Piezo-response in two-dimensional α-Tellurene films
Amey ApteSummayya KouserFarnaz Safi SamghabadiLong ChangLucas M. SassiDmitri LitvinovBoris I. YakobsonAnand B. PuthirathPulickel M. Ajayan
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Electrostriction
Ferrimagnetism
Magnetoelectric effect
Magnetism
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BiFeO3 simultaneously shows antiferromagnetic and ferroelectric order with high transition temperatures, i.e. T N ∼ 370°C and T C ∼ 830°C, respectively. Naturally, it has been inferred that coupling exists between the magnetic and ferroelectric order parameters like in the multiferroic manganites with low transition temperatures. A thorough investigation of the ferroelectric properties of BiFeO3 is therefore in line with the understanding of its multiferroic behaviour. Here, we review the ferroelectric properties of epitaxial (001) oriented BiFeO3 films grown by different techniques on several substrates. Structural characterization along with ferroelectric quantitative analysis point at the high quality of the films. Emphasis is put on identifying the various polarization variants and domain dynamics under an applied bias. In these studies, to unravel the intricate ferroelectric domain structure, piezo-force microscopy scans have been taken along the principal crystallographic directions. Two cases have been analysed. First, a 600 nm thick film grown on SrTiO3 (001) with a thin SrRuO3 underlayer exhibits a mosaic domain pattern due to the presence of both up and down polarization domains. Mainly four polarization domains have been identified in this case, which correspond to two structural domains. Second, epitaxial BiFeO3 films grown on DyScO3 (110) and miscut SrTiO3 (001) with a thin SrRuO3 underlayer show stripe patterns, with mainly two down polarization domains. A single structural domain of orthorhombic SrRuO3 epitaxial underlayer induces this changes in the domain structure of BiFeO3. The suppression of up domains by changing the substrate conditions prove the possibility of ferroelectric domain engineering. The three possible polarization switching mechanisms, namely 71 and 109° rotations, as well as 180° rotation, have been identified by following the domain dynamics in a two-domain epitaxial BiFeO3 film. Interestingly, 180° polarization reversal seems to be the most favorable switching mechanism in epitaxial films under an applied bias along [001]. The observation of both ferroelastic and ferroelectric switching processes open exciting possibilities for the optimization of BiFeO3's ferroelectric properties and investigation of magnetoelectric coupling in epitaxial films. A recent photoemission study using linearly polarized X-rays proved the coupling between the ferroelectric and antiferromagnetic domain structures.
Orthorhombic crystal system
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Electrostrictive and piezoelectric properties for a 1.3 μm thick film of a vinylidenefluoride/trifluoroethylene copolymer exhibiting a ferroelectric-to-paraelectric phase transition have been investigated as a function of temperature by means of electromechanical interferometry. The electrostriction remarkably increases in the vicinity of the phase transition temperature according to a sharp increase in the dielectric constant. For the unpoled sample in the ferroelectric phase with a polydomain structure in which the local spontaneous polarization is macroscopically cancelled out, the dependence of the electrostriction on the square of the applied electric field is nonlinear in the ferroelectric phase, while it is linear in the paraelectric phase. A theoretical model which takes into account the nonlinear dielectric constant can quantitatively explain the nonlinear contribution to the electrostriction in the vicinity of the ferroelectric–paraelectric phase transition, but it underestimates the contribution in the ferroelectric phase. Interactions within and/or between the ferroelectric domains in the polydomain structure are expected to contribute significantly to the nonlinear electrostriction. For poled samples a pronounced inverse-piezoelectric effect is measured. The achieved polarization and its temperature dependence are almost the same as for thicker films reported in earlier studies, obtained by different experimental techniques.
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The electrostriction effect of ferroelectric vinylidene fluoride (VDF) and trifluoroethylene copolymers was studied by measuring their dimensional variation under an external electric field changing beyond a coercive electric field. The relationship between strain and polarization for the copolymer which includes a 54 mol% VDF reveals a parabolic electrostriction relationship over the entire measurement range of the strain-polarization relationship. The relationship for the copolymer which includes a 73 mol% VDF, however, deviates from the parabolic relationship. The result is discussed in terms of the thermodynamics of a ferroelectric phase transition. Furthermore, the origin of the ferroelectricity of these copolymers is discussed.
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Ferroelectric Polymers
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Visualizing ferroelectric domains Multiferroic materials support intertwined ferromagnetic and ferroelectric orders, with the magnetic field capable of controlling the electric order and vice versa. Matsubara et al. used second harmonic generation microscopy to visualize what happens to the ferroelectric domains in the multiferroic TbMnO 3 when an externally applied magnetic field changes the direction of electric polarization by 90°. Unexpectedly, the domain walls, initially parallel to the polarization vector, did not change their shape or position. The resulting transition from neutral to charged domain walls may help in the development of future ferroelectric devices. Science , this issue p. 1112
Magnetoelectric effect
Piezoresponse force microscopy
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Perovskite ferroelectric (FE) materials have attracted considerable attention for a wide range of applications, such as dynamic random access memories (DRAM), microwave tunable phase shifters and second harmonic generators (SHGs). [1–3] Moreover, materials that have coupled electric, magnetic, and structural order parameters that result in simultaneous ferroelectricity, ferromagnetism, and ferroelasticity are known as multiferroics. [4–6] These multiferroics materials have attracted a lot of attention in recent years because they can potentially offer a whole range of new applications, including nonvolatile ferroelectric memories, novel multiple state memories, and devices based on magnetoeletric effects. Although there are some reports on the electrical and magnetic properties of perovskite-type ferroelectric and multiferroics materials, optical properties and electronic transitions have not been well investigated up to now. On the other hand, phase transition is one of the important characteristics for the ferroelectric/multiferroics system. As we know, the phase transition is strongly related to the structural variation, which certainly can result in the electronic band modifications. Therefore, one can study the phase transition of the above material systems by the corresponding spectral response behavior at different temperatures.
Ferroics
Ferroelasticity
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In this paper, we discuss the electrostriction enhancement mechanisms for ferroelectric polymers, including field enhancement mechanism in ferroelectric-dielectric composites, polarization enhancement mechanism and exchange coupling in ferroelectric nanocomposites, and percolation mechanism in ferroelectric-conductor composites.
Electrostriction
Ferroelectric Polymers
Percolation (cognitive psychology)
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Even a century after the discovery of ferroelectricity, the quest for the novel multifunctionalities in ferroelectric and multiferroics continues unbounded. Vertically aligned nanocomposites (VANs) offer a new avenue toward improved (multi)functionality, both for fundamental understanding and for real-world applications. In these systems, vertical strain effects, interfaces, and defects serve as key driving forces to tune properties in very positive ways. In this Perspective, the twists and turns in the development of ferroelectric/multiferroics oxide–oxide and unconventional metal–oxide VANs are highlighted. In addition, the future trends and challenges to improve classic ferroelectric/multiferroic VANs are presented, with emphasis on the enhanced functionalities offered by existing VANs, as well as those in emerging systems.
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We report here on the preservation of ferroelectricity down to 2 nm in BiFeO3 ultrathin films. The electric polarization can be switched reversibly and is stable over several days. Our findings bring insight on the fundamental problem of ferroelectricity at low thickness and confirm the potential of BiFeO3 as a lead-free ferroelectric and multiferroic material for nanoscale devices.
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