Phase Transitions in Nanoscale Ferroelectric Structures
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S.K. Streiffer and D.D. Fong Abstract Over decades of effort, investigations of the intrinsic phase transition behavior of nanoscale ferroelectric structures have been greatly complicated by materials processing variations and by the common and uncontrolled occurrence of spacecharge, which interacts directly with the polarization and can obscure fundamental behavior. These challenges have largely been overcome, and great progress in understanding the details of this class of phase transitions has been made, largely based on advances in the growth of high-quality, epitaxial ferroelectric films and in the theory and simulation of ferroelectricity. Here we will discuss recent progress in understanding the ferroelectric phase transition in a particular class of model systems: nanoscale perovskite thin-film heterostructures. The outlook for ferroelectric technology based on these results is promising, and extensions to laterally confined nanostructures will be described.
Origin of Nanoscale “Thin-Film” Effects
Introduction Ferroelectric materials display a wide range of useful properties and are employed in a great variety of applications, including nonvolatile memories, microelectromechanical systems, and infrared detectors.1–3 Ferroelectric properties are traditionally described as resulting from two different origins, usually termed intrinsic and extrinsic. Intrinsic contributions to behavior derive directly from the temperature, electromagnetic field, and electromechanical dependencies of the ferroelectric polarization.4 Extrinsic contributions arise from defects in the ferroelectric, including domain walls that participate in ferroelectric switching and the motion of which are a major contributor to bulk dielectric and piezoelectric response.5–11 Both intrinsic and extrinsic behaviors are modified when a ferroelectric system is reduced to nanoscale dimensions, either through direct creation of a ferroelectric nanostructure, such as a nanoscale epitaxial thin film, or through naturally occurring processes that give rise to nanoscale heterogeneity, as exemplified by the 832
structures is only now developing, despite decades of experimental and theoretical effort.22–24 While intrinsic effects have been effectively studied in superconducting and magnetic systems, similar investigations of ferroelectrics have been hindered by uncontrolled strain and charge effects that interact strongly with the polarization. Intrinsic contributions to ferroelectric properties ultimately relate back to the manner in which the polarization, the order parameter for a proper ferroelectric phase transition, evolves with temperature, and thus to the details of that ferroelectric phase transition. Because of the confounding issues described previously, the origins of differences in the paraelectric-to-ferroelectric phase transition in nanostructured ferroelectrics relative to their bulk counterparts and, consequently, of the properties that relate directly to the phase transition have been difficult to elucidate. This elucidation has argua
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