POLING OF THE SAMPLE
Before carrying out magneto-electric measurements, samples have to be poled both electrically and magnetically. For the electrical poling of ceramic pellets, contact poling method is efficient to align electric dipoles inside the material. A dc electric field of 1-3 kV/mm is used in this work to pole the prepared samples. Poling voltage is applied at a temperature of 150°C for half an hour, followed by cooling at the same voltage. Electric poling of this material is difficult due to the low resistance ferrite phase present in the composite. Ferrite phase provides a low resistive leakage path for the charged particles through the sample. In order to get effective poling in magneto-electric composites, samples should be optimized first. Results on studies on different particulate composites support low ferrite content of about 20% as the optimum composition [24]. The problems due to charge leakage and distraction of the sample could be avoided by limiting the current through it with a series resistance. Before carrying out the MD and magneto-electric measurements, the samples are not only poled electrically but also magnetically by a dc magnetic field of field strength IT for 30 minutes [25].
STRUCTURAL CHARACTERIZATION OF (Na0.5 К0.5)0.94 Li0.06 NbO3 - MFe2O4 BY X-RAY DIFFRACTION (XRD)
X-ray diffraction (XRD) is the most effective analytical technique used to identify the crystallographic structure and phases. When X-rays are applied to the powder samples, the diffraction intensity is assumed to be the sum of X-rays reflected from all the fine grains and the peaks are attributed to the Miller indices of the sample [26]. Lead-free magnetoelectric NCs of sodium potassium lithium niobate-nickel/cobalt ferrite (xNKLN-(l-x)MFO), with different molar weight percentages, have been characterized using XRD. Figure 11.4 shows the XRD spectra of the composites prepared. Panalytical X’Pert PRO high-resolution XRD with incident Cu-Ka radiation of A=1.54 A° is used for the structural characterizations of the composite. The XRD of both phases are compared with the results reported in literature and the spectra match well with them [21, 22]. In order to identify the (// к 1) planes, Pawley method has been used. The NKLN perovskite belongs to the space group symmetry Cm2m. The lattice parameters obtained are a = 5.637 A0, b = 5.669 A0 and c = 3.945 A°. Ferrite phase has face-centered cubic spinel structure. Spectrum can also be verified using standard ICSD file. The symmetry group of ferrites is identified as Fd3m. In the XRD spectra of composites, the peaks of individual phases are clearly visible and do not contain any additional peaks. This reveals that there is no chemical reaction between individual phases of the composite.
FIGURE 11.4 X-ray powder diffraction spectrum of (Nao;K0S)o94 Li006 Nb0j-MFe,04 (a) M is nickel (b) M is cobalt.
FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR) OF (Na0.5 К0.5)0.94 Li0.06 NbO3 - MFe2O4
Fourier transform infrared (FTIR) spectroscopy is generally used for chemical analysis. They are also useful in identifying typical molecular structures. Figure 11.5 shows the recorded FTIR spectra of the composite samples in the wavenumber range 4000 cm-1 to 400 cm-1. Magnetoelectric composite is a combination of different types of metal oxides. So the FTIR spectra will contain vibrational peaks of metal oxides. It is reported that in ferrites, the stretching vibration of the tetrahedral metal- oxygen bond in the range 600-550 cm-1 and octahedral metal-oxygen bond in the range 450-385 cm-1 provide confirmation for the formation of such bonds [27]. Tetrahedral metal-oxygen bond vibration found in our measurement range confirms the formation of metal oxides, which are clearly visible in Figure 11.5.
MAGNETIC CHARACTERIZATION BY VIBRATION SAMPLE MAGNETOMETRY
M-H hysteresis measurement can be used to study the magnetic properties of composites. Vibration Sample Magnetometer (VSM) is generally used to plot the magnetic hysteresis. A magneto-electric composite will perform efficiently only when the electric and magnetic properties are well maintained. The magnetic properties of such composites are due to the presence of magnetic ferrite phase. The individual ferrite grains act as the centers of magnetization and the saturation magnetization of the composites is the vector sum of all these individual contributions. The magnetic content increases with ferrite content and result in increase of net magnetization. But, the sample has to optimize the composition by taking care of the cross-linked chains of magnetic structures. This is because these low resistive ferrite chains will lead to electrical leakage paths that inhibit domain growth while poling, resulting in low piezoelectric and magnetoelectric properties. On considering magnetic properties, the ferroelectric materials incorporated into the ferrite phase acts as pores in the presence of applied magnetic field and break the magnetic circuit. This will result in the decrease of these magnetic parameters with increasing ferroelectric concentration. The magnetic behavior of ferrites depends on the number of parameters like cation distribution, site preference energies, covalence of bonds, and the molecular field [20, 28, 29]. But in a composite system, interfacial effects are also expected play a role on the magnetic interactions due to change in the distribution of magnetic ions and spin orientations [30]. Figure 11.6 is the magnetic hysteresis curves of the composites prepared, which is the conformation of magnetism in the composite material. Thin hysteresis curve of nickel ferrites based composites indicates soft magnetic behavior compared to cobalt ferrite composites.
FIGURE 11.5 FT-IR spectra of (Na0SK05)094 Li006 NbOj - MFe,04 composite showing metal-oxygen bond in which M is (a) nickel (b) cobalt.
FIGURE 11.6 M-H hysteresis of (Na05K05)094 Li0M1 Nb03-MFe,04 composite showing metal-oxygen bond in which M is (a) nickel (b) cobalt.
