We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

Methods are derived for measuring local strain, stress, and crystallographic texture (orientation) in polycrystalline samples when 1–10 grains are simultaneously illuminated by an energy scanable or broad-bandpass x-ray beam. The orientation and unit-cell shape for each illuminated grain can be determined from the diffracted directions of four Bragg reflections. The unit-cell volume is determined by measuring the energy (wavelength) of one reflection. The methods derived include an algorithm for simultaneously indexing the reflections from overlapping crystal Laue patterns and for determining the average strain and stress tensor of each grain. This approach allows measurements of the local strain and stress tensors which are impractical with traditional techniques.

To investigate the critical thickness of ferroelectric BaTiO3 (BTO) films, we fabricated fully strained SrRuO3∕BTO∕SrRuO3 heterostructures on SrTiO3 substrates by pulsed laser deposition with in situ reflection high-energy electron diffraction. We varied the BTO layer thickness from 3to30nm. By fabricating 10×10μm2 capacitors, we could observe polarization versus electric-field hysteresis loops, which demonstrate the existence of ferroelectricity in BTO layers thicker than 5nm. This observation provides an experimental upper bound of 5nm for the critical thickness. The BTO thickness-dependent scaling of the remanent polarization agrees with the predictions of recent first-principle simulations [J. Junquera and P. Ghosez, Nature 422, 506 (2003)].

Flexoelectricity can play an important role in the reversal of the self-polarization direction in epitaxial BiFeO3 thin films. The flexoelectric and interfacial effects compete with each other to determine the self-polarization state. In Region I, the self-polarization is downward because the interfacial effect is more dominant than the flexoelectric effect. In Region II, the self-polarization is upward, because the flexoelectric effect becomes more dominant than the interfacial effect.

SrTiO 3 is a well-known incipient ferroelectric (FE) material.1 As quantum fluctuation and antiferrodistortion compete with FE instability, SrTiO 3 remains in paraelectric phase even at low temperatures. Many researchers have used the strain engineering concept based on a lattice mismatch with different substrates to improve the FE properties.2,3 We recently successfully fabricated strontium titanate (STO) thin films that exhibited room-temperature ferroelectricity. 4 We introduced vacancy defects in the STO films by adjusting the growth conditions. Although we attributed the occurrence of ferroelectricity to the defects in the STO films, 4 it was unclear what types of defect play important roles in this ferroelectricity.In this paper, we describe the investigation of possible defects and their roles in FE STO films. From optical spectroscopy, we found that the occurrence of ferroelectricity in STO films has a strong correlation with the appearance of an absorption peak at around 1.3 eV. By performing first-principles calculations, we investigated changes in the electronic structure due to several complexes of Sr and O vacancies. We found that the Sr-O-O vacancy complex is the most probable candidate for defect dipoles that provide a localized state in the band gap, resulting in the optical absorption around 1.3 eV and ferroelectricity.For experimental investigation of the electronic structures of FE STO films, we grew 100-nm-thick STO films on both sides of polished SrTiO 3 (001) substrates for optical measurements using the same deposition conditions