Fyzikální ústav Akademie věd ČR

The seminal paper by Zhirnov (1958 Zh. Eksp. Teor. Fiz. 35 1175–80) explained why the structure of domain walls in ferroelectrics and ferromagnets is so different. We have recently realized that the antiparallel ferroelectric walls in rhombohedral ferroelectric BaTiO₃ can be switched between the Ising-like state (typical for ferroelectrics) and a Bloch-like state (unusual for ferroelectric walls but typical for magnetic ones) [see Fig. 1] by a compressive epitaxial stress. Phase-field simulations using a Ginzburg–Landau–Devonshire model allows to explore this strain-induced phase transition within the domain wall in detail [Fig. 2]. Strain-tunable chiral properties of ferroelectric Bloch walls promise a range of novel phenomena in epitaxial ferroelectric thin films. Results are published in JPCM [V. Stepkova et al., J. Phys.: Condens. Matter 24, 212201 (2012)].

Figure 1: Variation of polarization P within Bloch- and Ising-type domain walls. The Bloch-type structure (a,b) conserves the magnitude of the local vectorial order parameter. This is typically encountered in ferromagnetism. In contrast, ferroelectric materials usually have large anisotropy and the magnitude of the polarization can be more easily adjusted. Therefore, antiparallel walls in ferroelectrics are known to correspond to the achiral Ising solution (c). We show here that in rhombohedral BaTiO₃, both types of 180° domain walls can be stabilized.

Figure 2: Chiral order parameter. The extremal value of the Pt polarization component in the core of the domain wall is plotted as a function of the epitaxial stress. This quantity can be taken as an order parameter of this chiral phase transition within the domain wall. For the wall under study, the phase transition is continuous and it can be associated with the Landau free-energy sketched in the figure.