum value of 81 mV/cm∙Oe is observed for metglas/BTO/metglas laminates at lower Hdc. The present laminates offer promising opportunities of engineering environmental friendly ME laminate for applications in ME devices such as energy harvesters and magnetic field sensors.