Abstract:Integration of ferroelectric materials into novel technological applications requires low coercive field materials, and consequently, design strategies to reduce the ferroelectric switching barriers. In this first principles study, we show that biaxial strain, which has a strong effect on the ferroelectric ground states, can also be used to tune the switching barrier of hybrid improper ferroelectric Ruddlesden–Popper oxides. We identify the region of the strain-tolerance factor phase diagram where this intrins… Show more
“…3a ), suggesting that the spontaneous distortion, and therefore polarization of BFO membranes is smaller, and the switching-energy barrier between adjacent polarization states should be correspondingly reduced 43 . Notably, the 100-nm-thick BFO-clamped film has an in-plane lattice constant ( a = 3.95 Å) close to its bulk value ( a = 3.96 Å) 44 , indicating that the clamped film is nearly completely relaxed and that the effects from misfit strain (−1.35% when grown on STO) 45 are minimized at this thickness. Fig.…”
Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO3) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO3 films, to large (10’s of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.
“…3a ), suggesting that the spontaneous distortion, and therefore polarization of BFO membranes is smaller, and the switching-energy barrier between adjacent polarization states should be correspondingly reduced 43 . Notably, the 100-nm-thick BFO-clamped film has an in-plane lattice constant ( a = 3.95 Å) close to its bulk value ( a = 3.96 Å) 44 , indicating that the clamped film is nearly completely relaxed and that the effects from misfit strain (−1.35% when grown on STO) 45 are minimized at this thickness. Fig.…”
Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO3) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO3 films, to large (10’s of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.
“…3c, we show the strain -doping phase diagram of Sr 3 Sn 2 O 7 , calculated by fixing the in-plane lattice parameters and relaxing the out-of-plane one to simulate the boundary conditions on a thin film lattice matched to a substrate. Insulating, undoped Sr 3 Sn 2 O 7 is known to undergo a transition to a non-polar phase above ∼ ∓2% biaxial strain [55] like many other compounds [56]. Fig.…”
mentioning
confidence: 98%
“…3c shows that not only doping enhances polarization at fixed volume, but it also stabilizes the polar phase at wider strain ranges. The polar/non-polar transition induced by epitaxial strain is driven by the disappearance of one of two rotation modes in the polar phase [55]. The free electrons increase the stability of both rotation modes which make this phase transition occur at a higher strain value.…”
Doping ferroelectrics with carriers is often detrimental to polarization. This makes the design and discovery of metals that undergo a ferroelectric-like transition challenging. In this letter, we show from first principles that the oxygen octahedral rotations in perovskites are often enhanced by electron doping, and this can be used as a means to strengthen the structural polarization in certain hybrid-improper ferroelectrics -compounds in which the polarization is not stabilized by the long range Coulomb interactions but is instead induced by a trilinear coupling to octahedral rotations. We use this design strategy to predict a cation ordered Ruddlesden-Popper compound that can be driven into a metallic ferroelectric-like phase via electrolyte gating.
“…The main disadvantage is related to the magnitude of the polarization that, being a secondary order parameter, is usually small with values of few μC/cm −2 [8,[18][19][20]. Moreover, the switching mecha-nism might involve a quite complex energy landscape since, in order to switch the polarization, only one of the two (or more) antipolar distortions needs to change sign leaving the others invariant [15,18,21,22]. Nevertheless, these new mechanisms to achieve polar states have led to new exciting dipolar ordered phases such as the ferrochiral ordering in Ba(TiO)Cu 4 (PO 4 ) 4 [23] and the incommensurate helical dipole ordering in BiCu 0.1 Mn 6.9 O 12 [8], which open up the possibilities of complex noncollinear textures like polar skyrmions recently observed in (PbTiO 3 ) n /(SrTiO 3 ) n superlattices [24].…”
Hybrid improper ferroelectricity (HIF) allows the generation of an electrical polarization in the AA BB O 6 double perovskite materials thanks to the combination of two nonpolar octahedral distortions. Nevertheless, for selected combination of the A/A cations a nonpolar incommensurate phase is observed with average symmetry C2/m. Thanks to a detailed crystallographic description of the incommensurate phase, based on electron, neutron, and x-ray diffraction data, we show that the incommensurate modulation is related to an abrupt change of the out-of-phase tilting along the a and c axis, whereas the tilting along the b axis remains constant across the structure. By using group theory and symmetry analysis we show that we observe an incommensurate analog of HIF, which induces a hybrid improper dipolar density wave in NaLaCoWO 6 . The dipolar ordering is due also in this case to a trilinear invariant involving the commensurate and incommensurate octahedra tilting.
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