X-ray Photon Correlation Spectroscopy (XPCS) is a powerful technique for the study of nanoscale domain dynamics using synchrotron light. When an X-ray beam with a high degree of spatial coherence is used, the scattering from domains will have a speckle pattern which encodes information about the precise arrangement of domains. We have used this method to study both thermally and electrically driven domain reconfigurations.

Ferroelectric systems can easily get stuck in a non-equilibrium domain configuration during growth, particularly when a sample is quenched relatively rapidly to room temperature after growth. We studied evolution of metastable domain configurations with heating in a of system 20nm BaTiO₃ (BTO) films on top of ultra-thin PTO films on SrTiO₃ (STO) 001 substrates with epitaxial SrRuO₃ bottom electrodes. Dynamics other than continuous motion only appear in the BTO films grown on PTO which was ferroelectric during growth. The interesting dynamics arise because successfully moving toward a more favorable configuration requires a collective rearrangement of multiple domains within the ensemble. In attempting to go from one relatively favorable configuration to another the system may have to go through a number of less favorable configurations. Some rearrangement processes are not one-way; when a system back-tracks towards an earlier configuration it gives rise to the loop structures we see in the two-time correlation plots.

In another study, we report on the use of XPCS to study domain dynamics during electrical switching in three separate PbTiO₃ based superlattice systems, a ferroelectric-dielectric system, PbTiO₃/ SrTiO₃, a ferroelectric-metal system PbTiO₃/SrRuO₃, and a ``hybrid'' system that combines elements of both of the previous two. Meta-stable domain configurations in switching appear both as short-time scale features within each switching cycle and long-time scale correlations over a number of switching cycles.

PbTiO₃ is a material that exhibits a bulk paraelectric-ferroelectric phase transition at a critical temperature Tc of 765 K, with a polarisation that develops along the c-axis mostly due to ionic displacements. Theoretical studies in PbTiO₃ thin films have revealed complex phase diagrams with regions of distinct domain configurations as a function of different parameters. It has been demonstrated that it is possible to control the intrinsic domain pattern in terms of size and shape by tailoring the electrostatic boundary conditions, the film thickness, the deposition temperature, and the epitaxial strain of the substrate.
We study the domain configuration in PbTiO₃ heterostructures, epitaxially grown on (110)o-oriented DyScO₃ substrates, with bottom and top SrRuO₃ electrodes using a combination of atomic force microscopy, laboratory and synchrotron x-ray diffraction and high resolution scanning transmission electron microscopy. We observe that the anisotropic strain imposed by the orthorhombic substrate creates a large asymmetry in the domain configuration, with domain walls macroscopically aligned along one of the two in-plane directions. We show that the periodicity of the domain wall structure as a function of film thickness deviates from the Kittel law. As the ferroelectric film thickness increases, we find that the domain configuration evolves from flux-closure to an a/c-phase, with a larger scale arrangement of domains into superdomains. [APL Mater. 11, 061126 (2023)]
Above a critical value of PbTiO₃ thickness, we observe a modulation in the structure of the top SrRuO₃ electrode, demonstrating the possibility of domain engineering via structural coupling to ferroelastic domains. [APL Mater. 11, 101110 (2023)]
Moreover, piezoresponse force microscopy analysis of similar PbTiO₃ heterostructures without the top SrRuO₃ electrodes have been performed for direct imaging of the local polarisation and further investigation of the domain configuration.

In nanoscale ferroelectrics the polarization pattern is the result of a delicate balance to minimize electrostatic energy costs associated with the depolarization field effects. In recent years, new exotic polar textures featuring curled polarization patterns (vortices, skyrmions, merons, etc…) have been unraveled in ultrathin films, superlattices or nanostructures. The development of topological nanoelectronics on chips that would take advantage of such exotic textures requires their integration on silicon.
Here, we investigate the crystalline structure, defects, and polarization pattern in (BaTiO3/SrTiO3)n superlattices epitaxially grown on silicon substrates with different interlayer thicknesses. The superlattices were synthesized by molecular beam epitaxy on 4 nm SrTiO3-buffered Si (001) substrate and were characterized by X-ray diffraction and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Out-of-plane X-ray diffraction shows that the BaTiO3 and SrTiO3 layers are coherently grown. Synchrotron-based reciprocal space maps evidence the superlattice periodicity in the out-of-plane direction and show satellite peaks to the main diffraction spots in the lateral directions of the superlattices as well, which indicate a periodic polar ordering. From HAADF-STEM images, we extracted profiles of the out-of-plane and in-plane lattice parameters which show that the superlattice is subjected to tensile stress imparted by the silicon substrate upon cooling. The changes in tetragonality throughout the thickness of the superlattices will be discussed. Using a refined peak finding algorithm, we determined the atomic displacements of Ti atoms relative to the center of the unit cell (as defined by the Ba atoms). From the resulting dipole maps, periodic curled polar patterns are observed in the BaTiO3 layers with polarization extending into the SrTiO3 layers. The periodicity is identical to the one determined from the synchrotron X-ray measurements. The strain distribution in the superlattice determined from geometric phase analysis shows a peculiar modulation of the out-of-plane strain distribution within each BaTiO3 layer and an in-plane strain modulation which is commensurate with the curled nanodomains observed in HAADF-STEM. We will discuss the polar domain configuration for different BaTiO3 and SrTiO3 interlayer thicknesses. The evidence of curled polarization nanodomains in (BaTiO3/SrTiO3)n gives promises for a future integration of topological polar nanodomains into devices.

ABO₃ perovskites are a popular playground for both fundamental and applied research. They show a rich variety of structural, magnetic and orbitally ordered phase transitions. The properties of perovskites can be further manipulated by interfacing them with one another. As a thin film deposited epitaxially on a substrate, the properties are altered due to lattice parameter mismatch resulting in biaxial strain, mismatch in structural or magnetic modes at the interface, quantum confinement due to film thickness, polar discontinuities due to formal or ferroelectric polarisations of either material, and more, depending on the choice of materials. We present a preliminary investigation into the interface between two prototypical perovskites SrTiO₃ and RENiO₃ (RE corresponding to rare-earth), from first principles simulations based on density functional theory. Both materials exhibit important phase transitions with decreasing temperature, and polar phases have been observed under biaxial strain (SrTiO₃) and octahedral tilt control (NdNiO₃). Our first principles simulations allow us to not only find the ground states of SrTiO₃ / RENIO₃ heterostructures, but investigate interfacial effects individually. This allows us to disentangle the rich selection of phenomena that can occur in this system. We present preliminary results and highlight key differences with the current understanding of these two materials.