Direct demonstration of electric chirality control in a helimagnetic YMn$_6$Sn$_6$ by spin-polarized neutron scattering

Abstract

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.

Figures (9)

• Figure 1: Control and detection of helimagnetic chirality.a, Schematic illustration of the chirality control in a metallic helimagnet by electric current $\mathbf{j}$ and magnetic field $\mathbf{H}$. The left-handed chiral state is stabilized by applying $\mathbf{j}$ and $\mathbf{H}$ parallel to each other. The right-handed chiral state is, on the other hand, stabilized by applying antiparallel $\mathbf{j}$ and $\mathbf{H}$. b, Schematic illustration of the spin-polarized neutron diffraction from a helimagnetic structure. ${\mathbf q}$ is the magnetic propagation vector of the helimagnet, ${\mathbf s}_n$ is the incident neutron spin, ${\mathbf k}_i$ and ${\mathbf k}_f$ are the wave vectors of the incident and scattered neutrons, and ${\mathbf Q} = {\mathbf k}_i - {\mathbf k}_f$ denotes the scattering vector. For the left-handed chiral helimagnetic structure, the magnetic Bragg reflection with ${\mathbf Q} = +{\mathbf q}$ vanishes when ${\mathbf s}_n$ is parallel to ${\mathbf Q}$ and is nonzero when ${\mathbf s}_n$ is antiparallel to ${\mathbf Q}$. For the right-handed chiral helimagnetic structure, on the other hand, magnetic reflection is nonzero when ${\mathbf s}_n$ is parallel to ${\mathbf Q}$ and vanishes when ${\mathbf s}_n$ is antiparallel to ${\mathbf Q}$.
• Figure 1: Estimation of Joule heating.a, Resistivity of Sample A as a function of temperature. b, Resistivity as a function of applied dc electric current $I_{dc}$. The resistivity was measured using a lock-in technique with a superimposed ac current of 10 mA. The temperature of the sample stage was maintained at 150 K. A dc current of $I_0=+5.65$ A appears to heat the sample up to approximately 300 K.
• Figure 2: Helimagnetic order in YMn$_6$Sn$_6$.a, Crystal and magnetic structures of YMn$_6$Sn$_6$ at 2 KVenturini1996_YMnSnneutron. The arrows represent the directions of the Mn moments within the kagome plane. The magnetic structure corresponding to the predominant magnetic propagation vector $\mathbf{q}=(0,0,q_c)$ is shown. The crystal structure is drawn by VESTAVESTA. b, Magnetic susceptibility $M/H$, which is obtained by the magnetization $M$ divided by magnetic field $H$, as a function of temperature $T$ at $\mu_0H=0.1$ T for $H\parallel c$. Open and filled triangles denote the paramagnetic to antiferromagnetic transition temperature $T_N'$ and the antiferromagnetic to helimagnetic transition temperature $T_N$, respectively. This figure is reproduced from ref. Masuda2025_YMn6Sn6. c, $c^*$ component of the magnetic propagation vector $\mathbf{q}=(0,0,q_c)$ and that of additional propagation vector $\mathbf{q}'=(0,0,q_c')$ as a function of temperature $T$. Solid curves are guide to the eye. d, $(0,0,L)$ profiles of the neutron scattering intensity at selected temperatures. The triangles denote the positions of the magnetic reflections. The dashed lines connect the marker points for clarity. The data for 199.5 K, 99.6 K and 2.9 K are multiplied by 1/5.
• Figure 2: Sketch of the spin-polarized neutron scattering measurement configuration. Top-view sketch of the measurement configuration for the spin-polarized neutron scattering experiments in TAIKAN. Sample was attached to the goniometer so that the $c$ axis point rightward from the viewpoint of the incident beam, where the $c$ direction was defined by the positive magnetic field direction of the superconducting magnet used for the chirality control (Fig. \ref{['fig:fig3']}a). Spin-polarized incident neutron beam was provided by using spin polarizer and spin flipper installed in the upstream optics section of the beam line. The direction of the spin polarization axis was aligned along the $c$ axis using guide fields of $\sim1\times10^{-2}$ T. Guide field (3) was produced by permanent magnets fixed on the goniometer so that it is always parallel to the $c$ axis. Bottom panel shows the neutron spin directions through the beam line. Incident neutron spin was parallel ($\mathbf{s}_n=\uparrow$) and antiparallel ($\mathbf{s}_n=\downarrow$) to the $c$ axis when the spin flipper is ON and OFF, respectively.
• Figure 3: Demonstration of helimagnetic chirality control.a, Image of the setup for chirality control by electric current and magnetic field. A YMn$_6$Sn$_6$ single-crystal sample was fixed on a substrate and connected to copper ribbons using indium solder. Positive and negative electric currents $j_0$ and magnetic fields $H_0$ were applied along the $c$ axis. b, Top-view schematic illustration of the setup for spin-polarized neutron diffraction measurements. The scattering vector ${\mathbf Q}=\pm{\mathbf q}$ and incident neutron spin ${\mathbf s}_n$ were parallel or antiparallel to the $c$ axis. See Extended Data Fig. \ref{['fig:figS_setup']} for more details. c, Excess factor $P= \mp\frac{I_{\pm\mathbf{q},\uparrow}-I_{\pm\mathbf{q},\downarrow}}{I_{\pm\mathbf{q},\uparrow}+I_{\pm\mathbf{q},\downarrow}} \frac{1}{p_n}$ as a function of ${\rm sgn}(H_0j_0)$. $I_{\pm\mathbf{q},\uparrow}$ and $I_{\pm\mathbf{q},\downarrow}$ are the integrated intensities of $\mathbf{Q}=\pm\mathbf{q}$ magnetic reflections for ${\mathbf s}_n=\uparrow$ and ${\mathbf s}_n=\downarrow$, respectively, and $p_n=0.955(2)$ is the spin polarization of the incident neutron beamMorikawa2024_TAIKAN. d-h, $L$-scan profiles of the neutron scattering intensities around the $\mathbf{Q}=\pm\mathbf{q}$ magnetic reflections for Samples A-E at room temperature. The triangles denote the positions of the magnetic reflections. Solid curves are fit to a phenomenological function (see Methods).