S. Seki 교수

A magnetic skyrmion is a topologically stable particle-like object that appears as a vortex-like spin texture at the nanometer scale in a chiral-lattice magnet. Skyrmions have been observed in metallic materials, where they are controllable by electric currents. Here, we report the experimental discovery of magnetoelectric skyrmions in an insulating chiral-lattice magnet Cu(2)OSeO(3) through Lorentz transmission electron microscopy and magnetic susceptibility measurements. We find that the skyrm

The correlation between the dielectric and magnetic properties is investigated on the triangular-lattice antiferromagnets ACrO2 (A=Cu, Ag, Li, or Na) with a 120-degree spiral structure. For the A=Cu and Ag compounds with a delafossite structure, the ferroelectric polarization emerges with a spiral-spin order, implying strong coupling between ferroelectricity and the spiral-spin structure. For the A=Li and Na compounds with an ordered rock salt structure, on the other hand, no spontaneous polariz

The longitudinal spin Seebeck effect has been investigated for a uniaxial antiferromagnetic insulator Cr(2)O(3), characterized by a spin-flop transition under magnetic field along the c axis. We have found that a temperature gradient applied normal to the Cr(2)O(3)/Pt interface induces inverse spin Hall voltage of spin-current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr(2)O(3). The possible contribution of the anomalous Nernst effect is confirmed to b

The concept of a skyrmion, which was first introduced by Tony Skyrme in the field of particle physics, has become widespread in condensed matter physics to describe various topological orders. Skyrmions in magnetic materials have recently received particular attention; they represent vortex-like spin structures with the character of nanometric particles and produce fascinating physical properties rooted in their topological nature. Here, a series of noncentrosymmetric ferromagnets hosting skyrmi

Small-angle neutron scattering experiments were performed on a bulk single crystal of chiral-lattice multiferroic insulator Cu${}_{2}$OSeO${}_{3}$. In the absence of an external magnetic field, helical spin order with magnetic modulation vector $q\ensuremath{\parallel}\ensuremath{\langle}001\ensuremath{\rangle}$ was identified. When a magnetic field is applied, a triple-$q$ magnetic structure emerges normal to the field in the A phase just below the magnetic ordering temperature ${T}_{c}$, which

Measurements of polarized neutron scattering were performed on a S=1/2 chain multiferroic LiCu2O2. In the ferroelectric ground state with the spontaneous polarization along the c axis, the existence of transverse spiral spin component in the bc plane was confirmed. When the direction of electric polarization is reversed, the vector spin chirality as defined by C_(ij)=S_(i)xS_(j) (i and j being the neighboring spin sites) is observed to be reversed, indicating that the spin-current model or the i

Dielectric properties were investigated under various magnitudes and directions of magnetic field ($H$) for a chiral magnetic insulator Cu${}_{2}$OSeO${}_{3}$. We found that the skyrmion crystal induces electric polarization ($P$) along either in-plane or out-of-plane direction of the spin vortices depending on the applied $H$ direction. The observed $H$ dependence of $P$ in ferrimagnetic, helimagnetic, and skyrmion crystal state can be consistently described by the $d$-$p$ hybridization model,

Dielectric responses have been investigated on the triangular-lattice antiferromagnet $\mathrm{Cu}\mathrm{Fe}{\mathrm{O}}_{2}$ and its site-diluted analogs $\mathrm{Cu}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{O}}_{2}$ ($x=0.01$ and 0.02) with and without the application of the magnetic field. We have found a ferroelectric behavior at zero magnetic field for $x=0.02$. At any doping level, the onset field of the ferroelectricity always coincides with that of the noncollinear magne

In magnetic materials with chiral crystal structure, it has been predicted that quasiparticle flows propagating parallel and antiparallel to the external magnetic field can show different propagating character, with its sign of nonreciprocity dependent on the chirality of the underlying bulk crystal lattice. This unique phenomenon, termed magnetochiral nonreciprocity, has previously been demonstrated for the propagating light and conduction electrons but seldom for other quasiparticles. In this

Magnetic skyrmion is a topologically stable particle-like swirling spin texture potentially suitable for high-density information bit, which was first observed in noncentrosymmetric magnets with Dzyaloshinskii-Moriya interaction. Recently, nanometric skyrmion has also been discovered in centrosymmetric rare-earth compounds, and the identification of their skyrmion formation mechanism and further search of nontrivial spin textures are highly demanded. Here, magnetic structures in a prototypical s

Multiple-<i>q</i> spin order, i.e., a spin texture characterized by a multiple number of coexisting magnetic modulation vectors <i>q</i>, has recently attracted attention as a source of nontrivial magnetic topology and associated emergent phenomena. One typical example is the triple-<i>q</i> skyrmion lattice state stabilized by Dzyaloshinskii-Moriya interactions in noncentrosymmetric magnets, while the emergence of various multiple-<i>q</i> states of different origins is expected according to th

Magnetoelectric properties were investigated for an $S=1/2$ chain antiferromagnet ${\text{CuCl}}_{2}$, which turns out to be the first example of nonchalcogen based spiral-spin induced multiferroics. Upon the onset of helimagnetic order propagating along the $b$ axis under zero magnetic field $(H)$, we found emergence of ferroelectric polarization along the $c$ axis. Application of $H$ along the $b$ axis leads to spin-flop transition coupled with drastic suppression of ferroelectricity and rotat

Magnetic skyrmion, i.e. a topologically stable swirling spin texture, appears as a particle-like object in the two-dimensional (2D) systems, and has recently attracted attention as a candidate of novel information carrier. In the real three-dimensional (3D) systems, a skyrmion is expected to form a string structure along an extra dimension, while its experimental identification has rarely been achieved. Here, we report the direct visualization of 3D shape of individual skyrmion strings, for the

Terahertz time-domain spectroscopy was performed to directly probe the low-energy (1-5 meV) electrodynamics of triangular lattice antiferromagnets CuFe(1-x)Ga(x)O2 (x=0.00, 0.01, and 0.035). We discovered an electromagnon (electric-field-active magnon) excitation at 2.3 meV in the paraelectric ↑↑↓↓ collinear magnetic phase, while this electromagnon vanishes in the ferroelectric helimagnetic phase. Anticorrelation with noncollinear magnetism excludes the exchange-striction mechanism as the origin

Magnetic skyrmions with a topological particle nature have recently attracted attention as a potential information carrier for novel magnetic storage devices. For single-phase bulk crystals, skyrmions usually appear for a very narrow temperature region just below the magnetic ordering temperature ${T}_{c}$, and the stabilization of skyrmions for a wider temperature range remains an important challenge. Here, by investigating the impact of uniaxial tensile stress for a chiral magnet ${\mathrm{Cu}