論文

We investigated nonreciprocal electronic transport, a diode effect of electric current flow, in a helimagnet YMn6Sn6. We found that nonreciprocal resistivity, whose sign depends on the handedness of the magnetic spiral (chirality), was drastically enhanced in the vicinity of the helimagnetic transition magnetic field. This critical enhancement was observed in the wide temperature range below the helimagnetic transition temperature 335 K. We comparatively discuss three possible mechanisms of nonreciprocal electronic transport: band asymmetry, current-induced deformation of the magnetic structure, and asymmetric scattering, and suggest the most plausible mechanism is the last one.

The spiral handedness of magnetic moments, referred to as chirality, gives rise to emergent electromagnetic phenomena in helimagnets. In insulating helimagnets, known as multiferroics, the cycloidal spin structure induces electric polarization by utilizing the inverse Dzyaloshinskii-Moriya mechanism. Spin-polarized neutron diffraction experiments, which directly probe circular spin arrangements, clearly demonstrated that an electric field controlled the chirality in multiferroic helimagnets. On the other hand, it was unclear until recently how the chirality could be controlled in metallic helimagnets where a large electric field cannot be applied, while the chirality control technique in metallic helimagnets should enable the exploration of chirality-dependent spintronic functionalities. Recently, Jiang et al. succeeded in controlling the chirality of a spiral structure by the simultaneous application of a magnetic field and electric current in a metallic helimagnet, utilizing the nonreciprocal electronic transport as an indirect probe of chirality, highlighting the need for a neutron diffraction experiment that directly probes the chirality. Here, we directly demonstrate the chirality control in a metallic helimagnet YMn$_6$Sn$_6$ by means of spin-polarized neutron diffraction, which should give rise to a firm basis for the development of future helimagnetic spintronics.

Chiral phonons offer a route to transfer angular momentum without relying on magnetic order, but their electronic response in metals remains poorly understood from perspectives beyond spin-based scenarios. Using first-principles calculations, we show that coherent chiral lattice motion generates orbital accumulation and, through spin-orbit coupling, a smaller accompanying spin accumulation. Our approach evaluates orbital and spin expectation values directly from strain perturbed ab initio Hamiltonians in the long-wavelength limit, where the phonon perturbation is represented by symmetry adapted circular lattice distortions. We show that the response is controlled mainly by orbital character, near-degeneracies, and electron-phonon coupling, rather than by spin-orbit coupling alone. These results identify light transition metals as promising platforms for chiral-phonon-driven orbitronics.

We found signatures of current-induced sliding motion in helimagnetic $\mathrm{Mn}\mathrm{Au}_2$ thin films. An abrupt change in differential resistivity occurred at a threshold bias current in the helimagnetic state, whereas it was absent in the induced ferromagnetic state. Broadband voltage noise also emerged above the threshold current in the helimagnetic state. Based on the similarity to canonical charge/spin density wave systems, we ascribed the origin of these phenomena to the sliding motion of the helimagnetic structure.

Rectification phenomenon caused by the simultaneous breaking of time reversal and spatial inversion symmetries has been extended to a wide range of (quasi)particles and waves; however, the nonreciprocal diffraction, which is the imbalance of upward and downward deflections, was previously observed only for photons and remained to be extended to other (quasi)particles. In this study, we present evidence of the nonreciprocal diffraction of surface acoustic wave (SAW) utilizing a magnetoelastic grating on a SAW device. Asymmetric diffraction intensities were observed when the ferromagnetic resonance was acoustically excited. Based on a theoretical model, we attribute the microscopic origin of this phenomenon to the resonant scattering involving ferromagnetic resonance excitations. The novel property may pave an avenue to further development of SAW devices for various purposes, including microwave communications and quantum engineering applications.

Chiral magnetic textures give rise to unconventional magnetotransport phenomena such as the topological Hall effect and nonreciprocal electronic transport. While the correspondence between topology or symmetry of chiral magnetic structures and such transport phenomena has been well established, a microscopic understanding based on the spin-dependent band structure in momentum space remains elusive. Here, we demonstrate how a chiral magnetic superstructure introduces an asymmetry in the electronic band structure and triggers a nonreciprocal electronic transport in a centrosymmetric helimagnet α-EuP 3 . The magnetic structure of α-EuP 3 is highly tunable by a magnetic field and closely coupled to its semimetallic electronic band structure, enabling a systematic study across chiral and achiral magnetic phases on the correspondence between nonreciprocal transport and electronic band asymmetry. Our findings reveal how a microscopic change in the magnetic configuration of charge carriers can lead to nonreciprocal electronic transport, paving the way for designing chiral magnets with desirable properties.

Chirality in a helimagnetic structure is determined by the sense of magnetic moment rotation. We found that the chiral information did not disappear even after the phase transition to the high-temperature ferromagnetic phase in a helimagnet MnP. The 2nd harmonic resistivity rho(2f), which reflects the breaking down of mirror symmetry, was found to be almost unchanged after heating the sample above the ferromagnetic transition temperature and cooling it back to the helimagnetic state. The application of a magnetic field along the easy axis in the ferromagnetic state quenched the chirality-induced rho(2f). This indicates that the chirality memory effect originated from the ferromagnetic domain walls.

マルチフェロイック物質では複数の自由度が同時に強的な秩序を示す。またこの複数の秩序を外場などで制御することによって、新たな物質機能やデバイスへの応用が期待される。私たちはマルチフェロイック物質における圧電性とその応用である表面弾性波デバイスに注目した。私たちはBiFeO_3の単結晶を使った表面弾性波デバイスを作成し、マルチフェロイック物質において初めて、表面弾性波の伝搬を観測した。また磁気弾性結合を介したことによる、表面弾性波の磁場依存性も確認することができた

多くの磁性体における磁化ダイナミクスはマイクロ波領域の交流磁場によって励起できる。そのため、マイクロ波導波路上に磁性体をおくとマイクロ波の吸収が観測される。この吸収強度はマイクロ波透過方向に対して相反的であるが、自発電気分極を持った磁性体を使うことで、非相反性が現れる。今回我々は、電場で電気分極方向を反転できるヘキサフェライトを使い、電場によるマイクロ波非相反伝搬の制御に初めて成功した。

We observe the elastic stiffness and ultrasonic absorption of a Skyrmion crystal in the chiral-lattice magnet MnSi. The Skyrmion crystal lattice exhibits a stiffness 3 orders of magnitude smaller than that of the atomic lattice of MnSi, being as soft as the flux line lattice in type-II superconductors. The observed anisotropic elastic responses are consistent with the cylindrical shape of the Skyrmion spin texture. Phenomenological analysis reveals that the spin-orbit coupling is responsible for the emergence of anisotropic elasticity in the Skyrmion lattice.

We have investigated magnetic-field (H parallel to[100]) effect on a spin-frustrated tetragonally distorted spinelMn3O4 by measurements of magnetization, dielectric constant, and strain as well as synchrotron x-ray and neutron diffraction. In two low-temperature phases below 40 K, long-wavelength lattice modulations coupled with magnetic order have been confirmed, revealing spin-lattice coupling caused by the exchange striction in the geometrically frustrated Mn3+ spin moments on the distorted pyrochlore lattice. A magnetic-field-induced phase transition has been found at low temperatures, which involves an abrupt change in both structural and magnetic configurations. The magnetostructural phase transition by the application of a magnetic field H parallel to[100] is discussed in terms of interplay among spin, orbital, and lattice degrees of freedom.