Domain-Enhanced Interlayer Coupling in Ferroelectric/Paraelectric Superlattices

Abstract

We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength Λ and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.PACS numbers: 77.55.+f, 77.80.Dj, 77.80.Bh In the past decade refinements in deposition techniques have made it possible to fabricate nanoscale size oxide ferroelectric superlattices with the objective to merge and optimize the technological properties of the constitutive materials [1,2,3]. In designing such artificial structures an understanding of the physics of underlying processes is essential to determine whether the resulting characteristics are provided simply by the superposition of the bulk properties of the constituents or whether the interface and finite-size effects play a predominant role.Two competing types of phenomena that arise at the ferroelectric interface can affect the properties of the superlattices. The strain field, generated by the mechanical mismatch between the superlattice layers, influences the polarization orientation and generally increases the ferroelectric transition temperature T c [4]. In contrast, the electric depolarization field, produced by interfacial surface charges is unfavorable to the formation of the ferroelectric phase [5]. In fact, in cubic perovskite-like ferroelectrics the situation can be even more complex due the formation of both 180• ferroelectric [6] and 90• ferroelastic [4,7,8] domains. Although the properties of ferroelectric superlattices can be governed by domain structure, no systematic study of this effect has to our knowledge been performed.In the present paper, we address the question of ferroelectric domain formation in a periodic superlattice structure consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of equal nanometric width 2a f = 2a p . So as to avoid the complications of the effect of 90• ferroelastic domains we assume that the ferroelectric layers have either natural or strain-induced c-oriented uniaxial symmetry. We will show that the domain patterns formed in the different FE layers interact with each other across the PE layers via the spatially inhomogeneous depolarization electric field emerging from the domains of the neighboring FE layers as shown in Fig. 1. This proximity type effect is dependent critically on the thickness of the PE layers. Our interest has also been motivated by a recent experimental study of FE/PE superlattices of KTaO 3 /KNbO 3 [9] in which, as the superlattice wavelength Λ = 2a f + 2a p decreases, t...

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