Ferroelectric materials are analogous to many ways with ferromagnetic materials - both exhibit similar features such as spontaneous order parsmeter (polarization vs. magnetization), sensitivity to external stimuli such as applied bias or stress, complex domain structures and finally strain effects under an applied external fields. One of the chief applications for ferroelectrics is as actuators and sensors, where large electromechanical coefficients and high dielectric permittivites are desired as essential characteristics. It has been established that in the case of a traditional PbTiO₃ derived materials systems, these electromechanical coupling properties are high at the so-called morphotropic phase boundary (MPB). Typically, one can achieve the MPB through comprehensive mapping of composition with suitable dopants. In this project we mimic MPB properties, by deposition of alternating ferroelectric and antiferroelectric thin films over a suitable substrate. We find that such “heterolayering” creates unique mosaic patterns that yield to interesting electromechanical effects.

Figure: (Left) Vertical piezoresponse force image of the heterolayered PZT thin films (Right) corresponding lateral piezoresponse force image

Currently we are investigating the novel adaptive nanodomain structure (Fig.1) in heterolayered Pb(Zr,Ti)O₃ (PZT) thin films consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ on Pt/Ti/SiO₂/Si substrates. By constraining the top tetragonal (T) PbZr_0.3Ti_0.7O₃ (only 70 nm thick) with an underlying rhombohedral (R) PbZr_0.7Ti_0.3O₃ layer we create a complex ferroelastic domain hierarchy that is conventionally observed only in large grained bulk ceramics. These nanoscale microdomains resemble an adaptive ferroelectric state that is easily susceptible to external perturbation such as applied local DC bias. Piezoresponse Force Microscopy images demonstrate gross reconstruction of the domain orientations under local bias. This process creates giant piezoelectric properties confirmed by d33 measurements, where the heterolayered system shows values around 220 pm/V, which is nearly 300% of a constrained monodomain thin film. The observations are supported by a thermodynamic model, which predicts that a strong coupling between self-strains of the R and T phase should lead to such fine-twinned adaptive domain morphologies which acts similar to a morphotropic phase boundary.

Acknowledgements This work at UNSW is supported by UNSW Faculty Research Grant 2006-2007 and ARC DP 0666231. V. Anbusathaiah also acknowledges the University International Postgraduate Award (UIPA-Faculty).