Polarization Switching Dynamics Governed by the Thermodynamic Nucleation Process in Ultrathin Ferroelectric Films

Abstract

A long standing problem of domain switching process -how domains nucleate -is examined in ultrathin ferroelectric films. We demonstrate that the large depolarization fields in ultrathin films could significantly lower the nucleation energy barrier (U *) to a level comparable to thermal energy (kBT ), resulting in power-law like polarization decay behaviors. The "Landauer's paradox": U * is thermally insurmountable is not a critical issue in the polarization switching of ultrathin ferroelectric films. We empirically find a universal relation between the polarization decay behavior and U */kB T .PACS numbers: 77.22. Ej, 77.80.Fm Recent advances in complex oxide thin film synthesis and in first-principles calculations have intensified the basic research on ferroelectricity at nanoscale dimension [1,2]. The cooperative nature of ferroelectricity is expected to induce different polarization states at nanoscale due to low-dimensional boundary conditions and the strong interactions of polarization with strain, charge, and other electromechanical parameters [1,2,3,4,5,6,7]. Many workers have reported intriguing physical phenomena occurring in ultrathin ferroelectric (FE) films and other nanostructures, such as intrinsic size effects [1, 2, 3], strain-enhanced FE properties [4], unusual low-dimensional phases [5], and domain patterns [6,7]. On the other hand, the mechanism and domain dynamics of FE switching in ultrathin films have rarely been investigated in spite of their scientific and technological importance [8,9,10].Historically, the mechanism of polarization switching dynamics in ferroelectrics has been the subject of a great deal of research. It is now believed that the polarization switching takes place not by coherent switching, but by the nucleation and growth of new domains [8,9,10]. However, there still remains an unsolved issue: How do the domains nucleate? In the late 1950s, Landauer emphasized that a thermodynamic nucleation process cannot play a role in FE domain switching [11]. For a nucleus with reversed polarization to be created by a thermodynamic process, an energy barrier for nucleation U * should be thermally overcome, as shown in Fig. 1(a). However, Landauer's and later estimates showed that U * is practically insurmountable by the thermal activation process: U * > 10 8 k B T at an electric field E ∼ 1 kV/cm (a typical value of the coercive field for bulk ferroelectrics) [11] and U ∼ 10 3 k B T at E ∼ 100 kV/cm (a typical value of the coercive field for most FE thin films) [10,13], where k B is Plank's constant and T is temperature. According to these estimates, it is difficult to understand the observed domain nucleation in ferroelectrics. This problem has been known as the "Landauer's paradox". To overcome this difficulty, numerous workers have assumed that the nuclei could be formed inhomogeneously due to external effects, such as defects [12], long-range interaction between nuclei [13] and FE-electrode coupling [14]. However, in this Letter, we will demonstrate that U * could be thermally ...

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