Abstract
Two structural phase transitions are investigated in highly strained BiFeO3 thin films as a function of film thickness and temperature via synchrotron x-ray diffraction. Both transition temperatures (upon heating: monoclinic MC to monoclinic MA to tetragonal) decrease as the film becomes thinner. A film-substrate interface layer, evidenced by half-order peaks, contributes to this behavior, but at larger thicknesses (above a few nanometers), the temperature dependence results from electrostatic considerations akin to size effects in ferroelectric phase transitions, but observed here for structural phase transitions within the ferroelectric phase. For ultra-thin films, the tetragonal structure is stable to low temperatures.
Highlights
Two structural phase transitions are investigated in highly strained BiFeO3 thin films as a function of film thickness and temperature via synchrotron x-ray diffraction
The ferroelectric transition temperature (TC, “Curie temperature”) has long been known to decrease with decreasing film thickness under open-circuit conditions.[1,2,3,4,5]. This is well understood as a consequence of the Landau-Lifshitz-Kittel law[1,2,3,6,7,8,9] that describes the need for a decreasing domain size with decreasing film thickness in order to balance the energy costs associated with long-range electrostatic effects with the formation of domain walls
In contrast to electrostatic effects, which come into play at length scales of the same order of magnitude as ferroelectric domain sizes, the consequences of octahedral tilt coherence are typically restricted to just a few unit cells
Summary
Two structural phase transitions are investigated in highly strained BiFeO3 thin films as a function of film thickness and temperature via synchrotron x-ray diffraction. Origin of thickness dependence of structural phase transition temperatures in highly strained BiFeO3 thin films
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