Chirality is the fundamental property of symmetry in nature that influences the physical properties of both inorganic and organic substances. Consequently, it has gained significant attention in materials science in the last decade.

In magnetic chiral crystals, which lack both inversion and mirror symmetries, the effect of the antisymmetric Dzyaloshinskii–Moriya (DM) interaction leads to a helimagnetic ground state. Applying a magnetic field along the helical axis in such a system transforms this helimagnetic ground state into a topological spin structure, such as magnetic skyrmions and chiral solitons.

The existence of a chiral soliton lattice (CSL) state was first predicted theoretically and later confirmed in the monoaxial chiral helimagnet CrNb₃S₆, a transition metal compound. Such materials can offer new functionalities by tuning the CSL state, with potential applications in spintronics.

Recently, magnetization measurements of Yb(Ni_1-xCu_x)₃Al_9—a new monoaxial chiral helical magnet, have indicated the presence of a CSL state. Previous studies have shown that substituting nickel (Ni) with copper (Cu) results in an anomalous magnetization curve. Specifically, at 6% Cu doping, the magnetization resembles a CSL state, though it has not been directly observed.

In this study, we investigated the magnetic structure of Yb(Ni_1-xCu_x)₃Al₉ through resonant X-ray diffraction (RXD). We performed RXD measurements using both right-circularly polarized (RCP) and left-circularly polarized (LCP) X-rays on the right-handed (R) and left-handed (L) crystal specimens of Yb(Ni_1-xCu_x)₃Al₉ at different Cu concentrations.

Notably, the helicity of the magnetic structure depended on chirality of the crystal, as evident from the opposite behaviour of RCP and LCP X-rays for the R and L crystals in reciprocal space scans. This confirms the presence of an antisymmetric exchange interaction, i.e. the DM interaction.

Furthermore, when a magnetic field was applied perpendicular to the helical axis, second and third harmonic peaks were detected in the RXD pattern for crystals with 6% Cu doping. On increasing the magnetic field, the intensity of the first harmonic decreased while those of higher harmonic peaks increased. This behaviour, consistent with chiral Sine-Gorden model, provides compelling evidence for CSL state formation.

Remarkably, the period of the CSL is locked to the lattice when it is exactly three turns over eight unit-cells. These insights deepen our understanding of chirality-induced magnetic structures and their potential applications.

Our study is the first to demonstrate the presence of CSL state in Yb(Ni_1-xCu_x)₃Al₉ and was honoured with the Outstanding Paper Award of the Physical Society of Japan.

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