Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiOF films is controlled with precision by varying the reaction time. The fully fluorinated film (x ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated.
Synthetic techniques to prepare large-size, flexible, and high-quality singlecrystalline sheets of transition metal oxides are crucial to developing lowenergy consumption devices. One promising way is a lift-off and transfer technique using a heterostructure of polymer supporting oxide and Sr 3 Al 2 O 6 (SAO) layers grown on a single-crystalline substrate. By removing the water-soluble SAO and the supporting layers, the oxide sheet is obtained. Although some ferroelectric flexible sheets are prepared by this method, a simpler method for obtaining large-size sheets is required. Herein, a lift-off and transfer method is proposed without a supporting layer. With this simple method, single-crystalline SrRuO 3 and BaTiO 3 flexible sheets with a lateral size of a few millimeters are successfully prepared. The SrRuO 3 sheet exhibits high crystallinity and conductivity. Meanwhile, the ferroelectricity of the BaTiO 3 sheet is successfully observed via polarization hysteresis loop measurements. In addition to the simplicity, this method has low costs and the substrate is reusable. Accordingly, the proposed method could enhance the development of various kinds of large-size functional oxide sheets.
The antiferroelectric (AFE) phase, in which nonpolar and polar states are switchable by an electric field, is a recent discovery in promising multiferroics of hexagonal rareearth manganites (ferrites), h-RMn(Fe)O 3 . However, this phase has so far only been observed at 60−160 K, which restricts key investigations into the microstructures and magnetoelectric behaviors. Herein, we report the successful expansion of the AFE temperature range (10−300 K) by preparing h-DyFeO 3 films through epitaxial stabilization. Room-temperature scanning transmission electron microscopy reveals that the AFE phase originates from a nanomosaic structure comprising AFE P3̅ c1 and ferroelectric P6 3 cm domains with small domain sizes of 1− 10 nm. The nanomosaic structure is stabilized by a low c/a ratio derived from the large ionic radius of Dy 3+ . Furthermore, weak ferromagnetism and magnetocapacitance behaviors are observed. Below 10 K, the film exhibits an M-shaped magnetocapacitance versus magnetic field curve, indicating unusual magnetoelectric coupling in the AFE phase.
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