At lower temperatures (≈255 K [1] ), however, the original high-symmetry para electric-orthorhombic state is restored. Symmetry associated with this re-entrant phase transition has unusually, therefore, increased on cooling. Some observations show that this generates a local dip in the heat capacity, [1,2] stalling entropy reduction on decreasing temperature. [1] Strange symmetry transformations also occur in flux-grown barium titanate crystals, where highly ordered "Forsbergh Patterns" can first appear and then subsequently disappear, as temperature is monotonically varied. [3,4] Most recently, heating has been seen to cause high symmetry labyrinthine ferroelectric domain patterns to give way to lower symmetry stripe arrays: an effect classified as an "inverse transition". [5] Clearly, symmetry changes can therefore occasionally occur in the opposite sense to that normally seen. While fundamental thermodynamic laws are not broken, such cases are unusual, arresting, and worthy of note. [6]
the R3c space group [6] with polarization along the 〈111〉 c pseudocubic directions. [7] When grown on substrates with large inplane compressive strain (≈<-4.5%), [8] very thin BFO films can adopt a monoclinic unit cell that is approximately tetragonal (T) [8] with polarization along the 〈001〉 c pseudocubic axes. For thicknesses above ≈60 nm, the T-like phase becomes less favorable and there is a relaxation of the unit cell toward a more rhombohedral configuration. [8] This results in a mixed crystallographic phase microstructure, with alternating T-and R-like regions, as revealed by X-ray diffraction [9,10] and scanning transmission electron microscopy (STEM). [10] The strain-driven phase competition gives rise to an effective morphotropic phase boundary (MPB), similar to those commonly observed in solid solutions such as PbZr x Ti 1−x O 3 . In MPB materials, the different structural phases are separated by small energetic barriers, making them susceptible to phase transitions when subjected to external stimuli such as temperature, [11] electric field, [2] and mechanical stress. [12,13] Intriguingly, it is possible to deterministically and reversibly alter the phase population in BFO by application of electric field and nanoscale stress, [2,9,[14][15][16][17][18] thereby opening up a new route to further tune the hysteretic response during switching. However, challenges still remain in understanding the precise nature of the interplay of structural transitions and stress-mediated ferroelectric switching and the overall effect of this meshing of phenomena on the functional properties of the film. Such understanding could be pivotal to the development of future technological applications, such as pressure-sensors, photoresistive, [19] or magnetoelectronic devices, [1] exploiting the enhanced properties that can accompany the mixed phase microstructure and its tuning through different stimuli.Detailed studies of the effects of electric field on mixed phase BFO have previously been performed using scanning probe microscopy, revealing both R → T and T → R phase transitions. [2,3,9] The advent of band excitation piezoresponse force spectroscopy (BEPS) [20] has enabled the collection of nanoscale piezoelectric hysteresis loops [20] and detailed study of the accompanying phase transitions. [21,22] While electric field is relatively simple to exert on the nanoscale via an atomic force microscope (AFM) tip, it has been challenging to achieve controlled stress-induced phase transitions in a typical AFM setup.Epitaxially strained BiFeO 3 thin films with coexisting tetragonal-and rhombohedral-like phases exhibit a range of intriguing functional properties, often strongly related to the unique microstructure of the film. Here enhancements in electromechanical response are reported during simultaneous nanoscale application of electric field and localized stress. These enhancements manifest in the form of peaks, or humps, in the piezoresponse hysteresis loops obtained under a select polarity of applied electric field, correspond...
Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [L. Zhang et al., Science, 361, 494 (2018)], in which the switchable out-of-plane polarization was seen to be almost twice that of any previously known ferroelectric. To understand more about this system
Conducting domain walls (DWs) in ferroelectrics is an emerging research focus in nano-electronics [1,2]. Previously overlooked, these walls have recently been reported to possess diverse functional characteristics that are completely different from the domains that they delineate [3][4][5]. They can have their own distinct chemistry and magnetic behavior [6], and in turn represent a completely new sheet phase. The characteristics of these confined regions are believed to have the same exotic functional behaviours as seen in 2D materials such as graphene, opening up a plethora of possible electronic applications. In addition, the walls have the unique property of being 'agile'; they can be created or destroyed and even be controllably moved by an external field. However, this is an area of research at its very early stages, with a great deal of the fundamental physics still unknown. Since the region of interest (the domain wall) is atomically thin and dynamic, it is essential for the physical characterization to be at this scale and be time-resolved.
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