Functional properties of antiferroelectric (Pb,La)(Zr,Sn,Ti)O3 thin films and antiferroelectric/ferromagnetic multilayers

Abstract

Antiferroelectric materials possess multiple functionalities arising from possibility to induce a phase transformation between antiferroelectric and ferroelectric phases by application of electric field or mechanical stress. The objectives of this investigation are to develop high quality (Pb0.97La0.02)(Zr1-x-ySnxTiy)O3 antiferroelectric thin films and investigate their functional properties and suitability for device applications.

A series of process optimization was firstly done to prepare high quality single phase polycrystalline antiferroelectric (Pb0.97La0.02)(Zr1-x-ySnxTiy)O3 thin films on Si substrates by a chemical solution deposition process. Our optimized antiferroelectric thin films exhibited a large energy storage density up to 14 J/cm3 that is promising for application in electrical energy storage devices. The film with composition (Pb0.97,La0.02)(Zr0.90,Sn0.05,Ti0.05)O3, which exhibited the largest strain (0.49%), was then used to fabricate electromechanical cantilevers through bulk micro-machining process on silicon wafers. The antiferroelectric cantilevers showed the distinct digital actuation characteristics with the strain generated due to the antiferroelectric-ferroelectric phase transformation. The maximum displacement per unit voltage around the phase switching field reached 16.7 μm/V, significantly larger than the typical piezoelectric cantilevers. Moreover, the antiferroelectric PLZST cantilevers exhibited superior strain-fatigue resistance compared to similar piezoelectric microstructures. The results showed the suitability of antiferroelectric materials for micro electromechanical systems. The magnetoelectric coupling between ferromagnetic Ni and antiferroelectric (Pb0.97,La0.02)(Zr0.90,Sn0.05,Ti0.05)O3 thin films was then studied in multilayer membranes fabricated through a bulk micro-machining process. A novel AC-mode magneto-optical Kerr effect (MOKE) technique was developed to examine the magnetoelectric coupling of the membrane. The MOKE system was adapted to measure the oscillation of the magnetization vector of Ni due to the strain induced by an AC electric field applied to the PLZST layer of the sample at each applied steady magnetic field. The antiferroelectric to ferroelectric phase transformation of PLZST could induce a rotation of magnetization of about 0.5º in Ni driven by strain induced anisotropy of about 0.5 kJ/m3. Finally, we found that a ferroelastic strain in a magnetic shape memory (MSM) alloy Ni-Mn-Ga thin film deposited on top of the antiferroelectric layer could change crystal structure of antiferroelectric (Pb0.97,La0.02)(Zr0.90,Sn0.05,Ti0.05)O3 thin film, despite the existence of the substrate constraint. The ferroelastic strain in the Ni-Mn-Ga film resulted in antiferroelectric to ferroelectric phase transformation in the PLZST layer underneath. This finding indicates a different strategy to modulate the structure and function for multilayer thin films, and to create unprecedented devices with ferroic thin films. These unique properties in the antiferroelectric thin films, arising from the antiferroelectric to ferroelectric phase transformation, make them potential candidates for application in micro-electromechanical systems, high power energy delivery devices and multiferroic composite systems.