Tailoring the Non-Ising Internal Structure of Ferroelectric Domain Walls

In ferroelectric systems, the polarization is assumed to be strongly coupled to the crystal lattice. Any misalignment of the polarization with respect to the symmetry-allowed directions of the crystal would thus result in a significant increase in energy. Nevertheless, structures with swirling polarization textures such as vortices, flux closure domains, bubbles, and skyrmions have been recently reported in various ferroelectric systems. This unexpected degree of freedom of the polarization opens new perspectives for applications [1]. Furthermore, theoretical studies have established that highly complex internal structures can be expected at domain walls even in the simplest case of uniaxial ferroelectrics, which were hitherto assumed to present a locally centrosymmetric Ising-type configuration. The experimental evidence of the non-Ising character of ferroelectric domain walls, with Néel or chiral Bloch-type internal structure, has further impacted the development of this topic over the past years. Nevertheless, this exploration of the properties of ferroelectric domain walls and the study of their possible integration in advanced technological concepts [2] is still in its infancy stage as several scientific questions remain to be addressed. In particular, the complex interaction of the local polarization with defects, and the impact of electrostatics and strain gradients on the stability of the domain wall type remain to be elucidated.<br/>Due to the nanoscopic character of these structures and the small amplitude of the local polarization, it is challenging to study these systems experimentally with other techniques than ultrahigh-resolution transmission electron microscopy [3]. Our group in Strasbourg has demonstrated that nonlinear optical microscopy with polarization analysis can be used to detect the spectral signature of Néel or Bloch-type domain walls and topological structures such as Bloch lines [4]. We use second-harmonic generation microscopy and polarimetry analysis in conjunction with simulations accounting for the local symmetry lowering to investigate the three-dimensional internal structure of domain walls [5]. In this talk, I will present recent results showing how the internal polar structure changes locally depending on the three-dimensional domain wall morphology. Furthermore, I will address the possibility of modifying the internal structure of ferroelectric domain walls (from Bloch to Néel, or <i>vice versa</i>) in uniaxial ferroelectric crystals and discuss the mechanisms at play in such a process.<br/><br/><b>References</b><br/>[1] J. Seidel, Nature Materials <b>18</b>, 188 (2019); S. Das <i>et al.</i>, <i>APL Materials</i> <b>8</b>, 120902 (2020).<br/>[2] D. Meier, and S. M. Selbach, <i>Nat Rev Mater</i> (2021).<br/>[3] K. Moore et al., <i>APL Materials</i><b> 9</b>, 020703 (2021).<br/>[4] S. Cherifi-Hertel <i>et al.</i>, <i>Nat Commun</i> <b>8, </b>15768 (2017).<br/>[5] S. Cherifi-Hertel <i>et al.</i>, <i>Journal of Applied Physics</i> <b>129</b>, 081101 (2021), Y. Zhang, and S. Cherifi-Hertel, <i>Opt. Mater. Express</i> 11(11), 3736 (2021).