Formation and Piezoelectricity of Self-Assembled PbTiO3-CoFe2O4 Nanostructural Films

Abstract

Main tasks of our research include: (1) exploring optimum growth conditions for PLD deposition of self-assembled nanophase PbTiO3-CoFe2O4 films with different compositions and orientations; (2) analyzing morphologies and nanostructures of the two-phase films to clarify relative effects of elastic energy and interface energy on the self-assembled film formation; (3) investigating stress state and relaxation of stresses arising as a result of a paraelectric-ferroelectric transformation in PbTiO3; (4) exploring ferroelectric state in the confined PbTiO3 nanophase in the films with {110} and {111} orientations. Principal results of the research are: (1). Optimum PLD growth conditions to obtain high quality films with distinct separation of epitaxial PbTiO3 and CoFe2O4 nanophases are found after systematic studies. (2). Nano-facets along {111} plane between PbTiO3 and CoFe2O4 phases are found to be generic in addition to orientation dependent macroscopic interfaces. We have concluded that accounting of interface and surface energies is important for description of nano-faceting of interfaces and the near substrate zone of the films while the two-phase morphologies are determined by the elastic interactions; (3). The investigation of the stress state of the {001} film arising due to paraelectric-ferroelectric transition of PbTiO3 have discovered the polydomain nanostructure of the ferroelectric phase with ~50-60% c-domains. Piezoresponse of PbTiO3 should be reduced dramatically by combined effects of dissolution of Fe in PbTiO3, a domains and constraints. The relative large dzz from previous research must contain large extrinsic contribution due to movement of nano-domain walls. (4). Switching spectroscopy piezoresponse force microscopy (SS-PFM) is used to characterize local piezo- ferroelectric property of confined ferroelectrics in {110} and {111} films with composition of 1/3PbTiO3-2/3CoFe2O4. It is proved that PbTiO3 nano-inclusions exhibit ferroelectricity in both films. 180o domain switching is observed under measurement condition (<10V) for the {110} films but not for the {111} film. Quantitatively, both films yield a piezoresponse of about 15% compared to bulk single crystal PbTiO3. It is a reasonable value of intrinsic piezoeffect taking into account mechanical and electrical constraints (depolarizing field) as well as the effect of Fe dissolution and possible in-plane domains