Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

Abstract

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.

Polar-Graded Multiferroic SrMnO3 Thin Films

“…the emergence of a ferroelectric-ferromagnetic state in EuTiO 3 (ref. 14), the occurrence of incipient ferroelectricity in highly strained CaMnO 3 5 phenomenon where strain gradients are inversely proportional to size 19,20,22 . Interestingly, in contrast to piezoelectricity, flexoelectricity is a universal effect due to the relaxed restriction on crystal symmetry and might emerge even in centrosymmetric crystals 17 , where the inversion symmetry is disrupted by the strain gradient.…”

“…However, for certain tensile strain values, point defect formation is also a likely strain-relaxation mechanism; it has previously been predicted that tensile strain lowers the oxygen vacancy formation energy in biaxially strained CaMnO 3 35 . 19 . Here, the oxygen-pressure dependence on the perovskite phase stabilization is the direct evidence that the formation of oxygen vacancies during thin film growth plays a central role in its stability, since perovskite SMO is only stabilized under low oxygen pressure 28,36,37 .…”

“…In particular, strain-engineering of multiferroism has opened a path for the exploration of new phases with improved properties compared to their bulk parents [10][11][12][13][14] Recently, the coupling between strain gradients and polarization, known as flexoelectricity 15 , has been analyzed intensely due to its remarkable contribution to the polarization and permittivity of inhomogeneously strained ferroelectrics [16][17][18][19][20][21] . The most dramatic manifestations of flexoelectricity have been reported in thin films, as it is an inherently size-dependent 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 phenomenon where strain gradients are inversely proportional to size 19,20,22 . Interestingly, in contrast to piezoelectricity, flexoelectricity is a universal effect due to the relaxed restriction on crystal symmetry and might emerge even in centrosymmetric crystals 17 , where the inversion symmetry is disrupted by the strain gradient.…”

Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

“…Lee at al. [37] show that for HoMnO3 thin films, a large strain gradient can stabilise the single domain structure due to the flexoelectric effect. The absence Submitted to of domains for 20 and 40 nm a-TiO2/NGO films may be due to stabilization of single domain structure due to large strain variations and as the strain variation lessens the bi-domain structure arises for 60 nm films.…”

Giant flexoelectric effect through interfacial strain relaxation

“…In reality, however, inhomogeneous strain often occurs in films, where a strain gradient is inevitably present. Catalan et al [33,34] and Lee et al [35] noticed that a large strain gradient could occur in epitaxial thin films. Above a critical thickness of the strained epitaxial thin film, a lattice mismatch between the film and substrate can result in strain relaxation.…”