The electric coupling between surface ions and bulk ferroelectricity gives rise to a continuum of mixed states in ferroelectric thin films, exquisitely sensitive to temperature and external factors, such as applied voltage and oxygen pressure. Here we develop the comprehensive analytical description of these coupled ferroelectric and ionic ("ferroionic") states by combining the Ginzburg-Landau-Devonshire description of the ferroelectric properties of the film with Langmuir adsorption model for the electrochemical reaction at the film surface. We explore the thermodynamic and kinetic characteristics of the ferroionic states as a function of temperature, film thickness, and external electric potential. These studies provide a new insight into mesoscopic properties of ferroelectric thin films, whose surface is exposed to chemical environment as screening charges supplier.
Nonlinear electrostatic interaction between the surface ions of electrochemical nature and ferroelectric dipoles gives rise to the coupled ferroionic states in nanoscale ferroelectrics. Here, we investigated the role of the surface ions formation energy value on the polarization states and polarization reversal mechanisms, domain structure and corresponding phase diagrams of ferroelectric thin films. Using 3D finite elements modeling we analyze the distribution and hysteresis loops of ferroelectric polarization and ionic charge, and dynamics of the domain states.These calculations performed over large parameter space delineate the regions of single-and poly-domain ferroelectric, ferroionic, antiferroionic and non-ferroelectric states as a function of surface ions formation energy, film thickness, applied voltage and temperature. We further map the analytical theory for 1D system onto effective Landau-Ginzburg free energy and establish the correspondence between the 3D numerical and 1D analytical results. This approach allows performing the overview of the phase diagrams of ferroionic systems and exploring the specific of switching and domain evolution phenomena.Recently, we modified the SH approach allowing for the presence of the gap between the ferroelectric surface covered by ions and the SPM tip [34,35,36,37], and developed the analytic description for thermodynamics and kinetics of these systems. The analysis [34 − 36] leads to the elucidation of the ferroionic states, which are the result of nonlinear electrostatic interaction between the surface ions with the charge density obeyed Langmuir adsorption isotherm and single-domain ferroelectric polarization. The properties of ferroionic states were described by the system of coupled equation.Here, we study the stability of ferroionic states with respect to the domain structure formation and polarization reversal scenarios in thin ferroelectric films covered by ions. Our results, presented below, reveal unusual dependences of the film polar state and domain structure properties on the ion formation energies and their difference, and, even more, unexpected, on the applied voltage. That say one can expect to be faced with the electric field-induced phase transitions into ferroionic state in thin films covered with ion layer of electrochemically active nature.The manuscript is structured as following. Basic equations with boundary conditions are given in section II. Numerical results presented in section III demonstrate the effect of surface ions formation energy on stable polarization states, evolution of domain structure and surface charge during polarization reversal. To get insight into numerical results we presented simplified analytical modeling of ferro-ionic system behavior based on the free energy with renormalized coefficients in section IV. Distinctive features of polarization reversal in thin ferroelectric films covered by ions are discussed in section V.Electrostatic problem and derivation of the free energy with renormalized coefficients are given i...
The contrast formation mechanism in nanoscale Infrared (IR) Spectroscopy is analyzed. The temperature distribution and elastic displacement across the illuminated T-shape boundary between two materials with different IR-radiation absorption coefficients and thermo-physical and elastic properties located on a rigid substrate are calculated self-consistently for different frequencies f ∼ (1 kHz–1 MHz) of IR-radiation modulation (fully coupled problem). Analytical expressions for the temperature and displacement profiles across the “thermo-elastic step” are derived in the decoupling approximation for f = 0 (“static limit”), and conditions for approximation validity at low frequencies of IR-modulation are established. The step height was found to be thickness-independent for thick layers and proportional to the square of the thickness for very thin films. The theoretical results will be of potential interest for applications in the scanning thermo-ionic and thermal infrared microscopies for relatively long sample thermalization times and possibly for photothermal induced resonance microscopy using optomechanical probes.
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