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Register a new userModelling functional properties of ferroelectric oxide thin films with a three-domain structure
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The properties of a ferroelectric, (001)-oriented, thin film clamped to a substrate are investigated analytically and numerically. The emphasis is on the tetragonal, polydomain, ferroelectric phase, using a three domain structure, as is observed experimentally. The previously used, very restrictive set of boundary conditions, arising from the domain walls, is relaxed, creating more modes for energy relaxation. It is argued that this approach gives a more realistic description of the clamped ferroelectric film. It is shown that for the ferroelectric oxides PbZr_(1-x)Ti_xO_3} the tetragonal, polydomain phase is present over a wide range of substrate induced strains for x_Ti>0.5, corresponding to the tetragonal side of the bulk phase diagram. A polydomain, rhombohedral phase is present for x_Ti<0.5, at the bulk rhombohedral side. Phase-temperature diagrams, and ferroelectric, dielectric and piezoelectric properties, as well as lattice parameters, are calculated as function of substrate induced strain and applied field. The analytical formulation allows the decomposition of these properties into three different causes: domain wall motion, field induced elastic effects and piezoelectric effects. It is found that domain wall motion and polarization rotation of the in-plane oriented domains under an applied field contribute most to the properties, while the out-of-plane oriented domains hardly contribute.
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We present a segregrated strain model that describes the thickness-dependent dielectric properties of ferroelectric films. Using a phenomenological Landau approach, we present results for two specific materials, making comparison with experiment and with first-principles calculations whenever possible. We also suggest a smoking gun benchtop probe to test our elastic scenario.
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is presented, which enables the thermodynamic description of dense laminar polydomain states in epitaxial ferroelectric thin films. The theory explicitly takes into account the mechanical substrate effect on the polarizations and lattice strains in dissimilar elastic domains (twins). Numerical calculations are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic substrates. The misfit strain-temperature phase diagrams are developed for these films, showing stability ranges of various possible polydomain and single-domain states. Three types of polarization instabilities are revealed for polydomain epitaxial ferroelectric films, which may lead to the formation of new polydomain states forbidden in bulk crystals. The total dielectric and piezoelectric small-signal responses of polydomain films are calculated, resulting from both the volume and domain-wall contributions. For BaTiO3 films, strong dielectric anomalies are predicted at room temperature near special values of the misfit strain.
We investigated domain kinetics by measuring the polarization switching behaviors of polycrystalline Pb(Zr,Ti)O$_{3}$ films, which are widely used in ferroelectric memory devices. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of domain switching times. We viewed the switching process under an electric field as a motion of the ferroelectric domain through a random medium, and we showed that the local field variation due to dipole defects at domain pinning sites could explain the intriguing distribution.
In purely c-axis oriented PbZr$_{0.2}$Ti$_{0.8}$O$_3$ ferroelectric thin films, a lateral piezoresponse force microscopy signal is observed at the position of 180{deg}domain walls, where the out-of-plane oriented polarization is reversed. Using electric force microscopy measurements we exclude electrostatic effects as the origin of this signal. Moreover, our mechanical simulations of the tip/cantilever system show that the small tilt of the surface at the domain wall below the tip does not satisfactorily explain the observed signal either. We thus attribute this lateral piezoresponse at domain walls to their sideways motion (shear) under the applied electric field. From simple elastic considerations and the conservation of volume of the unit cell, we would expect a similar lateral signal more generally in other ferroelectric materials, and for all types of domain walls in which the out-of-plane component of the polarization is reversed through the domain wall. We show that in BiFeO$_3$ thin films, with 180, 109 and 71{deg}domain walls, this is indeed the case.
Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with SQUID measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.
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