Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films.

Perovskite oxides ABO3 continue to be a major focus in materials science. Of particular interest is the interplay between A and B cations as exemplified by intersite charge transfer (ICT), which causes novel phenomena including negative thermal expansion and metal-insulator transition. However, the ICT properties were achieved and optimized by cationic substitution or ordering. Here we demonstrate an anionic approach to induce ICT using an oxyhydride perovskite, EuVO2H, which has alternating layers of EuH and VO2. A bulk EuVO2H behaves as a ferromagnetic insulator with a relatively high transition temperature (TC) of 10 K. However, the application of external pressure to the EuIIVIIIO2H bulk or compressive strain from the substrate in the thin films induces ICT from the EuIIH layer to the VIIIO2 layer due to the extended empty V dxy orbital. The ICT phenomenon causes the VO2 layer to become conductive, leading to an increase in TC that is dependent on the number of carriers in the dxy orbitals (up to a factor of 4 for 10 nm thin films). In addition, a large perpendicular magnetic anisotropy appears with the ICT for the films of <100 nm, which is unprecedented in materials with orbital-free Eu2+, opening new perspectives for applications. The present results provide opportunities for the acquisition of novel functions by alternating transition metal/rare earth layers with heteroanions.

Fluorine-Assisted Low-Temperature Synthesis of GaN:ZnO-Related Solid Solutions with Visible-Light Photoresponse.

Wurtzite-structured Ga1-xZnx(N,O,F) was successfully synthesized by nitridation of mixtures of a Ga-containing oxide and ZnF2. The addition of ZnF2 lowered the nitridation temperature for the synthesis of Ga1-xZnx(N,O,F) to 823 K, even when bulk ZnGa2O4 was used as a paired precursor. This lowering of the synthesis temperature was ascribed to the enhancement of nitridation through the addition of fluorine. The low-temperature nitridation achieved by the addition of fluorine suppressed the volatilization of Zn compared with that during the synthesis of a GaN:ZnO solid solution by a conventional high-temperature ammonolysis reaction. The higher concentration of Zn, as well as the higher N concentration in Ga1-xZnx(N,O,F) achieved through the fluorine-assisted nitridation, led to a redshift of the absorption edge of Ga1-xZnx(N,O,F) to 560 nm compared with that of GaN:ZnO synthesized by the conventional ammonolysis reaction. The visible-light absorption of Ga1-xZnx(N,O,F) can be used to drive the photoelectrochemical oxidation of water.

Intrinsic defect structures of polycrystalline CaKFe4As4 superconductors.

We investigated the defect structures of polycrystalline CaKFe4As4 (CaK1144) superconductors by scanning transmission electron microscopy (STEM). The STEM studies revealed the presence of a one-layer CaFe2As2 (∼1 nm size) defect along the ab-plane, as observed in single crystalline CaK1144. Step-like CaFe2As2 defects are also observed. These nanoscale defects generate fine-sized stacking faults, a lattice mismatch, and stress field defects in the matrix of CaK1144 owing to the different sizes. Correlation of the defects in polycrystalline and single crystalline samples suggests that the defects type and their density depend on the synthesis conditions. A self-field critical current density (Jc) of 15.2 kA cm-2 was obtained at 5 K, and the curves were sustained above 30 K with a considerable Jc value of 1.4 kA cm-2. We investigated the relationship between the observed intrinsic defects and the behavior of the field dependence of Jc. The intrinsically intergrown planar defects, even in polycrystalline samples, are expected to be advantageous for various high-field applications of bulk CaK1144 superconductors.

Atomic-Scale Electrical Field Mapping of Hexagonal Boron Nitride Defects.

The distribution of electric fields in hexagonal boron nitride is mapped down to the atomic level inside a scanning transmission electron microscope by using the recently introduced technique of differential phase contrast imaging. The maps are calculated and displayed in real time, along with conventional annular dark-field images, through the use of custom-developed hardware and software. An increased electric field is observed around boron monovacancies and subsequently mapped and measured relative to the perfect lattice. The edges of extended defects feature enhanced electric fields, which can be used to trap diffusing adatoms. The magnitude of the electric field produced by the different types of edges is compared to monolayer areas, confirming previous predictions regarding their stability. These observations provide insight into the properties of this interesting material, serving as a suitable platform on which to test the limits of this technique, and encourage further work, such as dynamic experiments coupled with in situ techniques.

Synthesis of Three-Layer Perovskite Oxynitride K2Ca2Ta3O9N·2H2O and Photocatalytic Activity for H2 Evolution under Visible Light.

Substitution of oxide anions (O2-) in a metal oxide for nitrogen (N3-) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden-Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N·2H2O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N·1.6H2O. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N·2H2O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N·2H2O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N·2H2O modified with a Pt cocatalyst generated H2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N·2H2O was an order of magnitude lower than that of K2LaTa2O6N·1.6H2O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N·2H2O was shorter than that in K2LaTa2O6N·1.6H2O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N·2H2O.

An Artificial Z-Scheme Constructed from Dye-Sensitized Metal Oxide Nanosheets for Visible Light-Driven Overall Water Splitting.

Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h-1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

Synthesis of a Layered Niobium Oxynitride, Rb2NdNb2O6N·H2O, Showing Visible-Light Photocatalytic Activity for H2 Evolution.

Two-dimensional (2D) layered oxynitrides are promising candidates as visible-light-driven photocatalysts, but the actual examples are rare because of the difficulty in synthesizing the 2D oxynitrides. Here a phase-pure layered perovskite, Rb2NdNb2O6N·H2O, that belongs to a tetragonal P4/ mmm space group was successfully synthesized by thermal ammonolysis of a mixture of layered RbNdNb2O7 and Rb2CO3, as revealed by synchrotron X-ray diffraction, elemental analyses, and atomic-scale electron microscopy observation. The synthesized Rb2NdNb2O6N·H2O had an absorption edge at around 500 nm and a sufficiently high conduction-band potential to allow for proton reduction. With modification by a platinum cocatalyst, Rb2NdNb2O6N·H2O became photocatalytically active for H2 evolution in the presence of triethanolamine as an electron donor under visible light (λ > 400 nm).

Atomic number dependence of Z contrast in scanning transmission electron microscopy.

Annular dark-field (ADF) imaging by scanning transmission electron microscopy (STEM) is a common technique for material characterization with high spatial resolution. It has been reported that ADF signal is proportional to the nth power of the atomic number Z, i.e., the Z contrast in textbooks and papers. Here we first demonstrate the deviation from the power-law model by quantitative experiments of a few 2D materials (graphene, MoS2 and WS2 monolayers). Then we elucidate ADF signal of single atoms using simulations to clarify the cause of the deviation. Two major causes of the deviation from the power-law model will be pointed out. The present study provides a practical guideline for the usage of the conventional power-law model for ADF imaging.

Formation of Core-Shell Nanoparticles Composed of Magnetite and Samarium Oxide in Magnetospirillum magneticum Strain RSS-1.

Magnetotactic bacteria (MTB) synthesize magnetosomes composed of membrane-enveloped magnetite (Fe3O4) or greigite (Fe3S4) particles in the cells. Recently, several studies have shown some possibilities of controlling the biomineralization process and altering the magnetic properties of magnetosomes by adding some transition metals to the culture media under various environmental conditions. Here, we successfully grow Magnetospirillum magneticum strain RSS-1, which are isolated from a freshwater environment, and find that synthesis of magnetosomes are encouraged in RSS-1 in the presence of samarium and that each core magnetic crystal composed of magnetite is covered with a thin layer of samarium oxide (Sm2O3). The present results show some possibilities of magnetic recovery of transition metals and synthesis of some novel structures composed of magnetic particles and transition metals utilizing MTB.

Nanoscale magnetic characterization of tunneling magnetoresistance spin valve head by electron holography.

Nanostructured magnetic materials play an important role in increasing miniaturized devices. For the studies of their magnetic properties and behaviors, nanoscale imaging of magnetic field is indispensible. Here, using electron holography, the magnetization distribution of a TMR spin valve head of commercial design is investigated without and with a magnetic field applied. Characterized is the magnetic flux distribution in complex hetero-nanostructures by averaging the phase images and separating their component magnetic vectors and electric potentials. The magnetic flux densities of the NiFe (shield and 5 nm-free layers) and the CoPt (20 nm-bias layer) are estimated to be 1.0 T and 0.9 T, respectively. The changes in the magnetization distribution of the shield, bias, and free layers are visualized in situ for an applied field of 14 kOe. This study demonstrates the promise of electron holography for characterizing the magnetic properties of hetero-interfaces, nanostructures, and catalysts.

Real-space observation of skyrmion lattice in helimagnet MnSi thin samples.

Observing and characterizing the spin distributions on a nanometer scale are of vital importance for understanding nanomagnetism and its application to spintronics. The magnetic structure in MnSi thin samples prepared from a bulk, which undergoes a transition from a helix to a skyrmion lattice, was investigated by in situ observation using Lorentz microscopy. Stripe domains were observed at zero applied field below 22.5 K. A skyrmion lattice with 6-fold symmetry in real space appeared when a field of 0.18 T was applied normal to the film plane. The lattice constant was estimated to be 18 nm, almost identical to the helical period. In comparison with the marginally stable skyrmion phase in a bulk sample, the skyrmion phase was stable over a wide range of temperatures and magnetic fields in the thin samples.

Quantitative evaluation of magnetic flux density in a magnetic recording head and pseudo soft underlayer by electron holography.

The magnetic interaction between the pole tip of a single-pole head and a pseudo soft underlayer in perpendicular magnetic recording was observed by electron holography. The magnetic flux density inside the soft underlayer was quantitatively evaluated. The distribution of magnetic flux density was calculated using the finite element method, and the influences of the modulation of the reference wave and stray fields were investigated by comparison with experimental results. The flux density observed was found to be underestimated due to the modulation of the phase shift in reference wave. The magnetic flux measured experimentally was larger than that inside the specimen because of the relatively large stray fields above and below the specimen in the direction of the electron beam.