Keynote Talk  - Thursday, 16 September I 9:10 AM (CEST)

In ferroelectric thin films, the complex interplay between mechanical and electrostatic boundary conditions allows for the formation of a large variety of domain structures with fascinating properties[1]. Heterostructuring and careful tuning of the epitaxial strain allow for precise control of these boundary conditions, leading to the formation of novel domain structures such as polar merons in tensile-strained PbTiO3 films[2] and skyrmions in PbTiO3/SrTiO3 superlattices[3]. The structural coupling across PbTiO3 layers in these systems can also lead to the formation of complex three-dimensionally ordered supercrystal structures, recently observed in tensile-strained PbTiO3/SrTiO3 and PbTiO3/SrRuO3 superlattices[4,5]. Domain structures in ferroelectric systems not only change the properties of the ferroelectric itself, but can also be used to change the properties of other materials through electrostatic and structural coupling[6]. 

Here, we will show how one can use atomic force microscopy to study ferroelectric domains and domain walls in heterostructures of PbTiO3 deposited using off-axis RF magnetron sputtering. Through an overview of our work based on samples grown on SrTiO3 substrates – imposing purely out-of plane polarisation in PbTiO3 thin films, (Pb,Sr)TiO3 solid solutions and PbTiO3/SrTiO3 superlattices, we will see how to control the presence of intrinsic ferroelectric domains as well as their size and their stability. This can be achieved by tuning different parameters such as the electrostatic boundary conditions, the layer thickness, the concentration in the solid solutions and relative layer thickness in the superlattices, or even the deposition conditions. We will then focus on more recent results obtained on DyScO3 substrates, imposing a tensile strain that favours a ferroelastic domain structure, with PbTiO3 adopting both in-plane and out-of-plane polarization orientations. Using a combination of x-ray diffraction (XRD) and atomic force microscopy (AFM), we studied the domain structure in these systems as a function of PbTiO3 layer thickness. We found 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 above a certain critical thickness, the large structural distortions associated with the ferroelastic domains propagate through the top SrRuO3 layer, creating a modulated structure that extends beyond the ferroelectric layer thickness, with signatures observed both in XRD and AFM. Our results shine light on the complexity of ferroelastic domain structures in PbTiO3-based multilayers and their sensitivity to both electrostatic and mechanical boundary conditions. 

 

[1] Catalan et al. Rev. Mod. Phys 2012

[2] Wang et al. Nature Materials 2020

[3] Das et al. Nature 2019

[4] Stoica et al. Nature Materials 2019

[5] Hadjimichael, Li, et al. Nature Materials 2021

[6] Zubko et al. Nature 2016

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