Takashi Kurumaji

The University of Tokyo | Todai · Department of Advanced Materials Science

We report the polarization rotation of terahertz light resonant with the magnetoelectric (ME) spin excitation in the multiferroic (Fe,Zn)2Mo3O8. This resonance reflects the frequency dispersion of the diagonal ME susceptibility (axion term), with which we quantitatively reproduce the thermal and magnetic-field evolution of the observed polarization...

Formation of the triangular skyrmion-lattice is found in a tetragonal polar magnet VOSe$_2$O$_5$. By magnetization and small-angle neutron scattering measurements on the single crystals, we identify a cycloidal spin state at zero field and a N\'eel-type skyrmion-lattice phase under a magnetic field along the polar axis. Adjacent to this phase, anot...

Skyrmions in a frustrated magnet Skyrmions—tiny, topologically protected whirlpools of spin—have been investigated as potential information carriers in spintronic devices. Usually, skyrmions appear in noncentrosymmetric materials or at interfaces between materials. In contrast to this rule of thumb, Kurumaji et al. observed a skyrmion lattice phase...

We investigate the thermodynamic, magnetic, and electrical transport properties of a magnetic semimetal EuZnGe using single crystals grown from Eu-Zn flux in sealed tantalum tubes. Magnetic properties are found to be isotropic in the paramagnetic state while we observe an enhancement of in-plane magnetic susceptibility at the temperature near T*=11...

Hysteretic behaviour accompanies any first-order phase transition, forming a basis for many applications. However, its quantitative understanding remains challenging, and even a qualitative understanding of pronounced hysteresis broadening at low temperature, which is often observed in magnetic-field-induced first-order phase transition materials,...

It was discovered that crystal-field excitations involving spin-flip transitions play a crucial role in controlling switchable optomagnet effects in antiferromagnetic (Fe0.875Zn0.125)2Mo3O8. However, when the flipped spins are in excited states to frustrate the balanced spin moments, the photoinduced magnetization has not occurred yet. Only after t...

The in-plane (thermal) Hall effect is an unconventional transverse response when the applied magnetic field is in the (heat) current plane. In contrast to the normal Hall effect, the in-plane Hall effect requires the absence of certain crystal symmetries, and possibly manifests a nontrivial topology of quantum materials. An accurate estimation of t...

In some materials exhibiting field-induced first-order transitions (FOTs), the equilibrium phase-transition line is hidden by the hysteresis region associated with the FOT. In general, phase diagrams form the basis for the study of material science, and the profiles of phase-transition lines separating different thermodynamic phases include compreh...

The in-plane (thermal) Hall effect is an unconventional transverse response when the applied magnetic field is in the (heat) current plane. In contrast to the normal Hall effect, the in-plane Hall effect requires the absence of certain crystal symmetries, and possibly manifests a non-trivial topology of quantum materials. An accurate estimation of...

We investigate the crystallographic and magnetic properties of a chromium-based thiospinel CuGaCr$_{4}$S$_{8}$. From a synchrotron x-ray diffraction experiment and structural refinement, Cu and Ga atoms are found to occupy the tetrahedral $A$-sites in an alternate way, yielding breathing pyrochlore Cr network. CuGaCr$_{4}$S$_{8}$ undergoes a magnet...

Bose–Einstein condensation (BEC) in quantum magnets, where bosonic spin excitations condense into ordered ground states, is a realization of BEC in a thermodynamic limit. Although previous magnetic BEC studies have focused on magnets with small spins of S ≤ 1, larger spin systems potentially possess richer physics because of the multiple excitation...

We report an experimental demonstration of the magnetoelectrocaloric effect (MECE) in multiferroic $\mathrm{GdFeO_3}$. The temperature of the magnetic material changes when an external electric field is suddenly changed. MECE is the largest below the ordering temperature of Gd moments and modifiable by a magnetic field, suggesting that the ferroele...

Phase diagrams form the basis for the study of material science, and the profiles of phase-transition lines separating different thermodynamic phases include comprehensive information about thermodynamic quantities, such as latent heat. However, in some materials exhibiting field-induced first-order transitions (FOTs), the equilibrium phase-transit...

The centrosymmetric tetragonal itinerant magnet EuAl4 exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of EuAl4 by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray sca...

We study structural and magnetic properties of rare-earth based semimetals RAuGe (R = Y, Gd-Tm, and Lu) using flux-grown single crystals. These compounds belong to the noncentrosymmetric polar space group P63mc. We confirm the systematic structural evolution at room temperature as a function of ionic radius of rare earths to clarify the isopointal...

We investigated magnetic structures of the centrosymmetric skyrmion material Gd2PdSi3 by means of polarized neutron scattering near zero field with an isotope-Gd160-enriched single crystal. In a previous study, magnetic structures in Gd2PdSi3 at low temperatures were studied by resonant x-ray scattering measurements [T. Kurumaji et al., Science 365...

The centrosymmetric tetragonal itinerant magnet EuAl$_{4}$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a dilatometric investigation on EuAl$_{4}$ for three principal axes by means of the fiber-Bragg-grating technique, revealing anisotropic ma...

Frustrated magnets with a strong spin-lattice coupling can show rich magnetic phases and the associated fascinating phenomena. A promising platform is the breathing pyrochlore magnet CuInCr4S8 with localized S=3/2Cr3+ ions, which is proposed to be effectively viewed as an S=6 Heisenberg antiferromagnet on the face-centered-cubic lattice. Here we un...

We have investigated magnetic structures of the centrosymmetrric skyrmion material Gd2PdSi3 by means of polarized neutron scattering near zero field with an isotope-160Gd-enriched single crystal. In a previous study, magnetic structures in Gd2PdSi3 at low temperatures were studied by resonant X-ray scattering measurements [T. Kurumaji et al. Scienc...

We investigate the thermodynamic, magnetic, and electrical transport properties of a triangular-lattice antiferromagnet EuZnGe using single crystals grown from Eu-Zn flux in sealed tantalum tubes. Magnetic properties are found to be isotropic in the paramagnetic state while we observe an enhancement of in-plane magnetic susceptibility at the temper...

Frustrated magnets with a strong spin-lattice coupling can show rich magnetic phases and the associated fascinating phenomena. A promising platform is the breathing pyrochlore magnet CuInCr$_{4}$S$_{8}$ with localized $S$=3/2 Cr$^{3+}$ ions, which is proposed to be effectively viewed as an $S$=6 Heisenberg antiferromagnet on the face-centered-cubic...

On-demand spin orientation with long polarized lifetime and easily detectable signal is an ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe$_{1-x}$Zn$_{x}$)$_{2}$Mo$_{3}$O$_...

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta$_2$NiSe$_5$ and show that it can be controllably re...

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta2NiSe5 and show that it can be controllably reversed...

The topological Hall effect (THE) and its thermoelectric counterpart, the topological Nernst effect (TNE), are hallmarks of the skyrmion lattice phase (SkL). We observed the giant TNE of the SkL in centrosymmetric Gd2PdSi3, comparable in magnitude to the largest anomalous Nernst signals in ferromagnets. Significant enhancement (suppression) of the...

For the skyrmion-hosting intermetallic Gd2PdSi3 with centrosymmetric hexagonal lattice and triangular net of rare earth sites, we report a thorough investigation of the magnetic phase diagram. Our work reveals a magnetic phase with an isotropic value of the critical field for all orientations, where the magnetic ordering vector q is depinned from i...

For the skyrmion-hosting intermetallic Gd$_2$PdSi$_3$ with centrosymmetric hexagonal lattice and triangular net of rare earth sites, we report a thorough investigation of the magnetic phase diagram. Our work reveals a new magnetic phase with isotropic value of the critical field for all orientations, where the magnetic ordering vector $\mathbf{q}$...

The wave function of conduction electrons moving in the background of a noncoplanar spin structure can gain a quantal phase—the Berry phase—as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighboring...

Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity/vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on th...

We investigate the spin rotational structure of magnetic skyrmions in a tetragonal polar magnet VOSe$_2$O$_5$ via polarized small-angle neutron scattering (SANS). Spin polarization analysis of the scattered neutrons reveals the cycloidal spin modulation in all the incommensurate phases at zero and non-zero magnetic field along the $c$ axis, providi...

The topological Hall effect (THE), originating from non-zero scalar spin chirality defined in real-space, is a hallmark feature of the two-dimensional skyrmion lattice phase (SkL). However, the closely related thermoelectric response (topological Nernst effect, TNE) has never before been reported for the SkL. Here, we first confirm the presence of...

In this article, we focus on (1) type-II multiferroics driven by spiral spin orderings and (2) magnetoelectric couplings in multiferroic skyrmion-hosting materials. We present both phenomenological understanding and microscopic mechanisms for spiral spin state, which is one of the essential starting points for type-II multiferroics and magnetic sky...

On-demand spin orientation with a long polarized lifetime and an easily detectable signal is the ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe1−xZnx)2Mo3O8, from which we...

The wavefuntion of conduction electrons moving in the background of a non-coplanar spin structure can gain a quantal phase - Berry phase - as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighbouring...

slides for APS march meeting 2019. arXiv preprint is available at arXiv:1805.10719. Published in Science 365, 914-918, 10.1126/science.aau0968

Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric materials is a rewarding challenge, where the released helicity / vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. Several salient questio...

Geometrically frustrated magnets provide abundant opportunities for discovering complex spin textures, which sometimes yield unconventional electromagnetic responses in correlated electron systems. It is theoretically predicted that magnetic frustration may also promote a topologically nontrivial spin state, i.e., magnetic skyrmions, which are nano...

FY 2017 SPDR and FPR Research Report Session Poster Award Physics II Prize

Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of t...

DMI 2017 at Peterhof

Multiferroics, in which dielectric and magnetic orders coexist and couple with each other, attract renewed interest for their cross-correlated phenomena, offering a fundamental platform for novel functionalities. Elementary excitations in such systems are strongly affected by the lattice-spin interaction, as exemplified by the electromagnons and th...

A magnetic skyrmion is a nanometer-scale magnetic vortex carrying an integer topological charge. Skyrmions show a promise for potential application in low-power-consumption and high-density memory devices. To promote their use in applications, it is attempted to control the existence of skyrmions using low electric currents at room temperature (RT)...

APS March Meeting 2017 Published in Phys. Rev. Lett. 119, 077206.

We investigate a large variation in a diagonal component of the linear magnetoelectric (ME) susceptibility in a polar ferrimagnet Mn2Mo3O8 while changing the magnetic-ion site from orbital-quenched Mn2+ to Fe2+ with strong uniaxial anisotropy. In Mn2Mo3O8, the linear ME susceptibility is dominated by the non-relativistic exchange striction mechanis...

Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the...

The magnetoelectric (ME) effect, i.e., cross control of magnetization (electric polarization) by an external electric (magnetic) field, may introduce a new design principle for novel spin devices. To enhance the ME signal, control of a phase competition has recently been revealed as a promising approach. Here, we report the successful chemical-dopi...

In rare-earth (R) ferroborates, RFe3(BO3)(4) with R = Eu, Gd, and Tb, the magnetoelectric (ME) responses appear to stem from both the antiferromagnetic order of the iron (Fe) spins and the magnetic moments on the R ions. We measured the electric polarization (P) along the a axis while rotating a magnetic field (H) around the a axis and found that t...

Dielectric and magnetic properties have been investigated for single crystals of triangular-lattice antiferromagnets NiI2 and CoI2. For NiI2, the proper screw spin order with the magnetic modulation vector q∼(0.138,0,1.457) induces electric polarization (P) along the in-plane direction with respect to the triangular lattice basal plane. The P shows...

Multiferroic properties have been investigated for single crystals of the triangular-lattice antiferromagnet NiBr2 with long-wavelength (∼7 nm) cycloidal spin structure whose spin-spiral plane is parallel to (001). X-ray diffraction revealed a magnetoelastic lattice modulation with half the periodicity of the magnetic modulation below the cycloidal...

Magnetic and dielectric properties with varying magnitude and direction of magnetic-field H have been investigated for a triangular-lattice helimagnet MnI_{2}. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-pla...

Magnetoelectric properties were investigated for transition-metal dihalide MX2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. We discovered the emergence of ferroelectric polarization (P) in helimagnetic state for several compounds such as CuCl2 with edge-shared S=1/2 chain and MnI2 with stacked t...

Magnetoelectric properties were investigated for an S=1/2 chain antiferromagnet CuCl2, 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. Ap...