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Article Dans Une Revue Nature Année : 2020

Inverse transition of labyrinthine domain patterns in ferroelectric thin films

Résumé

Phase separation is a cooperative process whose kinetics underpins the orderly morphogenesis of domain patterns on mesoscopic scales 1,2. Systems of highly degenerate frozen states may exhibit the rare and counter-intuitive inverse symmetry breaking phenomenon 3. Already hypothesized a century ago 4 , inverse transitions have been found experimentally in very disparate materials , ranging from polymeric and colloidal compounds to high-T c superconductors, proteins, ul-trathin magnetic films, liquid crystals and metallic alloys 5,6 , with the notable exception of ferro-electric oxides, despite the widespread theoretical and experimental work on the latter. Here we show that subsequent to a subcritical quench, the non-equilibrium self-assembly of ferroelectric domains in ultrathin films of Pb(Zr 0.4 Ti 0.6)O 3 (PZT) results in a maze, or labyrinthine pattern, featuring meandering stripe domains. Furthermore, upon increasing temperature, this highly degenerate labyrinthine phase undergoes an inverse transition whereby it transforms into the less-symmetric parallel stripe domain structure, before the onset of paraelectricity at higher temperatures. We find that this phase sequence is ascribed to enhanced entropic contribution of domain walls, and that domain straightening and coarsening is predominantly driven by the relaxation and diffusion of topological defects. The computational modeling and the experimental observation of the inverse dipolar transition in BiFeO 3 (BFO) suggest the universality of the phenomenon in ferroelectric oxides. The multitude of self-patterned states and the various topological defects they embody may be put at use to leap beyond current domain and domain-wall-based 7 technologies by enabling fundamentally new design principles and topologically enhanced func-tionalities within ferroelectric films. To investigate polarization self-patterning, we use an ab-initio based effective Hamiltonian approach 35 and examine ultrathin films of Pb(Zr 0.4 Ti 0.6)O 3 (see Methods section), for these widely used quasi-two dimensional ferroelectric systems are already known to exhibit various modulated phases depending on the interplay between strain and the amount of screening of surface charges 8,9,34-36,43. It is worthwhile noting that two underlying nested symmetry-breaking processes are at play in these systems and involve two distinct dynamical length scales. While compressive strain introduces crystalline anisotropy and favors dipoles with orientation perpendicular to the film-plane 1,35 (cubic symmetry is reduced to a quasi-Z 2 symmetry), the depolarizing field arising from incomplete screening of surface charges essentially imposes zero net polarization, and favors instead the formation of multiple mesoscopic domains as a result of the spontaneous breaking of the residual discrete symmetry. These domains of opposite polarization alternate along in-plane directions, and consist each of ferroelectrically ordered ensembles of dipoles. More precisely, while an individual dipole retains the freedom to flip between [001] and [001] out-of-plane directions, an individual domain, as an emergent mesoscopic degree of freedom, has the propensity to align along either [100] (horizontal) or [010] (vertical) in-plane tetragonal directions, due to the underlying square lattice geometry 1. Naturally, the dynamics pertaining to the motion and relaxation of domains is slower than that of individual dipole fluctuations, and this very fact poses important questions as to what extent domain dynamics and their morphology will be kinetically constrained. One manifestation of this kinetic constraint resides in the possibility of obtaining two distinct modulated phases at low temperatures depending on the rate of cooling. While the well-known parallel stripe domain pattern (Fig. 1a) emerges as the ground state upon adiabatically cooling (annealing) the system 34,35 , the labyrinthine domain polarization pattern (Fig. 1b) onsets upon abruptly cooling (sub-critical quenching) the system. The latter pattern consists of convoluted stripes and meandering domains and has a very close internal energy that is only 0.6% higher than that of the ground state. Interest
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Dates et versions

hal-02483177 , version 1 (18-02-2020)

Identifiants

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Y. Nahas, S. Prokhorenko, J. Fischer, B. Xu, C. Carretero, et al.. Inverse transition of labyrinthine domain patterns in ferroelectric thin films. Nature, 2020, ⟨10.1038/s41586-019-1845-4⟩. ⟨hal-02483177⟩
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