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Magnetic excitations in the noncentrosymmetric magnet Sr$_2$MnSi$_2$O$_7$ were investigated through inelastic neutron scattering measurements. Major magnetic excitations are limited up to the energy transfer of 0.5 meV, and two magnon branches under zero magnetic field were well explained in the framework of linear spin-wave theory. The magnitudes of the square-lattice in-plane and inter-plane nearest-neighbor interactions, spin anisotropy term, and the Dzyaloshinskii-Moriya interaction are respectively estimated to be $J_1=45.54(5)$ $\mu$eV, $J_2=0.52(1)$ $\mu$eV, $\Lambda=4.98(11)$ $\mu$eV, $D_{xy}=0.02(9)$ $\mu$eV, and $D_z=4.10(1)$ $\mu$eV, and calculations using these parameters reproduce experimental data quite well. Sr$_2$MnSi$_2$O$_7$ appears to have the smallest energy scale among the melilite-type compounds, and the small $J_2/J_1=0.0114(2)$ indicates the sufficient two-dimensionality., Comment: 6 pages, 5 figures

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., Comment: 15 pages, 12 figures

Magnetoelectric mutual control in multiferroics, which is the electric control of magnetization, or reciprocally the magnetic control of polarization has attracted much attention because of its possible applications to spintronic devices, multi-bit memories, and so on. While the required working temperature for the practical application is much higher than room temperature, which ensures stable functionality at room temperature, the reported working temperatures were at most around room temperature. Here, we demonstrated magnetic control of ferroelectric polarization at 432 K in ferroelectric and ferroelastic Tb$_2$(MoO$_4$)$_3$, in which the polarity of ferroelectric polarization is coupled to the orthorhombic strain below the transition temperature 432 K. The paramagnetic but strongly magnetoelastic Tb$^{3+}$ magnetic moments enable the magnetic control of ferroelectric and ferroelastic domains; the ferroelectric polarization is controlled depending on whether the magnetic field is applied along [110] or [$\bar{1}$10]. This result may pave a new avenue for designing high-temperature multiferroics., Comment: 13 pages, 7 figures

Chiral magnetic textures give rise to unconventional magnetotransport phenomena such as the topological Hall effect and nonreciprocal electronic transport. While the correspondence between real-space magnetic topology/symmetry 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 structure in real space introduces an asymmetry in the electronic band structure and triggers a nonreciprocal electronic transport in a centrosymmetric helimagnet {\alpha}-EuP$_3$. The magnetic structure of {\alpha}-EuP$_3$ is highly tunable by a magnetic field and closely coupled to its semi-metallic 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., Comment: 31 pages, 12 figures, 1 table

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., Comment: 18 pages, 16 figures

In a magnetic texture, the spin of a conduction electron is forced to be aligned to the localized moment. As a result, the topology of the magnetic texture affects the electron dynamics in nontrivial ways. A representative example is the topological Hall effect in noncoplanar spin textures with finite spin chirality. While propagating in the noncoplanar spin texture, electrons acquire Berry phase, and their motion is deflected as if they were in a magnetic field. Here, we report a distinct Berry phase effect in a coplanar helimagnet: the spin moment of the conduction electron is polarized under electric currents depending on the chirality of the helimagnet. The accumulated spin polarization works as a source of spin current, and the chirality can be detected by the inverse spin Hall mechanism. The functionality allows us to read out the chirality without magnetic fields, and therefore paves the way to future helimagnet-based spintronics.

We have studied acoustic topological edge modes in a honeycomb phononic crystal composed of metallic nano-pillars on a LiNbO$_3$ substrate. Acoustic band calculations show that the topological surface acoustic wave (SAW) mode inhabits the edge of the honeycomb phononic crystal in spite of the hybridization with the internal acoustic modes of the substrate. Pulse-type microwave impedance microscopy realized clear visualization of the gigahertz topological edge mode between two mutually inverted topological phononic crystals. A frequency-dependent image showed that the edge mode evolves as the bulk SAW modes are suppressed owing to the energy gap formation, consistent with the topological nature. The realization of a topological waveguide in a simple pillar structure on a substrate might pave a new path to the development of topological SAW devices., Comment: 13 pages, 6 figures

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality (or helicity) corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control sequence have to be solved in practical applications. Another issue is the thin film fabrication that enables the development of spintronic devices based on helimagnets. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu$_2$ helimagnetic conductor. The result demonstrates the feasibility of helimagnet-based spintronics that can overcome all the above problems., Comment: 22 pages, 9 figures

Breaking of spatial inversion symmetry (SIS) induces unique phenomena in condensed matter. Besides the classic examples such as natural optical activity and piezoelectricity, the spin-orbit interaction further enriches the effect of SIS breaking, as exemplified by the Rashba effect. In particular, by combining this symmetry with magnetic fields or another type of time-reversal symmetry (TRS) breaking, noncentrosymmetric materials can be made to exhibit nonreciprocal responses, which are responses that differ for rightward and leftward stimuli. For example, resistivity becomes directionally dependent; that is to say, rectification appears in noncentrosymmetric materials in a magnetic field. However, the effect of SIS breaking on thermal transport remains to be elucidated. Here we show nonreciprocal thermal transport in the multiferroic helimagnet TbMnO3. The longitudinal thermal conductivity depends on whether the thermal current is parallel or antiparallel to the vector product of the electric polarization and magnetization. This phenomenon is thermal rectification that is controllable with external fields in a uniform crystal. This discovery may pave the way to thermal diodes with controllability and scalability., Comment: 12 pages, 5 figures

One of the major strategies to control magnetism in spintronics is to utilize the coupling between electron spin and its orbital motion. The Rashba and Dresselhaus spin-orbit couplings induce magnetic textures of band electrons called spin momentum locking, which produces a spin torque by the injection of electric current. However, joule heating had been a bottleneck for device applications. Here, we propose a theory to generate further rich spin textures in insulating antiferromagnets with broken spatial inversion symmetry (SIS), which is easily controlled by a small magnetic field. In antiferromagnets, the ordered moments host two species of magnons that serve as internal degrees of freedom in analogy with electron spins. The Dzyaloshinskii-Moriya interaction introduced by the SIS breaking couples the two-magnon-degrees of freedom with the magnon momentum. We present a systematic way to design such texture and to detect it via magnonic spin current for the realization of antiferromagnetic memory., Comment: 14 pages, 7 figures

Emergent phenomena and functions arising from topological electron-spin textures in real space or momentum space are attracting growing interest for new concept of states of matter as well as for possible applications to spintronics. One such example is a magnetic skyrmion, a topologically stable nanoscale spin vortex structure characterized by a topological index. Real-space regular arrays of skyrmions are described by combination of multi-directional spin helixes. Nanoscale configurations and characteristics of the two-dimensional skyrmion hexagonal-lattice have been revealed extensively by real-space observations. Other three-dimensional forms of skyrmion lattices, such as a cubic-lattice of skyrmions, are also anticipated to exist, yet their direct observations remain elusive. Here we report real-space observations of spin configurations of the skyrmion cubic-lattice in MnGe with a very short period (~3 nm) and hence endowed with the largest skyrmion number density. The skyrmion lattices parallel to the {100} atomic lattices are directly observed using Lorentz transmission electron microscopes (Lorentz TEMs). It enables the first simultaneous observation of magnetic skyrmions and underlying atomic-lattice fringes. These results indicate the emergence of skyrmion-antiskyrmion lattice in MnGe, which is a source of emergent electromagnetic responses and will open a possibility of controlling few-nanometer scale skyrmion lattices through atomic lattice modulations.

Right- and left-handed circularly polarized light interact differently with electronic charges in chiral materials. This asymmetry generates the natural circular dichroism and gyrotropy, also known as the optical activity. Here we demonstrate that optical activity is not a privilege of the electronic charge excitations but it can also emerge for the spin excitations in magnetic matter. The square-lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$ offers an ideal arena to test this idea, since it can be transformed to a chiral form by application of external magnetic fields. As a direct proof of the field-induced chiral state, we observed large optical activity when the light is in resonance with spin excitations at sub-terahertz frequencies. In addition, we found that the magnetochiral effect, the absorption difference for the light beams propagating parallel and anti-parallel to the applied magnetic field, has an exceptionally large amplitude close to 100%. All these features are ascribed to the magnetoelectric nature of spin excitations as they interact both with the electric and magnetic components of light.

Small-angle neutron scattering (SANS) profiles revealed the existence of magnetic textures like liquid crystals in the low-temperature helical phase of Fe0.7Co0.3Si, isomorphous of MnSi., Comment: 10 pages, 4 figures