Magneto-electric coupling in multiferroic nanocomposites of the type x (Na0.5K0.5)0.94Li0.06NbO3- (1−x) CoFe2O4: Role of ferrite phase
Introduction
Magneto-electric (M-E) materials are smart possessing magnetic field induced electric polarization and the opposite of electric field induced magnetization. They have the unique property of controllability in magnetization with electric field and vice versa. This makes them promising ones in the field of miniaturization of devices like sensors, spintronic memory devices, phase shifters etc. The story of magneto-electric materials began in 1894 at the time when Curie predicted that an asymmetric body can polarize directionally in the presence of a magnetic field [1]. On the basis of origin and structure, magneto-electric materials can be classified into single phase and composite materials. The conditions for occurrence of magneto-electric behavior in a natural single phase material are the presence of ferroelectric type ionic movements, magnetic interaction path ways, usually of super exchange type, and the fulfillment of symmetry conditions [2]. Unfortunately single phase magneto-electric materials are very rare and their room temperature magneto-electric coefficient is too low for any practical application. Necessity of new materials of this type gave birth to artificial composite materials. Composites have greater design flexibility and multi-functionality at room temperature. They are multiphase materials with one phase piezoelectric and the other magneto-strictive. Individual phases of the composite need not be magneto-electric in nature; magneto-electric property is the product of inherent properties of combining different phases. Magneto-electric output depends on the strength of interaction between the subsystems [3]. Hence, in order to get large magneto-electric coefficient, the mechanical coupling of the phases should be optimum. The magneto-electric property in a composite is usually strain mediated because in indirect magneto-electric effect, the polarization induced on the piezoelectric phase is by the strain developed on the magneto-strictive phase on applying an external magnetic field. Different connectivity schemes are used to prepare composites; the common schemes being 0–3 particulate, 2-2 laminate, 1–3 fiber matrix etc [4]. Of late a great deal of effort is underway in different research laboratories to develop composites with high magneto-electric coupling. A special interest is there for developing such composites that are free of elemental lead, owing to concerns for environment.
In this work we investigate the preparation of 0–3 lead-free ferrite-perovskite particulate composites with good magneto-electric property. Many lead based systems with good magneto-electric properties have been reported, because the performance of leaded piezoelectrics is magnificent. But development of environment friendly alternatives to leaded piezoelectric materials is a challenging problem to the scientific community. In this work we have chosen (Na0.5K0.5)0.94Li0.06NbO3 (NKLN), which has comparatively good piezoelectric property, with reported value of d33 coefficient of 235 pC/N [5], [6]. Among magnetic materials ferrites exhibit unique physical, electrical and magnetic properties. The ferrite phase selected in this work is Cobalt ferrite (CFO), which belongs to an important class of ferrites with good magneto-striction properties. Cobalt ferrites are hard magnetic materials with comparatively high coercivity and saturation magnetization [7]. The value of magneto-striction coefficient of Cobalt ferrite, reported in literature is −167 ppm [8].
The magnetic nanoparticles of CFO have been synthesized following a simple chemical co-precipitation route and the piezoelectric phase by physical process. Normal sintering method is used to prepare composites. Sintering route has the benefit of flexibility for combining the phases of widely different crystal structures, and controllability on grain growth and other physical parameters [9]. More details of sample preparation are given in the next section of this paper.
In this work ferrite particles are embedded in the piezoelectric matrix to synthesize the M-E composites. According to theory, increased ferrite content leads to higher magneto-electric coefficient. But, at the same time, the conducting or semiconducting ferrites could deteriorate the insulation property of the composite [10]. This may cause charge leakage and depreciate the alignment of dipoles and growth of domains during electrical poling. So, as a compromise, we prepared only four different compositions of the samples with the general formula, x NKLN-(1−x) CFO with molar weight fractions x=0.7, 0.75, 0.8, and 0.85 and conducted systematic studies on these. The samples attained a density close to 90% of theoretical density for these four compositions. The physical characterizations of the samples are done by powder X-ray diffractometry (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Transmission electron microscopy (TEM). Dielectric and magnetic properties of the samples are measured, discussed and reported. Alignments of electric and magnetic dipoles are accomplished by electric and magnetic poling of the samples before magneto-electric characterization.
The M-E properties obtained for x NKLN-(1−x) CFO have been compared with those obtained for the corresponding x NKLN-(1−x) NFO samples. It is found that NFO based samples have better M-E properties compared to CFO based samples in every respect. This is attributed to the magnetic softness of nickel compared to cobalt.
Section snippets
Experimental method
Cobalt ferrite nanoparticles were synthesized by wet chemical co-precipitation route. Aqueous solutions of 0.4 M Ferric chloride and 0.2 M Cobalt chloride were mixed and stirred well. Freshly prepared NaOH solution was added drop wise to the resultant salt mixture under vigorous stirring to obtain a pH of 11. Few drops of Oleic acid were then added as surfactant and stirring continued keeping the temperature at about 90 °C. In co-precipitation synthesis, the size and distribution of the
Results and discussion
Fig. 1 shows the XRD spectra of x NKLN–(1−x) CFO composites. The diffraction peaks of different phases of the composites can be clearly indexed. In the Fig., x represents the NKLN phase and y represents the CFO phase. The individual phases are identified by standard ICSD files. The symmetry groups and lattice parameters of NKLN and CFO are Cm2m with a=5.637 A°, b =5.669 A°, c=3.945 A° and Fd3m with a =8.377 A° respectively. No extra phase is seen in the XRD spectra of the composites. From the
Conclusions
We prepared x NKLN – (1−x) CFO composites with four different molar concentrations. The characterizations were done using XRD and FTIR. The dielectric response of the samples with frequency, temperature and magnetic field was studied. The resonance/ relaxation peaks gave the evidence for piezoelectric evolution in poled samples. These peaks were also observed in the AC conductivity curves due to the small polaron conduction. The magneto-electric properties of the samples were measured and the
Acknowledgements
Work supported by Department of Science and Technology, New Delhi under the Nanomission scheme (SR/NM/NS-30/2010). Authors thank Dr. P.A. Joy (Materials Chemistry Division, NCL, Pune) for help with magnetic measurements and Dr. Nandakumar Kalarikkal (School of Pure and Applied Physics, MG University, Kottayam) for M-E measurements. We also thank SAIF, Cochin University of Science and Technology for XRD, TEM and FTIR characterization.
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