Gijsbert Rispens

Flexoelectric rotation of polarization in ferroelectric thin films

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.

Polar domains in lead titanate films under tensile strain

Thin films of PbTiO3, a classical ferroelectric, have been grown under tensile strain on single-crystal substrates of DyScO3. The films, of only 5 nm thickness, grow fully coherent with the substrate, as evidenced by synchrotron x-ray diffraction. A mapping of the reciprocal space reveals intensity modulations (satellites) due to regularly spaced polar domains in which the polarization appears rotated away from the substrate normal, characterizing a low-symmetry phase not observed in the bulk material. This could have important practical implications since these phases are known to be responsible for ultrahigh piezoelectric responses in complex systems.

Giant tunnel electroresistance with PbTiO3 ferroelectric tunnel barriers

The persistency of ferroelectricity in ultrathin films allows their use as tunnel barriers. Ferroelectric tunnel junctions are used to explore the tunneling electroresistance effect—a change in the electrical resistance associated with polarization reversal in the ferroelectric barrier layer—resulting from the interplay between ferroelectricity and quantum-mechanical tunneling. Here, we use piezoresponse force microscopy and conductive-tip atomic force microscopy at room temperature to demonstrate the resistive readout of the polarization state through its influence on the tunnel current in PbTiO3 ultrathin ferroelectric films. The tunnel electroresistance reaches values of 50 000% through a 3.6 nm PbTiO3 film.

Fine tuning epitaxial strain in ferroelectrics: PbxSr1−xTiO3 on DyScO3

Epitaxial strain can be efficiently used to modify the properties of ferroelectric thin films. From the experimental viewpoint, the challenge is to fine-tune the magnitude of the strain. We illustrate here how, by using a suitable combination of composition and substrate, the magnitude of the epitaxial strain can be controlled in a continuous manner. The phase diagram of PbxSr1−xTiO3 films grown epitaxially on (110)-DyScO3 is calculated using a Devonshire–Landau approach. A boundary between in-plane and out-of-plane oriented ferroelectric phases is predicted to take place at x≈0.8. A series of PbxSr1−xTiO3 epitaxial films grown by molecular beam epitaxy shows good agreement with the proposed phase diagram.

X-Ray Diffraction of Ferroelectric Nanodomains in PbTiO3 Thin Films

ABSTRACT X-ray diffraction constitutes a powerful technique with which to characterise ferroelectric domains. Here we describe the principles of ferroelectric nanodomain diffraction and present some results for PbTiO3 thin films grown under tensile strain on two different substrates, with thicknesses below and above the critical thickness for strain relaxation. The combination of conventional and grazing incidence diffraction and the analysis of the scattering between Bragg peaks allowed the identification of a new polar symmetry in ultra-thin films with only anti-parallel 180° domains. Thick films showed tetragonal 90° ferroelectric/ferroelastic domains instead, with a depressed TC and a domain periodicity largely independent of temperature.

ULTRA-THIN LEAD TITANATE FILMS GROWN BY MOLECULAR BEAM EPITAXY

ABSTRACT The growth of atomically-flat thin films of ferroelectric PbTiO3 on SrTiO3 substrates, using molecular beam epitaxy, is reported. The main issue in the growth of these materials is the high volatility of lead. This can be largely overcome by using PbO, instead of Pb, as a source and by using atomic oxygen during growth. The continuous decrease of the out-of-plane lattice parameter with increasing temperature in the investigated range (RT-650?C), indicates that the films are still ferroelectric at the growth temperature (Tg = 600?C), as expected according to the theoretical prediction of TC = 765?C (compared to Tbulk C = 490?C) for the present mismatch strain values.