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.

Excellent electrochemical properties, Li ion dynamics and room temperature work function of Li2MnO3cathode thin films.

We report on excellent electrochemical properties, Li ion diffusion coefficient and work function of Li2MnO3thin films fabricated by RF sputtering technique. The x-ray diffraction study confirms the formation of Li2MnO3thin film in layered structure. The x-ray photoelectron spectroscopy study confirms the existence of Li and Mn ions and their appropriate oxidation states in Li2MnO3thin film. Work function of the Li2MnO3thin film has been measured using scanning Kelvin probe microscopy and is found to be 5.51 eV. Electrochemical studies show that the Li2MnO3thin film exhibits oxidation and reduction peaks at 2.98 V and 2.81 V respectively with the discharging capacity of 10µAh cm-2in the first cycle and 9µAh cm-2in the 100th cycle at aCrate of 25µA cm-2. Electrochemical stability of Li2MnO3thin film is probed by measuring the charge discharge profile with high sweep rate of 500 mV s-1. Li ion diffusion coefficient value is seen to 1.6 × 10-14cm2s-1and 2.56 × 10-14cm2s-1before and after the cycling respectively. Electrochemical studies indicate that Li2MnO3thin films can be utilized as a promising cathode layer in all-solid thin film battery fabrication.

Cation-Deficiency-Induced Crystal-Site Engineering for ZnGa2O4:Mn2+ Thin Film.

Zn-deficient spinel-type ZnGa2O4:Mn2+ phosphor thin films were prepared using pulsed laser deposition. With an increase (decrease) in the Zn deficiency (concentration) of the films, changes in lattice constant, optical band gap, and photoluminescence spectra were observed. All films without γ-Ga2O3:Mn showed green luminescence attributable to the transition from the 4T1 state to the 6A1 state. In addition, the spectral shape changed depending on the temperature. The luminescence spectra have two peaks resulting from the Mn2+ ions located in the tetrahedral and octahedral sites. These peaks had different thermal quenching temperatures, which were around 320 and 260 K, respectively. Therefore, the spectral shape changed with increasing temperature. The spectral shape also depended on the Zn concentration. With an increase (decrease) in the Zn concentration (deficiency) of the films, the intensity of emission from Td increased in comparison with that from Oh. Therefore, the position of Mn2+ was controlled by Zn deficiency similarly to the effect of crystal-site engineering.

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+.

Enhancement of Ultrahigh Rate Chargeability by Interfacial Nanodot BaTiO3 Treatment on LiCoO2 Cathode Thin Film Batteries.

Nanodot BaTiO3 supported LiCoO2 cathode thin films can dramatically improve high-rate chargeability and cyclability. The prepared BaTiO3 nanodot is <3 nm in height and 35 nm in diameter, and its coverage is <5%. Supported by high dielectric constant materials on the surface of cathode materials, Li ion (Li+) can intercalate through robust Li paths around the triple-phase interface consisting of the dielectric, cathode, and electrolyte. The current concentration around the triple-phase interface is observed by the finite element method and is in good agreement with the experimental data. The interfacial resistance between the cathode and electrolyte with nanodot BaTiO3 is smaller than that without nanodot BaTiO3. The decomposition of the organic solvent electrolyte can prevent the fabrication of a solid electrolyte interface around the triple-phase interface. Li+ paths may be created at non solid electrolyte interface covered regions by the strong current concentration originating from high dielectric constant materials on the cathode. Robust Li+ paths lead to excellent chargeability and cyclability.

Ferroelectric BaTaO2N Crystals Grown in a BaCN2 Flux.

Perovskite-type oxynitride BaTaO2N has been attracting attention for its large dielectric constant, which is almost independent of the temperature by measurements on its ceramics. Its dielectric characteristics are attributed to polar nanoregions (PNRs) in the average cubic crystal structure. Polarization saturation to produce a butterfly-like piezoresponse force microscopy (PFM) signal was observed on BaTaO2N crystals in the present study. Reddish crystallites of BaTaO2N of up to 3.1 µm in size were grown using a BaCN2 flux. Grain growth proceeded through the formation of a Ruddlesden-Popper-type oxynitride from the reaction between BaTaO2N powder and molten BaCN2. Their electrical property was studied using PFM with special care because of the small size of the crystals. They were found to be much more highly insulating than its ceramics. Ferroelectricity with complete phase inversion was observed on an oxynitride perovskite crystal for the first time. A large coercivity of 50-60 V was observed in the measurement. Such ferroelectricity is ascribed to the PNRs induced by the polar linkages between cis-type TaO4N2 octahedra.

Evaluation of ferroelectricity in a distorted wurtzite-type structure of Sc-doped LiGaO2.

Since the discovery of ferroelectricity in a wurtzite-type structure, this structural type has gathered much attention as a next-generation ferroelectric material due to its high polarization value combined with its high breakdown strength. However, the main targets of wurtzite-type ferroelectrics have been limited thus far to simple nitride/oxide compounds. The investigation of new ferroelectric materials with wurtzite-type structures is important for understanding ferroelectricity in such structures. We therefore focus on ß-LiGaO2 in this study. Although AlN and ZnO possess well-known wurtzite-type structures (P63mc), ß-LiGaO2 has a distorted wurtzite-type structure (Pna21), and there are no reports of ferroelectricity in LiGaO2. In this study, we have revealed that LiGaO2 exhibits relatively high barrier height energy for polarization switching, however, Sc doping effectively reduces that energy. Then, we conducted thin film preparation and evaluation for Sc-doped LiGaO2 to observe its ferroelectric properties. We successfully observed ferroelectric behavior by using piezoresponse force microscopy measurements for LiGa0.8Sc0.2O2/SrRuO3/(111)SrTiO3.

Design of a KNN-BZT Ceramic with High Energy Storage Properties and Transmittance under Low Electric Fields.

With the advancement of science and technology, single-function ceramics have been difficult to meet the rapid development of electronic components. It is of great significance to find and develop multifunctional ceramics with excellent performance and environmental friendliness (such as good energy storage and transparency). Especially, the realization of its excellent performance under low electric fields has more reference and practical value. In this study, by Bi(Zn0.5Ti0.5)O3 (BZT) modification in (K0.5Na0.5)NbO3 (KNN), reducing grain size, and increasing band gap energy, the purpose of improving energy storage performance and transparency has been achieved under low electric field. The results show that the submicron average grain size decreased to 0.9 µm and the band gap energy (E g) increased to 2.97 eV for 0.90KNN-0.10BZT ceramics. The transparency is up to 69.27% in the near-infrared region (1344 nm) and the energy storage density is 2.16 J/cm3 under 170 kV/cm. Moreover, the 0.90KNN-0.10BZT ceramic exhibits a power density (P D) of 17.50 MW/cm3 and the stored energy can be discharged in 1.60 µs at 140 kV/cm. This revealed a potential application of KNN-BZT ceramic as an energy storage and transparent capacitor in the electronics industry.

Unveiling a Chemisorbed Crystallographically Heterogeneous Graphene/L10-FePd Interface with a Robust and Perpendicular Orbital Moment.

A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 µB → @Gr/FePd interface: Ml = 0.32 µB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

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.

Suppression Mechanisms of the Solid-Electrolyte Interface Formation at the Triple-Phase Interfaces in Thin-Film Li-Ion Batteries.

Side reactions of the charge/discharge in Li-ion batteries (LIBs) generate a solid-electrolyte interface (SEI) onto an electrode surface, resulting in the degradation of the lifetime of a cell. The suppression of SEI formations has attracted much attention for achieving longer cyclable LIBs. Our research group has previously reported that few SEI were observed at triple-phase interfaces (TPIs) consisting of BaTiO3, LiCoO2, and electrolyte interfaces in LIBs with excellent cyclability and ultrahigh-speed chargeability. An investigation on the suppression mechanisms of SEI formations at TPIs should yield important information on understanding the undesirable side reactions. Therefore, we have explored the suppression mechanisms of SEI formations by preparing epitaxial thin films and evaluating the surface of the samples after the electrochemical treatment. The results of X-ray photoelectron spectroscopy and scanning electron microscopy with energy-dispersive X-ray analysis measurements suggested that the decomposition of LiPF6 was suppressed at TPIs, implying that the generation of PF5 via the decomposition of LiPF6 contributed to SEI formation.

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3.

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Short range biaxial strain relief mechanism within epitaxially grown BiFeO3.

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO3 films grown on La0.6Sr0.4MnO3/SrTiO3 and YAlO3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO3 thin films is rhombohedral by showing the rhombohedral signature Bragg's reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO3 grown on La0.6Sr0.4MnO3/SrTiO3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO3 grown on YAlO3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

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.

Superconducting transition temperatures in the electronic and magnetic phase diagrams of Sr2VFeAsO3-δ , a superconductor.

We elucidate the magnetic phases and superconducting (SC) transition temperatures (T c) in Sr2VFeAsO3-δ (21113V), an iron-based superconductor with a thick-blocking layer fabricated from a perovskite-related transition metal oxide. At low temperatures (T < 37.1 K), 21113V exhibited a SC phase in the range 0.031 ⩽ δ ⩽ 0.145 and an antiferromagnetic (AFM) iron sublattice in the range 0.267 ⩽ δ ⩽ 0.664. Mixed-valent vanadium exhibited a dominant AFM phase in 0.031 ⩽ δ ⩽ 0.088, and a partial ferrimagnetic (Ferri.) phase in the range 0.124 ⩽ δ ⩽ 0.664. The Ferri. phase was the most dominant at a δ value of 0.267, showing an AFM phase of Fe at T < 20 K. Increasing the spontaneous magnetic moments reduced the magnetic shielding volume fraction due to the SC phase. This result was attributed to the magnetic phase of vanadium, which dominates the superconductivity of Fe in 21113V. The T c-δ curve showed two maxima. The smaller and larger of T c maxima occurred at δ = 0.073 and δ = 0.145, respectively; the latter resides on the phase boundary between AFM and the partial Ferri. phases of vanadium. 21113V is a useful platform for verifing new mechanisms of T c enhancement in iron-based superconductors.

Tensile stress effect on epitaxial BiFeO3 thin film grown on KTaO3.

Comprehensive crystal structural study is performed for BiFeO3 (BFO) film grown on KTaO3 (KTO) substrate using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Nano-beam electron diffraction (NBED) combined with structure factor calculation and high resolution TEM images clearly reveal that the crystal structure within BFO thin film is rhombohedral BFO, i.e., bulk BFO phase. Epitaxial relationship found by NBED indicates the BFO film grows in a manner that minimizes lattice mismatch with KTO. It further suggests BFO film is under slight biaxial tensile stress (~0.35%) along in-plane direction. XRD reveals BFO lattice is under compressive stress (~1.6%), along out-of-plane direction as a result of the biaxial tensile strain applied along in-plane direction. This leads to Poisson's ratio of ~0.68. In addition, we demonstrate (1) why hexagonal notation rather than pseudocubic one is required for accurate BFO phase evaluation and (2) a new XRD method that shows how rhombohedral BFO can readily be identified among other phases by measuring a rhombohedral specific Bragg's reflection.

Lead-Free Multilayer Piezoceramic Composites: Effect of Cosintering on Electromechanical Properties.

The macroscopic electromechanical behavior of lead-free multilayer composites was characterized from room temperature to 150 °C. The polar seed component consisted of a nonergodic relaxor (Bi1/2Na1/2)TiO3-7BaTiO3, with an electric-field-induced long-range ferroelectric order, whereas the nonpolar matrix was an ergodic relaxor Bi0.5(Na0.75K0.25)0.5 TiO3-6BiAlO3 that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of the effects of cosintering are demonstrated through examination of grain size, interdiffusion, and pore structure. By manipulating the sintering interactions between the two constituents, namely, diffusion paths and residual stresses, both internal mechanical and electrical fields, as well as compositional gradients can be used to enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems.

Elucidation of crystal and electronic structures within highly strained BiFeO3 by transmission electron microscopy and first-principles simulation.

Crystal and electronic structures of ~380 nm BiFeO3 film grown on LaAlO3 substrate are comprehensively studied using advanced transmission electron microscopy (TEM) technique combined with first-principles theory. Cross-sectional TEM images reveal the BiFeO3 film consists of two zones with different crystal structures. While zone II turns out to have rhombohedral BiFeO3, the crystal structure of zone I matches none of BiFeO3 phases reported experimentally or predicted theoretically. Detailed electron diffraction analysis combined with first-principles calculation allows us to determine that zone I displays an orthorhombic-like monoclinic structure with space group of Cm (=8). The growth mechanism and electronic structure in zone I are further discussed in comparison with those of zone II. This study is the first to provide an experimentally validated complete crystallographic detail of a highly strained BiFeO3 that includes the lattice parameter as well as the basis atom locations in the unit cell.

In-situ observation of ultrafast 90° domain switching under application of an electric field in (100)/(001)-oriented tetragonal epitaxial Pb(Zr0.4Ti0.6)O3 thin films.

Ferroelastic domain switching significantly affects piezoelectric properties in ferroelectric materials. The ferroelastic domain switching and the lattice deformation of both a-domains and c-domains under an applied electric field were investigated using in-situ synchrotron X-ray diffraction in conjunction with a high-speed pulse generator set up for epitaxial (100)/(001)-oriented tetragonal Pb(Zr0.4Ti0.6)O3 (PZT) films grown on (100) c SrRuO3//(100)KTaO3 substrates. The 004 peak (c-domain) position shifts to a lower 2θ angle, which demonstrates the elongation of the c-axis lattice parameter of the c-domain under an applied electric field. In contrast, the 400 peak (a-domain) shifts in the opposite direction (higher angle), thus indicating a decrease in the a-axis lattice parameter of the a-domain. 90° domain switching from (100) to (001) orientations (from a-domain to c-domain) was observed by a change in the intensities of the 400 and 004 diffraction peaks by applying a high-speed pulsed electric field 200 ns in width. This change also accompanied a tilt in the angles of each domain from the substrate surface normal direction. This behaviour proved that the 90° domain switched within 40 ns under a high-speed pulsed electric field. Direct observation of such high-speed switching opens the way to design piezo-MEMS devices for high-frequency operation.

Epitaxial LiCoO2 films as a model system for fundamental electrochemical studies of positive electrodes.

Epitaxial LiCoO2 (LCO) thin films of different orientations were fabricated by pulsed laser deposition (PLD) in order to model single-crystal behavior during electrochemical reaction. This paper demonstrates that deposition of conductive SrRuO3 between a SrTiO3 (STO) substrate and an LCO film allows (1) epitaxial growth of LCO with orientation determined by STO and (2) electrochemical measurements, such as cyclic voltammetry and impedance spectroscopy. Scanning transmission electron microscopy (S/TEM and SEM) has demonstrated an orientation relationship between LCO and STO of three orientations, (111), (110) and (100), and identified a LCO/electrolyte surface as consisting of two crystallographic facets of LCO, (001) and {104}. The difference in the orientation of LCO accounts for the difference in the exposed area of {104} planes to the electrolyte, where lithium ions have easy access to fast diffusion planes. The resistance for lithium ion transfer measured by electrochemical impedance spectroscopy had inverse correlation with exposed area of {104} plane measured by TEM. Chemical diffusivity of lithium ions in LCO was measured by fitting electrochemical impedance spectroscopy data to a modified Randles equivalent circuit and allowed us to determine its dependence on film orientation.