Marios Hadjimichael: Nanoscale domains in metal-ferroelectric-metal heterostructures

During the past years, researchers have observed interesting polarisation textures in nanoscale ferroelectric materials by carefully tailoring their electrostatic and mechanical boundary conditions using various thin film deposition methods. Recent examples of ferroelectric-dielectric superlattices and multilayers have shown that large depolarising fields lead to the formation of complex polarisation textures, from nanoscale stripe domains to bubble-like structures and vortices, which are characterised by the continuous rotation of the ferroelectric polarisation [1-5]. The dynamics of these domains and domain walls can give rise to enhanced macroscopic properties and even negative capacitance behaviour [6].

The nature of the electrostatic boundary conditions changes when metal-ferroelectric heterostructures are considered. The free charges in the metal partially screen the ferroelectric polarisation, yet the finite screening length is still responsible for the appearance of a large depolarising field and therefore for the appearance of ferroelectric domains. While domain formation was often invoked to explain the macroscopic properties of ultrathin ferroelectric capacitors, the evidence for domains in these systems is usually indirect.

In this talk, I will show how we use PbTiO3-SrRuO3 ferroelectric-metal superlattices to characterise the structural and dielectric properties of the ultrathin ferroelectric layers. When the superlattices are subject to tensile strain, we demonstrate the appearance of a stable supercrystal phase comprising a three-dimensional ordering of nanoscale domains with tailored periodicities. A combination of laboratory and synchrotron X-ray diffraction, piezoresponse force microscopy, scanning transmission electron microscopy and phase-field simulations reveals a complex hierarchical superstructure, which consists of alternate vertical and horizontal ferroelectric flux-closure patterns, which form to minimize the elastic and electrostatic energy. Large local deformations of the ferroelectric lattice are accommodated by periodic lattice modulations of the correlated metallic layers, presenting a new paradigm for engineering correlated materials with tailored modulated structural and electronic properties.