Theory of phase transitions in ferroelectric superlattices / Lim Kok Geng
Ferroelectric superlattices comprising two or more different layers are currently a topic of active research because of their potential applications in memories and fundamental scientific interest. A thermodynamic model based on the Landau-Ginzburg theory is proposed to study the phase transition...
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| Format: | Thesis |
| Published: |
2015
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| Subjects: | |
| Online Access: | http://studentsrepo.um.edu.my/6551/1/thesis.pdf http://studentsrepo.um.edu.my/6551/ |
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| Summary: | Ferroelectric superlattices comprising two or more different layers are currently a
topic of active research because of their potential applications in memories and fundamental
scientific interest. A thermodynamic model based on the Landau-Ginzburg theory
is proposed to study the phase transitions in ferroelectric superlattices. An interface energy
term is introduced in the free energy to describe the formation of intermixed layer
with properties different from those of both layers. These intermixed layers are mutually
coupled through the local polarization at interfaces.
The effects of electrostatic coupling and interface intermixing on the internal electric
field and polarization of superlattices which composed of alternate layers of ferroelectrics
and paraelectrics are discussed. As an illustration, the model is applied to a superlattice
consisting of a ferroelectric layer as PbTiO3 (PT) and a paraelectric layer as SrTiO3 (ST)
on a ST substrate. Appropriate electrostatic boundary conditions are considered for the
case of superlattice with polarization perpendicular to the surface or interface. The effect
of interface intermixing and modulation period on the internal electric field and polarization
are studied by changing the volume fraction or thickness ratio of the PT/ST superlattice.
In addition, the spatially-varying internal electric field, dielectric susceptibility
and polarization of these ferroelectric superlattices are calculated. Effects of modulation
period and temperature on the internal electric field, dielectric susceptibility and polarization
of these superlattices with inhomogeneous properties are examined. The polarization
reversal in PT/ST superlattices with “switchable” polarization in intermixed layers is also
studied. The dependence of polarization and internal electric field on an applied electric
field is discussed. The polarization and internal electric field profiles at certain applied
electric field are examined. Besides that, the effects of alternating interface charges, �s
of the ferroelectric superlattices are studied by taking into account the intermixing at the
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interfaces between layers. The alternating interface charges enhance the polarization of
ferroelectric superlattices, and lead to the formation of an effective internal electric field
in the superlattices.
Lastly, the thermodynamic model is extended to study the effect of composition
and interface intermixing on ferroelectric properties of BaTiO3=BaxSr1 |
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