Atomic Mapping of Domain Configurations in Ferroelectric Thin Films

Abstract

Atomic-scale information is of critical importance for understanding intrinsic characteristics of advanced functional materials such as ferroelectrics, magnets, superconductors and catalysts. For example, it is often the small deviations from symmetry in atom positions and the resultant strains that allow ferroelectric oxides, used in computer memory chips, to store charge/information or to resonate with magnets as composite multiferroics. Ferroelectric crystals feature asymmetric or polar structures that are switchable under an external field, holding promise for random access memories, thin-film capacitors and actuators [1].PbTiO 3 is a ferroelectric with a tetragonal structure. We have grown a number of PbTiO 3 /SrTiO 3 multilayered samples on orthorhombic GdScO 3 (110) substrates by pulsed laser deposition (PLD), using a Lambda Physik LPX 305i KrF (λ=248 nm) excimer laser. We observe the as-grown epitaxial PbTiO 3 /SrTiO 3 multilayer films in which the electric dipoles at domain-walls of ferroelectric PbTiO 3 are characterized by means of aberration-corrected scanning transmission electron microscopy (AC-STEM) [2][3][4][5][6]. We also rationalize the experimental observation via phase-field modeling [3].Besides the well-accepted head-to-tail 90º uncharged domain-walls (UCDWs), we have identified not only head-to-head positively charged but also tail-to-tail negatively charged domain-walls (CDWs). The widths, polarization distributions, and strains across these charged domain-walls are mapped quantitatively at atomic scale, where remarkable difference between these domain-walls is presented [4].We observe not only the atomic morphology of the flux-closure quadrant but also a periodic array of flux-closures in ferroelectric PbTiO 3 films, mediated by tensile strain on a GdScO 3 substrate. We directly visualize an alternating array of clockwise and counter-clockwise flux-closures, whose periodicity depends on the PbTiO 3 film thickness. In the vicinity of the core, the strain is sufficient to rupture the lattice, with strain gradients up to 10 9 /m [5]. We also observe a 3D polarization texture of a four-fold flux closure domain. Ferroelectric displacement analysis based on aberration-corrected scanning transmission electron microscopic imaging reveals highly inhomogeneous strains with strain gradient above 10 7 /m. These giant disclination strains significantly broaden the 90° domain walls, while the flexoelectric coupling at 180° domain wall is less affected [3].We find that the coupling of 90° and 180° domain walls with dislocations may induce the formation of 90° charged domain walls and some other novel metastable domain configurations. Such domain patterns are observed to relax with 180° domain walls annihilation and the reversal of polarization directions of a-and c-domain. The configurations of both dislocations and stacking faults may lead to a dramatic change of the domain patterns. Unusual dislocation strain field is identified for a dislocation with the burgers vector 45° apart from a 90° domain wal...