Magnetic Phase Transition-Induced Modulation of Ferroelectric Properties in Hexagonal RFeO3 (R = Tb and Ho).

Hexagonal rare-earth iron oxides (h-RFeO3) exhibit spontaneous magnetization and room-temperature ferroelectricity simultaneously. However, achieving a large magnetoelectric coupling necessitates further exploration. Herein, we report the impact of the magnetic phase transition on the ferroelectric properties of epitaxial h-RFeO3 (R = Tb and Ho) films prepared by pulsed laser deposition. The metastable h-RFeO3 phase is successfully stabilized with high crystallinity and low leakage current due to the ITO buffer layer, making it possible to investigate the ferroelectric properties. The h-TbFeO3 film exhibits a magnetic-field-induced transition from antiferromagnetic (AFM) to weak ferromagnetic (wFM) phases below 30 K, while also exhibiting ferroelectricity at 300 K. The dielectric constants change with the magnetic phase transition, demonstrating hysteresis in the magnetocapacitance. In contrast, the h-HoFeO3 film exhibits antiferroelectric-like behavior and an AFM-wFM phase transition. Notably, the h-HoFeO3 film shows a rapid increase in the remnant polarization during the AFM-wFM phase transition accompanied by an increase in the ferroelectric component. Considering the strong connection between the antiferroelectric behavior in the h-RFeO3 system and the ferroelectric domain wall motion, this considerable modification of ferroelectric properties during the magnetic phase transition is probably due to the faster movement of the ferroelectric domain walls in the wFM phase induced by the clamping effect. Our findings indicate the effectiveness of magnetic phase transitions in enhancing the magnetoelectric coupling, particularly when utilizing domain wall clamping properties.

Atomic layer deposition of Y2O3 films using a novel liquid homoleptic yttrium precursor tris(sec-butylcyclopentadienyl)yttrium [Y(sBuCp)3] and water.

Atomic layer deposition (ALD) of Y2O3 thin films was studied using a novel homoleptic yttrium ALD precursor: tris(sec-butylcyclopentadienyl)yttrium [Y(sBuCp)3]. Y(sBuCp)3 is a liquid at room temperature. The thermogravimetry curve for Y(sBuCp)3 is clean, with no indication of decomposition or residue formation. Thermogravimetry-differential thermal analysis measurements showed that Y(sBuCp)3 is stable for 18 weeks at 190 °C. Y(sBuCp)3 has a homoleptic structure. Thus, a reduction in manufacturing costs is expected compared to those associated with heteroleptic precursors because additional chemical synthesis steps are usually necessary to produce heteroleptic compounds. In addition, ALD of Y2O3 was demonstrated using Y(sBuCp)3 and water as a co-reactant. The deposition temperature was varied from 200 to 350 °C. The growth rate was 1.7 Å per cycle. In addition, neither carbon nor nitrogen contamination was detected in the Y2O3 films by X-ray photoelectron spectroscopy. Furthermore, smooth films were confirmed by X-ray secondary-electron microscopy. The root-mean-square roughness was measured to be 0.660 nm by atomic force microscopy. Metal-insulator-semiconductor (MIS) Pt/Y2O3/p-Si devices were fabricated to evaluate the electrical properties of the Y2O3 films. An electric breakdown field of -6.5 MV cm-1 and a leakage current density of ∼3.2 × 10-3 A cm-2 at 1 MV cm-1 were determined. The permittivity of Y2O3 was estimated to be 11.5 at 100 kHz. Therefore, compared with conventional solid precursors, Y(sBuCp)3 is suitable for use in ALD manufacturing processes.

Synthesis and transparent conductivity of crack-free La:BaSnO3 epitaxial flexible sheets.

La-doped BaSnO3 (LBSO), which exhibits both high electron mobility and visible-light transparency, is a promising transparent electrode/transistor material that does not require expensive elements such as indium. However, because a high crystal orientation is necessary for high mobility, the development of a synthetic technique is crucial for next-generation optoelectronic applications. One promising method for achieving this is the lift-off and transfer method. Epitaxial films are first deposited on single-crystal substrates, peeled off from the substrates, and subsequently transferred onto other substrates. However, such transferred sheets typically contain a high density of cracks. Therefore, LBSO sheets with flexibility, high mobility, and transparency have not yet been reported. In this study, we successfully synthesized crack-free LBSO epitaxial sheets via a lift-off and transfer method using a water-soluble Sr3Al2O6 sacrificial layer and amorphous (a-)Al2O3 protection layer. The LBSO sheet simultaneously exhibited a high electron mobility of 80 cm2 V-1 s-1 and a wide optical bandgap of 3.5 eV owing to the epitaxial crystallinity of the sheet. Moreover, two types of LBSO sheets were prepared, flat and rolled sheets, by tuning the lift-off process. The flat sheet had a lateral size of 5 mm × 5 mm, whereas the rolled sheet had a tube shape with a height of 5 mm and a diameter of 1 mm. Such large crack-free area and flexibility were achieved in LBSO sheets owing to the use of the a-Al2O3 protection layer.

Significant Suppression of Cracks in Freestanding Perovskite Oxide Flexible Sheets Using a Capping Oxide Layer.

Flexible and functional perovskite oxide sheets with high orientation and crystallization are the next step in the development of next-generation devices. One promising synthesis method is the lift-off and transfer method using a water-soluble sacrificial layer. However, the suppression of cracks during lift-off is a crucial problem that remains unsolved. In this study, we demonstrated that this problem can be solved by depositing amorphous Al2O3 capping layers on oxide sheets. Using this simple method, over 20 mm2 of crack-free, deep-ultraviolet transparent electrode La:SrSnO3 and ferroelectric Ba0.75Sr0.25TiO3 flexible sheets were obtained. By contrast, the sheets without any capping layers broke. The obtained sheets showed considerable flexibility and high functionality. The La:SrSnO3 sheet simultaneously exhibited a wide bandgap (4.4 eV) and high electrical conductivity (>103 S/cm). The Ba0.75Sr0.25TiO3 sheet exhibited clear room-temperature ferroelectricity with a remnant polarization of 17 µC/cm2. Our findings provide a simple transfer method for obtaining large, crack-free, high-quality, single-crystalline sheets.

Room-Temperature Antiferroelectricity in Multiferroic Hexagonal Rare-Earth Ferrites.

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 rare-earth manganites (ferrites), h-RMn(Fe)O3. 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-DyFeO3 films through epitaxial stabilization. Room-temperature scanning transmission electron microscopy reveals that the AFE phase originates from a nanomosaic structure comprising AFE P3̅c1 and ferroelectric P63cm 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 Dy3+. 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.

Large Polarization Switching and High-Temperature Magnetoelectric Coupling in Multiferroic GaFeO3 Systems.

GaFeO3-type iron oxides are promising multiferroics due to the coexistence of large spontaneous magnetization and polarization near room temperature. However, the high leakage current and difficulties associated with synthesizing single crystals make it difficult to achieve two important features in the system: a large ferroelectric polarization switching and magnetoelectric coupling at a high-temperature region. Herein, we report successful achievement of these features by preparing high-quality Sc-doped GaFeO3 single crystals (ScxGa1-x/2Fe1-x/2O3 with x = 0-0.3) using the floating zone method. The x ≥ 0.05 crystals exhibit a leakage current 104 times lower than the x = 0 crystals, highlighting the importance of Sc doping. Because of the reduced leakage current, the Sc-doped crystals exhibit large ferroelectric polarization switching along the c-axis with a remanent polarization of 22-25 µC/cm2, which is close to the theoretically predicted polarization value of 25-28 µC/cm2. In addition, the Sc-doped crystals exhibit ferrimagnetism with magnetic anisotropy along the a-axis. Furthermore, a magnetic-field-induced modulation of polarization is observed in the x = 0.15 crystal even at a relatively high temperature, i.e., 100 K.

Modulating the Structure and Magnetic Properties of ε-Fe2O3 Nanoparticles via Electrochemical Li+ Insertion.

ε-Fe2O3, a metastable phase of iron oxide, is widely known as a room-temperature multiferroic material or as a superhard magnet. Element substitution into ε-Fe2O3 has been reported in the literature; however, the substituted ions have a strong site preference depending on their ionic radii and valence. In this study, in order to characterize the crystal structure and magnetic properties of ε-Fe2O3 in the Fe2+/Fe3+ coexisting states, Li+ was electrochemically inserted into ε-Fe2O3 to reduce Fe3+. The discharge and charge of Li+ into/from ε-Fe2O3 revealed that Li+ insertion was successful. X-ray magnetic circular dichroism results indicated that the reduced Fe did not exhibit site preference. Increasing the Li+ content in ε-Fe2O3 resulted in decreased saturation magnetization and irregular variation of the coercive field. We present a comprehensive discussion of how magnetic properties are modified with increasing Li+ content using transmission electron microscopy images and considering the Li+ diffusion coefficient. The results suggest that inserting Li+ into crystalline ε-Fe2O3 is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe2+/Fe3+.

Reactive solid phase epitaxy of layered aurivillius-type oxyfluorides Bi2TiO4F2 using polyvinylidene fluoride.

Aurivillius-type oxyfluorides are promising ferroelectric and photocatalytic materials. However, their thin films have yet to be fabricated because of the difficulty of synthesis when using both conventional high-temperature gas-phase processes and low-temperature topotactic methods. Here, we present reactive solid phase epitaxy of a layered Aurivillius-type oxyfluoride Bi2TiO4F2 from room-temperature fabricated Bi2TiOx using polyvinylidene fluoride (PVDF) as a fluorine source. Bi2TiO4F2 epitaxial films are obtained by reacting a room-temperature fabricated precursor with PVDF at 330 °C under an Ar flow. However, crystallization does not proceed through PVDF treatment in air, indicating that a reduced atmosphere is crucial to removing oxide ions from the precursor and incorporating fluoride ions. The Bi2TiO4F2 film shows a peak at 240 K in the dielectric constant-versus-temperature curve, which originates from the tilting of Ti(O,F)6 octahedra. This peak temperature is lower than that of the bulk (284 K), suggesting that the local structural distortion is suppressed because of the epitaxial strain from the substrate. Reactive solid phase epitaxy using PVDF as described in this paper should provide a new means of synthesizing transition-metal oxyfluorides in the epitaxial thin-film form.

Selective fluorination of perovskite iron oxide/ruthenium oxide heterostructures via a topotactic reaction.

We demonstrated selective fluorination of a SrFeO2.5 layer in a SrFeO2.5/SrRuO3 epitaxial bilayer film via a topotactic reaction with polyvinylidene difluoride while maintaining the epitaxial relationship. Physical property measurements of the SrFeO2F/SrRuO3 heterostructure confirmed that the SrRuO3 layer could work as a good bottom electrode.

Epitaxial Growth of Orthorhombic GaFeO3 Thin Films on SrTiO3 (111) Substrates by Simple Sol-Gel Method.

A Sol-gel method assisted with spin-coating has been successfully used to grow orthorhombic GaFeO3 epitaxial films on SrTiO3 (111) substrates for the first time. The film with Pna21 crystal structure has been grown along the c-axis. The rocking curve of (004) reflection shows that the Full-Width at Half-Maximum (FWHM) value could be determined to be 0.230°, indicating good single crystallinity and high quality. X-ray Φ scan reveals a three-fold symmetry of the substrate and a six-fold symmetry of the film, respectively. The in-plane domains rotate 60° from each other in the film. Uniform film with dense structure, columnar grains with similar grain size was obtained. The thickness of the film was evaluated to be ~170 nm. The roughness value (RMS) measured by AFM was 4.5 nm, revealing a flat film. The in-plane Magnetization versus Magnetic field (M-H) curve at 5 K performs a typical ferri- or ferromagnetic hysteresis loop with a saturated magnetization (Ms) value of 136 emu/cm³. The Curie temperature could be determined to be 174 K. Compared to Pulsed Laser Deposition (PLD), the sol-gel method can prepare large area films with low cost. These new films show promising applications in multiferroic devices.

Fabrication of Fluorite-Type Fluoride Ba0.5Bi0.5F2.5 Thin Films by Fluorination of Perovskite BaBiO3 Precursors with Poly(vinylidene fluoride).

Metal fluorides are gathering significant interest for use in many applications, such as optical glasses, chemical sensors, and solid electrolytes using fluoride ion batteries, due to their high transparency over a wide wavelength range (ultraviolet to infrared) and fast fluoride ion conductivity. Here, we present a topotactic route for synthesizing thin films of fluorite-type Ba0.5Bi0.5F2.5 (BBF), a promising fluoride ion conductor, from perovskite-type BaBiO3 (BBO) precursor films by fluorination using poly(vinylidene fluoride). The fluorination reaction fully converted BBO to BBF without stopping at the oxyfluoride stage. The BBF films obtained at relatively low reaction temperatures (150-200 °C) showed Ba/Bi cation ordering in the [001] direction, indicating that the cation framework of perovskite BBO was maintained during the fluorination reaction. Meanwhile, increasing the fluorination temperature led to mixtures of cations, resulting in random distribution of Ba and Bi. This demonstrates that the degree of cation ordering in BBF can be controlled by adjusting the fluorination temperature.

Reversible Changes in Resistance of Perovskite Nickelate NdNiO3 Thin Films Induced by Fluorine Substitution.

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 NdNiO3 epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO3-xFx 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 NdNiO3, 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 NdNiO3 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.