Shintaro Ishiwata 교수

The mutual control of the electric and magnetic properties of a solid is currently of great interest because of the possible application for novel electronic devices. We report on the low-magnetic-field (for example, B values of +/-30 milliteslas) control of the polarization (P) vector in a hexaferrite, Ba2Mg2Fe12O22, which shows the helimagnetic spin structure with the propagation vector k0 parallel to [001]. The B-induced transverse conical spin structure carries the P vector directing perpend

Complete magnetoelectric (ME) phase diagrams of orthorhombic $R{\text{MnO}}_{3}$ with and without magnetic moments on the $R$ ions have been established. Three kinds of multiferroic ground states, the $ab$-cycloidal, the $bc$-cycloidal, and the collinear $E$-type phases, have been identified by the distinct ME responses. The electric polarization of the $E$-type phase dominated by the symmetric spin exchange $({\mathbit{S}}_{i}\ensuremath{\cdot}{\mathbit{S}}_{j})$ is more than ten times as large

A helical spin texture is of great current interest for a host of novel spin-dependent transport phenomena. We report a rich variety of nontrivial helimagnetic phases in the simple cubic perovskite ${\mathrm{SrFeO}}_{3}$ under magnetic fields up to 42 T. Magnetic and resistivity measurements revealed that the proper-screw spin phase proposed for ${\mathrm{SrFeO}}_{3}$ can be subdivided into at least five kinds of ordered phases. Near the multicritical point, an unconventional anomalous Hall effe

A new triclinic perovskite, BiNiO3, has been synthesized at a high pressure of 6 GPa in an oxidizing atmosphere. Structure refinements based on synchrotron X-ray powder diffraction and bond-valence calculations revealed a disproportionation of the Bi ions into Bi3+ and Bi5+, both being located in distorted oxygen octahedra. Because of the presence of Bi5+, the oxidation state of Ni was +2, rather than +3 as had been expected; BiNiO3 thus showed insulating behavior with localized spins of S = 1.

The authors unveil topological spin structures composed of skyrmion strings in a cubic centrosymmetric iron oxide that provides the missing experimental link between centrosymmetric lattices and a rich variety of helimagnetic order. This is the first observation of a three-dimensional topological spin texture in a bulk perovskite-type oxide, one of the most ubiquitous and important systems for oxide electronics. The study enlarges the family of topological spin textures and offers a promising ro

Magnetic structure of a multiferroic Y-type hexaferrite ${\text{Ba}}_{2}{\text{Mg}}_{2}{\text{Fe}}_{12}{\text{O}}_{22}$ with helical-spin structures propagating along [001] below 195 K has been extensively studied, using polarized and unpolarized neutron diffractions under transverse magnetic fields $B$ $(\ensuremath{\perp}[001])$ up to 4.5 T. At zero magnetic field, a longitudinal-conical spin state was found to emerge below about 50 K, which enables the low-$B$ control of electric polarization

Orthorhombic perovskites RMnO(3) are representative of spin-driven ferroelectrics. When the radius of the rare-earth ion R is smaller than that of Dy, for instance in YMnO(3), the orthorhombic phase becomes metastable at ambient pressure, which impedes the crystal growth; thus, the detailed magnetic and multiferroic properties of the metastable phase have not been characterized. In this work, we successfully obtained single crystals of orthorhombic YMnO(3) using quasi-hydrothermal conditions und

Metal-insulator transitions strongly coupled with lattice were found in ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{NiO}}_{3}$. Synchrotron x-ray powder diffraction revealed that pressure ($P\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, $T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), temperature ($T\ensuremath{\sim}340\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $x=0.05$), and La substitution ($x\ensuremath{\sim}0.075$, $T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) caused

A new cobalt oxide, SrCo6O11, was synthesized by reacting γ-NaxCoO2 with SrCl2 under high pressure. This material crystallizes in the R-type hexagonal ferrite structure containing Co3O8 Kagomé layers separated by two kinds of triangularly arranged Co−O pillars. One of the pillars, CoO5 bipyramids, produces interesting magnetism such as plateaus in magnetization.

Microscopic origin of magnetic-field $(B)$ induced electric polarization $(P)$ potentially up to near room temperature has been investigated for helimagnets ${\text{Ba}}_{2}{({\text{Mg}}_{1\ensuremath{-}x}{\text{Zn}}_{x})}_{2}{\text{Fe}}_{12}{\text{O}}_{22}$ with controlled magnetic anisotropy by revealing $B$- and $x$-dependent changes of magnetoelectric responses. As Zn concentration $(x)$ increases, the $B$-induced $P$ rapidly diminishes, accompanying the change in the magnetic-easy surface f

The synthesis, crystal structure and magnetic properties of Ba1.2Mn8O16, obtained using a KCl mineralizer, are reported. This material crystallizes in the monoclinic hollandite structure, containing Mn–O double chains (space group: I 2/m, a = 10.052(1) A, b = 2.8579(2) A, c = 9.7627(10) A, β = 89.96(1)°), and undergoes a magnetic transition at 40 K with the magnetization having a small ferromagnetic component. The ordering temperature is nearly 10 times lower than expected from the Weiss tempera