Ferromagnetic-like closure domains in ferroelectric ultrathin films

TL;DR

This study uses first-principles simulations to reveal the existence and stabilization mechanisms of ferromagnetic-like closure domains in ultrathin ferroelectric BaTiO3 films, highlighting atomic relaxations and defect pinning effects.

Contribution

It demonstrates the stabilization of closure domains in ultrathin ferroelectric films and elucidates atomic relaxation and defect pinning mechanisms involved.

Findings

Closure domains are stable down to two unit cells.

In-plane atomic relaxation stabilizes the domains.

Charged defect pinning sites are identified at interfaces.

Abstract

We simulate from first-principles the energetic, structural, and electronic properties of ferroelectric domains in ultrathin capacitors made of a few unit cells of BaTiO$_3$ between two metallic SrRuO$_3$ electrodes in short circuit. The domains are stabilized down to two unit cells, adopting the form of a domain of closure, common in ferromagnets but only recently detected experimentally in ferroelectric thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, that behaves more like SrO in highly polarizable SrTiO$_3$ than in metallic SrRuO$_3$. Even if small, these lateral displacements are essential to stabilize the domains, and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in films fatigue.