Lye-Hock Ong

Hysteresis loops of ferroelectric bilayers and superlattices

A ferroelectric superlattice with an antiferroelectric interfacial coupling is considered; the same model describes a bilayer with antiferroelectric coupling. By mapping minimum points in the Landau free energy expression and plotting them against the applied electric field, a triple hysteresis loop pattern is obtained. The loop patterns vary between typically ferroelectric and typically antiferroelectric depending on the layer thicknesses and the magnitude of the interfacial-coupling constant. This work suggests the possibility of designing multilayer elements for computer memories with four or more different storage states.

Phase transitions in hydrogen-bonded phenol–amine adducts: analysis by ferroelastic theory

Abstract Phase transitions in a recently discovered class of hydrogen-bonded organic crystals are analyzed by Landau theory. It is shown that the transitions are ferroelastic. Two types of transition, orthorhombic-to-monoclinic and monoclinic-to-triclinic, are analysed using an expansion of the free energy in symmetry-allowed powers of the strain components. For three examples, new data are presented for the crystallographic parameters in a range of temperature including the phase transition point Tc. These data show that the transitions are second order and the temperature dependences are consistent with the mean-field analysis. We propose that the making and breaking of the hydrogen bond interactions with variation of temperature accounts for these reversible ferroelastic phase transitions. Expressions are derived for the entropy and the specific-heat discontinuity at Tc. It is pointed out that the transitions should be accompanied by ‘soft-mode’ behavior in a phonon of appropriate symmetry.

Effects of extrapolation length δ on switching time and coercive field

The Landau free energy expression for a ferroelectric thin film studied by Tilley and Zeks [Solid State Commun. 49, 823 (1984)] (Tilley–Zeks model) and the Landau–Khalatnikov dynamic equation are used to study the surface effects (represented by the extrapolation length ±δ) on properties of polarization reversal, namely, switching time τS and coercive field eC. Positive δ models a decrease in the local polarization at surfaces, and negative δ an increase, with a smaller absolute value of δ giving a stronger surface effect. For positive δ, τS and eC decrease with decreasing ∣δ∣ while for negative δ, τS and eC increase with decreasing ∣δ∣. Strong surface effects, represented by smaller ∣δ∣, are more profound in thin FE films.

Electrostatic coupling and interface intermixing in ferroelectric superlattices

A thermodynamic model is developed to study electrostatic coupling and interface intermixing in superlattices comprising alternate layers of ferroelectrics and paraelectrics. Interface intermixing leads to inhomogeneous internal electric field and polarization in superlattices. The spatial distribution of polarization extends into the layer over a distance governed by its correlation length. Periodic modulation of the internal electric field and polarization in superlattices are correlated. Interface intermixing enhances the depolarization field of superlattices; however, it has a negligible effect on polarization and transition temperature. The internal electric field, originating from the electrostatic coupling between ferroelectric layers, plays a dominant role in determining the properties of superlattices. Copyright c � EPLA, 2012

Tilley-Zeks Model in Switching Phenomena of Ferroelectric Films

We use the Tilley-Zeks model of Landau Devonshire free energy and the Landau Khalatnikov equation of motion to elucidate the phenomena of polarization reversal of second-order ferroelectric films, particularly the characteristics of hysteresis loops by the applied field and the response of bipolar pulses by the films. It is shown that at constant temperature, the size of hysteresis loops increases with increasing film thickness for + δ and the reverse is true for –δ. For a film of fixed thickness, the size of hysteresis loop decreases with increasing temperature for both cases of +δ and –δ. The model has also demonstrated the effects of magnitude and frequency of the applied field on the hysteresis loops are similar to the experimental results.

Theory of far-infrared reflection and transmission by ferroelectric thin films

We present a detailed formulation to describe far-infrared reflection and transmission from a ferroelectric film at normal incidence. The formalism begins with the Gibbs free energy per unit area and the Landau–Khalatnikov equations of motion. To take size and surface effects in the film into account, boundary values and possible spatial variation in polarization are included in the free-energy expression, which leads to the existence of two additional parameters, D and δ. The inclusion of D leads to the presence of another spin-wave type of optical mode in addition to the normal polariton type of waves in the dispersion curves and the appearance of the spin-wave mode fringes in the reflectivity curves. Reflection curves for various values of δ are illustrated, and the results show that the effects of these two parameters are distinctively different in reflectivity. Far-infrared reflectivity measurements are proposed here as a tool to determine surface and size effects in ferroelectric thin films.

Influence of dielectric stiffness, interface, and layer thickness on hysteresis loops of ferroelectric superlattices

We examined the influence of dielectric stiffness, interface, and layer thickness on the hysteresis loops, including the remanent polarization and coercive field of a superlattice comprising alternate layers of ferroelectric and dielectric, using the Landau-Ginzburg theory. An interface energy term is introduced in the free energy functional to describe the formation of interface “dead” layers that are mutually coupled through polarization (or induced-polarization). Our studies reveal that the hysteresis loop is strongly dependent on the stiffness of the dielectric layer, the strength of the interface coupling and layer thickness. The intrinsic coupling at the interface between two neighboring layers reduces the coercive field, though the corresponding remanent polarization is significantly enhanced by a soft dielectric layer.

Thickness dependence of switching time and coercive field in ferroelectric thin films

The switching time and the coercive field in the polarization reversal of ferroelectric (FE) films have been investigated theoretically using the Landau Devonshire free energy expression and the Landau Khalatnikov dynamic equation in this work. Our numerical data show that the switching time is an exponential function of the applied field, and the function implies that there is a definite coercive field in the switching of a FE film. The effects of the thickness and the surface parameter δ on the switching time and the coercive field have been studied, and we found that the coercive field and the switching time could either decrease or increase with decreasing film thickness in film with surface conditions of positive δ or negative δ, respectively. These results are consistent with the switching phenomena reported in experiments regarding the film thickness dependence of the coercive field and the switching time.

Charge compensation phenomena for polarization discontinuities in ferroelectric superlattices

Based on the Landau-Ginzburg theory, the screening charge compensation phenomena for polarization discontinuities at interface in ferroelectric superlattices are investigated. Interface mixing leads to polarization continuity or discontinuity in superlattices. Free charge with equal but of opposite sign for alternate interface, driven by internal electric field, builds up to counteract the depolarization effect in superlattices. The charge density depends on the degree of interface mixing, and it approaches a saturated value when the mismatch of internal field or polarization at interface vanishes. Using PbTiO3/SrTiO3 (PT/ST) as a model system, the charge compensation phenomena for polarization discontinuities in superlattices is investigated by examining the polarization and internal electric field profiles. Dependence of local polarization and internal field at the interfaces on screening charge density is also examined in detail.

First Order Temperature Dependent Phase Transition in a Monoclinic Polymorph Crystal of 1,6-Hexanedioic Acid: An Interpretation Based on the Landau Theory Approach

Crystals of 1,6-hexanedioic acid (I) undergo a temperature-dependent reversible phase transition from monoclinic P21/c at a temperature higher than the critical temperature (Tc) 130 K to another monoclinic P21/c at temperature lower than Tc. The phase transition is of first order, involving a discontinuity and a tripling of the b-axis at Tc whereas the other unit cell parameters vary continuously. The transition is described by the phenomenological Landau theory. The crystal structure analyses for data collected at 297(2) K and 120.0(1) K show that there is half of a molecule of (I) in the asymmetric unit at 297(2) K whereas there are one and a half molecules of (I) in the asymmetric unit at 120.0(1) K. At both temperatures, 297(2) and 120.0(1) K, intermolecular O-H···O hydrogen bonds link the molecules of I into infinite 1D chains along [101] direction. However there are significantly more O-H···O hydrogen bonds presented in the 120.0(1) K polymorph, thereby indicating this phase transition is negotiated via hydrogen bonds. The relationship of the conformational changes and hydrogen bonding for these two polymorphs are explained in detail.