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. (C) 2011 American Institute of Physics. [doi:10.1063/1.3630016

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. (C) 2000 American Institute of Physics. [S0003-6951(00)03843-2]

Far-field properties of a vortex airy beam

Analytical far-field expressions for the transverse electric mode and transverse electric magnetic mode terms, and the energy flux distributions of vortex Airy beams are derived based on the vector angular spectrum of the beam and the stationary phase method. The physical pictures of vortex Airy beams from the vectorial structure are illustrated and the energy flux distributions are demonstrated in far-field. The influences of the beam parameters, especially the exponential factor, on the energy flux distributions of vortex Airy beams and its transverse electric mode and transverse electric magnetic mode terms are discussed. This work provides a new understanding of the propagation behaviors and applications of a vortex Airy beam

Interface-induced modifications of polarization in nanoscale ferroelectric superlattices

A thermodynamic model to study the effect of interface intermixing on polarization of ferroelectric superlattices composed of a ferroelectric and paraelectric layers is developed. Interface intermixing forms an intermixed layer with property different from its individual layers, leading to inhomogeneous ferroelectric properties in the superlattices. Polarization induces near the interface of paraelectric layers, which extends into the layer over a distance governed by its characteristic length. Dependence of polarization on periodic thickness indicates an interface-induced modification of ferroelectricity in superlattices. Enhancement in polarization of superlattices is shown to be possible, if certain interface property and periodic thickness met

Polarization modulation profiles in ferroelectric superlattices

A Landau-Ginzburg continuum model for describing a periodic superlattice consisting of alternating ferroelectric and paraelectric layers is presented. An interface coupling term is introduced to describe the interaction at the interface between the two constituent layers. Explicit expressions to describe the order parameter profiles were derived and illustrated. Our study reveals that the interface-induced polarizations in the paraelectric layer plays an important role to determine the polarization modulation profile of a ferroelectric superlattice

Tuning of polarization, internal electric field, and hysteresis loop behaviours in BaTiO₃/Ba_xSr_1-xTiO₃ superlattices

A thermodynamic model based on the Landau-Ginzburg theory is developed to study the polarization and hysteresis loop behaviors in ferroelectric superlattices with technologically important BaxSr1-xTiO3 (BST) solid solutions. Our study shows that the polarization, coercive field, and hysteresis loop behaviors can be tailored by changing the thickness ratio and the Ba/Sr content of BST. The study also found that the sign of the internal fields depends sensitively on both thickness ratio and Ba/Sr content of BST. Both results imply that the internal electric field of superlattice can be tuned to yield zero polarizing or depolarizing field via manipulation of thickness ratio and composition. These findings could pave the way to enhance the efficiency of ferroelectric photovoltaic devices by manipulating the internal electric field through thickness and composition

First-principles study on the atomistic corrosion processes of iron

The corrosion of iron presents an important scientific problem and a serious economic issue. It is also one of the most important subjects in materials science because it is basically an electrochemical process and closely related to other topics such as the electrocatalysis of the oxygen reduction reaction. So far, many studies have been conducted to address the corrosion of iron, a very complicated process that occurs when iron is exposed to oxygen and water. An important question is, at which site of the iron surface the corrosion starts and how it results in the final stage of the corrosion. In the present study, as an example of superficial defects, Fe dimers sticking out of Fe(100) surfaces are considered in order to understand the iron corrosion process from first-principles using density functional theory. We found that the Fe dimers spontaneously react with O 2 and H 2 O to form Fe 2 (OH) 4 + 4OH - . Here, it is interesting to note that the Fe dimer plays the role of a water splitting catalyst, because the space above it is always vacant and can accept oxygen molecules many times for reacting with the surrounding water molecules. Then, if the Fe 2 (OH) 4 molecules are detached from the surface, they react with O 2 to form Fe 2 O(OH) 4 without an activation barrier, and, in turn, the Fe 2 O(OH) 4 and H 2 O molecules react to form Fe 2 (OH) 6 complexes with an activation energy of 0.653 eV. If these complexes further dissociate into Fe(OH) 3 molecules, they react with each other to form Fe 2 O 3 ·2H 2 O with an activation energy of 0.377 eV. This work may provide useful information on possible iron corrosion processes by water in the air

Diffuse phase transition and dielectric tunability of Ba(ZryTi1-y)O-3 relaxor ferroelectric ceramics

Barium zirconate titanate Ba(ZryTi1-y)O-3 (BZT, y = 0.2, 0.25, 0.30, 0.35) ceramics have been prepared by a sol-gel process. The temperature dependence of dielectric permittivity of the ceramics has been investigated. The results show that the phase transition temperature T-C (or T-m) is suppressed with increasing Zr content. The degree of diffuseness of the phase transition is more pronounced for higher Zr content, implying the existence of a composition-induced diffuse phase transition of the ceramics with 0.20 less than or equal to y less than or equal to 0.35. For BZT ceramics with y > 0.25, a relaxor ferroelectric characteristic is observed. High turtability and figure of merit are obtained in BZT compositions investigated, which imply that these ceramics are promising materials for tunable capacitor applications. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved