A. H. G. Vlooswijk

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.

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.

Smallest 90° domains in epitaxial ferroelectric films

Periodic ferroelectric-ferroelastic 90° domain patterns with an unprecedented small domain periodicity of 27 nm were observed in thin PbTiO3 films grown on DyScO3 substrates. These patterns contain the narrowest possible a domains 6 nm wide that allow to preserve the lateral coherence in the films, producing highly ordered patterns visible by x-ray diffraction.

Substrate influence on the shape of domains in epitaxial PbTiO3 thin films

Epitaxial PbTiO3 thin films were grown on SrTiO3(001) and DyScO3(110) substrates by pulsed laser deposition. We used high-resolution transmission electron microscopy to investigate the 90° domain structure in the films. They were found to have a predominant fraction of c domains along with a certain minor volume fraction of a domains that is clearly higher in case of the DyScO3 substrates. In PbTiO3 on SrTiO3 the a domains were found to have a wedge shape, whereas in PbTiO3 on SrRuO3∕DyScO3 they have a nearly uniform width. The presence of steps in the domain walls has been observed in the films on both substrates, but the steps are clearly more dominant in the case of SrTiO3 than of SrRuO3∕DyScO3 and are responsible for the observed wedge shape. The observed difference in the films induced by the two substrates is attributed to a higher stiffness of SrTiO3 than of SrRuO3∕DyScO3 as we corroborated with nanoindentation experiments.

Thickness scaling of ferroelastic domains in PbTiO3 films on DyScO3

We report on the thickness dependence of the ferroelastic domains of PbTiO3 films grown on (110)-DyScO3 with low thicknesses (up to 240 nm), which fall outside the validity range of the square root law proposed by Roytburd. For slow-grown films, the data reveal the linear thickness dependence predicted by Pertsev and Zembilgotov, using a complete elastic description, while a 2/3 scaling exponent is found for fast-grown films. Extremely long domains running all through the samples have been observed in the latter case, compared to the short domains observed in slow-grown films. These differences are ascribed to the in-plane anisotropy for domain wall nucleation, which is likely caused by the anisotropic elastic modulus of the substrate.

Growth of flat SrRuO3 (111) thin films suitable as bottom electrodes in heterostructures

Thin film growth of ferroelectric or multiferroic materials on SrTiO3(111) with a buffer electrode has been hampered by the difficulty of growing flat electrodes on this polar orientation. We report on the growth and characterization of SrRuO3 thin films deposited by Pulsed laser deposition on SrTiO3(111). We show that our SrRuO3(111) films are epitaxial and display magnetic bulk-like properties. Films presenting a thickness between 20 and 30 nm are found to be very flat and therefore suitable as bottom electrodes in heterostructures

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.

Evidence of Substrate-Induced Ferroelectric Phase Transition in SrTiO3 by Means of Thermal Measurements

The paper reports the results of the temperature dependence measurements of the specific heat for a SrTiO3 thin (104 nm) film deposited on a DyScO3 substrate. Two anomalies of the specific heat have been observed. They can be associated with the strain-induced ferroelectric phase transition (at 304K) and the shifted antiferrodistortive structural phase transition (at 173 K).

Comment on “Nanometer resolution piezoresponse force microscopy to study deep submicron ferroelectric and ferroelastic domains” [Appl. Phys. Lett. 94, 162903 (2009)]

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