Magnetoelectric Effect in Lead-Free Multiferroic Composites and Thin Films
The functional behavior of two-phase composites consisting of piezoelectric and magnetostrictive components is a manifestation of the coupled nature of their constituent phases, known as magnetoelectric (ME) effect. It has a promise in it to a progress in the modern electronic industry by not only improving the existing properties but also reducing the size of microelectronic components. However, the demand for miniaturization has not seized because it is now accompanied by the demand of environment-friendly materials. Therefore, the primary aims of this dissertation are to develop new lead-free multiferroic materials and to study the relationship between the intrinsic properties of piezoelectric and magnetostrictive components and the resulting ME coupling between them. The ferroelectric component with excellent piezoelectric properties combined with the component with excellent magnetostrictive properties would lead to enhanced ME coupling and value-added understanding of the science behind it. At first, a piezoelectric composition in the system of (1-x)(Ba,Ca)TiO3 -xBa(Zr,Ti)O3 with x = 0.5 was identified (BCT50-BZT) with piezoelectric coefficient of value greater than 500 pC/N in order to utilize it as the piezoelectric phase. The converse ME coefficient of 0.5(Ba,Ca)TiO3 - 0.5Ba(Zr,Ti)O3-15% CoFe2O4 was found to be around 6 ps/m. It was discovered that the constituent electromechanical coupling factor and magnetomechanical coupling factor might play a role on the ME response. To understand the role of magnetostrictive phase, it was modified with a non-magnetic Al3+ ion. The composite of 0.5(Ba,Ca)TiO3 - 0.5Ba(Zr,Ti)O3 - 50% Co(Fe1.5Al0.5O4 was found to possess 3-times larger converse ME coefficient compared to its unmodified counterpart. This enhancement was attributed to the improved effective piezomagnetic coefficient d/dH and increased initial permeability of the Al3+ modified CFO. Secondly, using the knowledge from the BCZT-CFO composites, NiFe2O4 (NFO) was introduced as the magnetostrictive phase which has initial magnetic permeability of 39 that is much larger than that of CFO. This provided an enhancement of around 400 % in the ME coupling in BCZT-30%NFO (converse ME coefficient of 45 ps/m) sample as compared with that of BCZT-50%CFO(Al) (11 ps/m). Hence it gave a substantial evidence that the most important properties to be considered to improve the ME response of two-phase composites are the initial magnetic permeability of the ferrite phase and the maximal elastic compliance of the piezoelectric phase. This discovery has a potential to open new horizons for tailoring of new ME materials. Thirdly, a two-phase system consisting of relaxor ferroelectric Ba(Ti, Sn)O3 combined with CFO was studied. This system is a unique example of two-phase systems in which the dielectric properties were almost entirely retained even in the composite, which is in contrast to the case in other non-relaxor-magnetostrictive composites reported here. The ME coupling of the composite consisting of Ba(Ti0.7Sn0.3)O3-20%CFO was much better than the normal ferroelectric-magnetostrictive composites studied for their magnetodielectric properties, previously. The correlation of the maximum of elastic compliance versus temperature and the induced ME versus temperature curves was highlighted, providing another evidence of the vital role electromechanical softness of the ferroelectric material on the stress/strain mediated ME coupling. Lastly, when the connectivity was changed from (0-3) to (2-2) which is a bilayer geometry, improved local direct ME coupling was observed, due to the better interface between the magnetostrictive phase and the ferroelectric phase. Bilayered thin films of CFO and BaTiO3 (BTO) were synthesized with pulsed laser deposition on two differently oriented substrates. TEM analyses revealed that the CFO layer had a better interface when it was grown on (111) oriented substrate in comparison to that of (100) oriented substrate. This is a discovery as there are no reports in literature on the growth bilayered thin films on (111) oriented substrate. Direct ME coupling studied with PFM under an applied dc magnetic field. The bilayered thin film on (111) oriented substrate has an improved ME coupling in comparison to that of bilayered thin film grown on the (100) oriented substrate.
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