Flexoelectric rotation of polarization in ferroelectric thin films

• Published in: Nature materials Vol. 10; no. 12; pp. 963 - 967
• Main Authors: Catalan, G., Lubk, A., Vlooswijk, A. H. G., Snoeck, E., Magen, C., Janssens, A., Rispens, G., Rijnders, G., Blank, D. H. A., Noheda, B.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.10.2011; Nature Publishing Group
• Subjects: 639/301/119/544; 639/301/119/996; Biomaterials; Chemistry and Materials Science; Condensed Matter Physics; Crystal structure; Electric properties; Ferroelectric materials; Ferroelectricity; Ferroelectrics; Horizontal; Materials Science; Mechanical properties; Nanotechnology; Optical and Electronic Materials; Phase boundaries; Polarization; Strain; Strain distribution; Substrates; Thin films; X-ray diffraction
• ISSN: 1476-1122; 1476-4660
• DOI: 10.1038/nmat3141

Strain engineering enables modification of the properties of thin films using the stress from the substrates on which they are grown. Strain may be relaxed, however, and this can also modify the properties thanks to the coupling between strain gradient and polarization known as flexoelectricity. Here we have studied the strain distribution inside epitaxial films of the archetypal ferroelectric PbTiO 3 , where the mismatch with the substrate is relaxed through the formation of domains (twins). Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal an intricate strain distribution, with gradients in both the vertical and, unexpectedly, the horizontal direction. These gradients generate a horizontal flexoelectricity that forces the spontaneous polarization to rotate away from the normal. Polar rotations are a characteristic of compositionally engineered morphotropic phase boundary ferroelectrics with high piezoelectricity; flexoelectricity provides an alternative route for generating such rotations in standard ferroelectrics using purely physical means. The close relationship between crystal structure and electric polarization in ferroelectrics means that strain strongly influences their properties. The demonstration of how strain gradients leading to a higher-order effect, flexoelectricity, can be used to rotate electric polarization in thin films indicates new ways of controlling piezoelectricity by purely mechanical means.