Prabahar, K and Ramadurai, Ranjit
(2019)
Growth and Characterization of lead free Multiferroic
Ba0.85Ca0.15Zr0.10Ti0.90O3 –CoFe2O4 Nano Composite Thin Films.
PhD thesis, Indian institute of technology Hyderabad.
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Abstract
Multiferroic materials simultaneously possess more than one ferroic orders. The most important multiferroic materials are those that show simultaneous ferroelectric and ferromagnetic properties which is called magneto-electric materials. Magneto-electric materials manifest a change in polarization under external magnetic field or change in magnetization under an electric field. Magnetoelectricity can be found both in single phase and composite materials. Single phase materials have both electric and magnetic orders and the magnetoelectric effect originate from the coupling of ferroelectric and magnetic dipoles. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetization by an electric field and vice versa. Magneto- electric thin film nano composites have drawn significant interest because of the easy incorporation in to the integrated magnetic and electric devices, such as sensors, microelectromechanical system and spintronics devices which cannot be realized in the bulk counterpart. This dissertation focused on the growth and characterization of lead free multiferroic Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) – CoFe2O4 (CFO) nano composite thin films deposited on Pt substrate through pulsed laser deposition. BCZT thin film was chosen as piezoelectric materials because of recent development in lead-free piezoelectric ceramics in which piezoelectric properties are quite comparable to that of lead based PZT ceramics which is non-environment friendly due the toxicity of lead. Similarly, CFO thin films were preferred because of its high magnetostriction value among the oxide ferromagnetic materials. First, the piezoelectric thin film was systematically optimized for different oxygen pressures and substrate temperatures. The effect of these process parameter on structure, microstructure, ferroelectric and mechanical properties has been studied. The piezoelectric domains and the ferroelectric switching of dipoles with applied electric field were also studied. From the above experiments the optimum growth conditions were realized as O2 Pressure – 13.3 Pa and Substrate temperature 700°C to achieve high quality BCZT thin films. In the same way, the effect of different oxygen pressures on structure, microstructure, magnetic and mechanical properties were studied for CFO film. In addition, magnetic annealing was carried out to increase the magnetostriction and decrease the strain sensitivity in-order to enhance the total magnetoelectric coupling in the nano composites. Enhanced magnetostrictive properties and uniaxial anisotropy has been observed for magnetic annealed film compared to the nonmagnetic annealed and as deposited films. By using the above optimized conditions, multilayers of BCZT/CFO/BCZT nano composites were deposited for different thickness ratios to obtain growth control morphology at the interface since there is a continuous change in the morphology texture and surface topography as a function of film thickness. The interface between the piezoelectric and ferromagnetic layer plays a key role in transferring the stress and to achieve high magnetoelectric coupling. In general, the polycrystalline ferroelectric thin film was grown with columnar grains and spinel ferrite with granular grains. This difference in the morphology at the interface reduces the coupling between the two phases. Here we propose a new structure in the polycrystalline 2-2 multilayered structure which stacks in a single continuous column like a superlattice structure which provides more flexibility for strain transfer and reduces the substrate clamping effect by altering the deposition conditions within the frame work of Structure Zone Model (SZM). Morphologically coherent continuous columnar structure was obtained for the nanocomposite having thickness ratio of BCZT (900nm)/CFO (180nm)/BCZT (900). The grains between the BCZT/CFO and CFO/BCZT interface grew in a grain-over grain pattern with same crystallographic orientation such that the films are in local epitaxy registry over a wide spread of different orientation at the interface. The effect of thickness on the structure, ferroelectric, magnetic and magneto-dielectric properties were studied along with microstructure and Raman studies at the interface. A high dielectric constant change of 21% obtained by an induced applied magnetic field attained in continuous columnar growth indicating the high strain coupling.
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