Tetsuya Tohei

Negative magnetocaloric effect at the antiferromagnetic to ferromagnetic transition of Mn3GaC

The magnetocaloric effect of Mn3GaC, which shows an antiferromagnetic to ferromagnetic transition at 165 K has been investigated. In this compound, magnetocaloric effect obtained at the transition is opposite to that of ordinary ferromagnetic systems, namely, negative magnetocaloric effect. It was found that a large magnetic entropy change, ΔSmag, of 15 J/kg K is obtained under an applied field of 2 T. The adiabatic temperature change, ΔTad, reaches 5.4 K in a field change of 2 T. At higher magnetic fields, both ΔSmag and ΔTad retain a large value over wide temperature range, exhibiting characteristic temperature dependence of a trapezoidal shape. These features are attributed to a sharp first-order transition retained in high magnetic fields as well as small magnetocrystalline anisotropy.

Large magnetoelectric coupling in magnetically short-range ordered Bi5Ti3FeO15 film

Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. However, single-phase materials with such cross-coupling properties at room temperature exist rarely in nature; new design of nano-engineered thin films with a strong magneto-electric coupling is a fundamental challenge. Here we demonstrate a robust room-temperature magneto-electric coupling in a bismuth-layer-structured ferroelectric Bi5Ti3FeO15 with high ferroelectric Curie temperature of ~1000 K. Bi5Ti3FeO15 thin films grown by pulsed laser deposition are single-phase layered perovskit with nearly (00l)-orientation. Room-temperature multiferroic behavior is demonstrated by a large modulation in magneto-polarization and magneto-dielectric responses. Local structural characterizations by transmission electron microscopy and Mössbauer spectroscopy reveal the existence of Fe-rich nanodomains, which cause a short-range magnetic ordering at ~620 K. In Bi5Ti3FeO15 with a stable ferroelectric order, the spin canting of magnetic-ion-based nanodomains via the Dzyaloshinskii-Moriya interaction might yield a robust magneto-electric coupling of ~400 mV/Oe·cm even at room temperature.

A new layered iron arsenide superconductor: (Ca,Pr)FeAs2.

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

Homologous series of iron pnictide oxide superconductors (Fe2As2)[Can+1(Sc,Ti)nOy] (n=3,4,5) with extremely thick blocking layers

We have discovered homologous series of iron pnictide superconductors (Fe2As2)[Can+1(Sc,Ti)nOy] (n=3,4,5). These compounds have extremely thick blocking layers up to quintuple perovskite layers sandwiched by the Fe2As2 layers. These samples exhibited bulk superconductivity with relatively high Tc up to 42 K. The relationship between Tc and the iron-plane interlayer distance suggested that Tc of the iron based superconductor is basically determined by the local structure of Fe2As2 layer.

A new homologous series of iron pnictide oxide superconductors (Fe2As2)(Can + 2(Al, Ti)nOy) (n = 2, 3, 4)

We have discovered a new homologous series of iron pnictide oxides (Fe2As2)(Can + 2(Al, Ti)nOy) (n = 2, 3, 4). These compounds have perovskite-like blocking layers between Fe2As2 layers. The structure of the new compounds is tetragonal with space groups of P4/nmm for n = 2 and 4 and P4mm for n = 3, which are similar to those of (Fe2As2)(Can + 1(Sc, Ti)nOy) (n = 3, 4, 5) found in our previous study. Compounds with n = 3 and 4 have new crystal structures with three and four sheets of perovskite layers, respectively, including a rock salt layer in each blocking layer. The a-axis lengths of the three compounds are approximately 3.8??, which are close to those of FeSe and LiFeAs. (Fe2As2)(Ca6(Al, Ti)4Oy) exhibited bulk superconductivity in magnetization measurement with Tc(onset) ~ 36?K and resistivity drop was observed at ~ 39?K. (Fe2As2)(Ca5(Al, Ti)3Oy) also showed large diamagnetism at low temperatures. These new compounds indicate that considerable room still remains for new superconductors in layered iron pnictides.

Origins of Hole Doping and Relevant Optoelectronic Properties of Wide Gap p-Type Semiconductor, LaCuOSe

LaCuOSe is a wide band gap (∼2.8 eV) semiconductor with unique optoelectronic properties, including room-temperature stable excitons, high hole mobility ∼8 cm(2)/(Vs), and the capability of high-density hole doping (up to 1.7 × 10(21) cm(-3) using Mg). Moreover, its carrier transport and doping behaviors exhibit nonconventional results, e.g., the hole concentration increases with decreasing temperature and the high hole doping does not correlate with other properties such as optical absorption. Herein, secondary ion mass spectroscopy and photoemission spectroscopy reveal that aliovalent ion substitution of Mg at the La site is not the main source of hole doping and the Fermi level does not shift even in heavily doped LaCuOSe:Mg. As the hole concentration increases, the subgap optical absorption becomes more intense, but the increase in intensity does not correlate quantitatively. Transmission electron microscopy indicates that planar defects composed of Cu and Se deficiencies are easily created in LaCuOSe. These observations can be explained via the existence of a degenerate low-mobility layer and formation of complex Cu and Se vacancy defects with the assistance of generalized gradient approximation band calculations.

Multivariate Statistical Characterization of Charged and Uncharged Domain Walls in Multiferroic Hexagonal YMnO3 Single Crystal Visualized by a Spherical Aberration-Corrected STEM

A state-of-the-art spherical aberration-corrected STEM was fully utilized to directly visualize the multiferroic domain structure in a hexagonal YMnO3 single crystal at atomic scale. With the aid of multivariate statistical analysis (MSA), we obtained unbiased and quantitative maps of ferroelectric domain structures with atomic resolution. Such a statistical image analysis of the transition region between opposite polarizations has confirmed atomically sharp transitions of ferroelectric polarization both in antiparallel (uncharged) and tail-to-tail 180° (charged) domain boundaries. Through the analysis, a correlated subatomic image shift of Mn-O layers with that of Y layers, exhibiting a double-arc shape of reversed curvatures, have been elucidated. The amount of image shift in Mn-O layers along the c-axis is statistically significant as small as 0.016 nm, roughly one-third of the evident image shift of 0.048 nm in Y layers. Interestingly, a careful analysis has shown that such a subatomic image shift in Mn-O layers vanishes at the tail-to-tail 180° domain boundaries. Furthermore, taking advantage of the annular bright field (ABF) imaging technique combined with MSA, the tilting of MnO5 bipyramids, the very core mechanism of multiferroicity of the material, is evaluated.

Fluorine in Shark Teeth: Its Direct Atomic-Resolution Imaging and Strengthening Function**

Atomic-resolution imaging of beam-sensitive biominerals is extremely challenging, owing to their fairly complex structures and the damage caused by electron irradiation. Herein, we overcome these difficulties by performing aberration-corrected electron microscopy with low-dose imaging techniques, and report the successful direct atomic-resolution imaging of every individual atomic column in the complex fluorapatite structure of shark tooth enameloid, which can be of paramount importance for teeth in general. We demonstrate that every individual atomic column in shark tooth enameloid can be spatially resolved, and has a complex fluorapatite structure. Furthermore, ab initio calculations show that fluorine atoms can be covalently bound to the surrounding calcium atoms, which improves understanding of their caries-reducing effects in shark teeth.

Titanium enrichment and strontium depletion near edge dislocation in strontium titanate [001]/(110) low-angle tilt grain boundary

Dislocations are linear lattice defects in a crystalline solid. Since the unusual atomistic environment of the dislocation may greatly influence various material properties, control of the composition would offer more opportunities to obtain unique one-dimensional structures. In the present study, we have characterized the structure of dislocations in a low-angle tilt grain boundary of strontium titanate (SrTiO3). High-spatial resolution elemental mapping by electron energy loss spectroscopy combined with scanning transmission electron microscopy has enabled visualization of the enrichment of titanium (Ti) and the depletion of strontium (Sr) near the dislocation cores. The Ti enrichment and the Sr depletion have been observed at all of the dislocations, and the grain boundary is considered to be Ti excess. The extra Ti ions are located on the positions different from the normal perovskite lattice, suggesting that the local structure is largely reconstructed. It has been proposed that tensile strain at the dislocations may be a cause of the Ti enrichment.

A new iron pnictide oxide (Fe2As2) (Ca5 (Mg, Ti)4Oy) and a new phase in the Fe―As―Ca―Mg―Ti―O system

A new layered iron arsenide oxide (Fe2As2)(Ca5(Mg, Ti)4Oy) and its structural derivative were found in the Fe?As?Ca?Mg?Ti?O system. The crystal structure of (Fe2As2)(Ca5(Mg, Ti)4Oy) is identical to that of (Fe2As2)(Ca5(Sc, Ti)4Oy), which was reported in our previous study. The lattice constants of this compound are a = 3.86(4)?? and c = 41.05(2)??. In addition, another phase with a thicker blocking layer was found. The structure of the compound was tentatively assigned through scanning transmission electron microscope observation as (Fe2As2)(Ca8(Mg, Ti)6Oy) with sextuple perovskite-type sheets divided by a rock salt layer. The interlayer Fe?Fe distance of this compound is ~ 30??. These compounds exhibited bulk superconductivity, as found from magnetization and resistivity measurements.