Junaidah Osman

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.

Calculation of Nonlinear Susceptibility Tensor Components in Ferroelectrics

We present the formalism within which all second and third order nonlinear susceptibility coefficients can be calculated from the Landau expansion for an isotropic paraelectric phase by means of the Landau-Khalatnikov dynamical equations. Both first-order and second-order phase transitions are considered. Detailed results are given for all the second-order coefficients in the ferroelectric phase and for third harmonic generation, intensity-dependent refractive index and d.c. Kerr effect in both paraelectric and ferroelectric phases.

First-order phase transitions in ferroelectric films

Abstract We discuss a version of the Landau–Devonshire model for films undergoing a first-order phase transition. The spatial variation P(z) of the order parameter is described by an Euler–Lagrange equation with associated boundary condition. We define a general numerical scheme for finding P(z) and evaluating the resulting thermodynamic functions. Results are presented for the thickness, temperature and boundary-condition dependence of P(z), the free energy and entropy and the superheating, supercooling and thermodynamic critical temperatures.

Calculation of nonlinear-susceptibility tensor components in ferroelectrics: cubic, tetragonal, and rhombohedral symmetries

We present the formalism for the calculation of all second- and third-order nonlinear susceptibility coefficients based on the Landau–Devonshire free-energy expansion for cubic symmetry in the high-temperature paraelectric phase and the Landau–Khalatnikov dynamical equations. Second-order phase transition and single-frequency input waves are considered. Detailed results are given for all nonvanishing tensor elements of the second- and third-order nonlinear optical effects in the paraelectric and the tetragonal and rhombohedral ferroelectric phases.

Calculation of intrinsic hysteresis loops of ferroelectric thin films

A general numerical scheme is applied to calculate the dielectric equation of state (mean polarisation versus applied field) and resulting hysteresis loop for ferroelectric films. The scheme is sufficiently general to apply to films in which the phase transition from the paraelectric state is either first or second order, and possible variation of the polarisation P near the surfaces is included. It is shown that by design of the film thickness and of the variation of P the hysteresis loop in a film of first-order material can be varied between forms which resemble those of the bulk material in various temperature intervals.

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.

Theory of Switching in Bulk First-Order Ferroelectric Materials

A detailed account of switching and related properties of bulk first-order ferroelectric materials is given. The ferroelectric is described by the Landau-Devonshire free energy and all the results are given in terms of dimensionless variables so that they are generally applicable. In the first part of the paper the static properties are discussed. The forms of the dielectric hysteresis loop in various temperature ranges are presented and analytic results are derived for the two coercive fields that are needed for a description of the hysteresis. Switching is described by means of the Landau-Khalatnikov dynamical equation, and analytic expressions for the maximum and minimum switching currents are derived. The equation is solved numerically to find switching characteristics for an applied step field and the hysteretic response to a sinusoidal field.

Calculation of nonlinear magnetic susceptibility tensors for a uniaxial antiferromagnet

In this paper, we present a derivation of the nonlinear susceptibility tensors for a two-sublattice uniaxial antiferromagnet up to the third-order effects within the standard definition by which the rf magnetization m is defined as a power series expansion in the rf fields h with the susceptibility tensors as the coefficients. The starting point is the standard set of torque equations of motion for this problem. A complete set of tensor elements is derived for the case of a single-frequency input wave. Within a circular polarization frame (pnz) expressions are given for the first-order susceptibility, second-harmonic generation, optical rectification, third-harmonic generation and intensity-dependent susceptibility. Some of the coefficients with representative resonance features in the far infrared are illustrated graphically and we conclude with a brief discussion of the implications of the resonance features arising from the calculations and their potential applications.

Surface aided polarization reversal in small ferroelectric particles

Polarization reversal in ferroelectric particles driven by a pulsed electric field is examined theoretically using Landau–Devonshire–Khalatnikov theory. A significant reduction in reversal times is shown to be possible if certain surface properties and size criteria are met. The surface properties are also shown to control the magnitude of the applied field needed for irreversible switching. An interesting signature of surface effects is found in the switching current. The theory predicts that the switching current for small ferroelectric particles can exhibit double peaks as a function of time. The size and relative times of the peaks provide specific information on the magnitude and rate of surface reversal dynamics.

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.