The spins of magnetic atoms often form a spin-swirling particle-like structure and then organize themselves into a lattice. This spectacular state is known as a magnetic skyrmion lattice, which is usually a triangular lattice. The skyrmion lattice is represented by a superposition of three helimagnetic waves propagating at 120°angle from each other. Since the helimagnetic structures have either a right-handed or left-handed helicity, the Dzyaloshinskii–Moriya-type antisymmetric interaction, which governs helicity selection, has been considered as necessary for the formation of a magnetic skyrmion lattice. In fact, many magnetic skyrmion lattices have been observed in magnetic materials with chiral crystal structures lacking both inversion and mirror symmetries, such as MnSi and EuPtSi. Recently, however, magnetic skyrmion lattices have been observed in materials with inversion symmetry, such as Gd₂PdSi₃ and GdRu₂Si₂, although these materials lack antisymmetric interactions. This observation raises an interesting question about magnetic materials, such as EuNiGe₃, that lack inversion symmetry but possess mirror planes: What kind of magnetic skyrmion lattice is formed when opposite antisymmetric interactions coexist?

In this study, we focused on the magnetic field-induced ordered phase of the tetragonal EuNiGe₃, which might be a magnetic skyrmion lattice phase. We searched for the three magnetic propagation vectors. Furthermore, the magnetic helicity was investigated using circularly polarized resonant X-ray diffraction. In the helimagnetic ordered phase at zero field, it was clarified that two of the four domains have positive helicity, while the other two have negative helicity, reflecting the fourfold and mirror symmetries of the crystal.

When a magnetic skyrmion lattice is formed in a magnetic field, the three helimagnetic components propagating in different directions couple together. We revealed that all three helimagnetic components of the magnetic skyrmion lattice have the same helicity. Surprisingly, the helicity of one of these components is reversed from that in the zero-field helical phase. Another component, which should originally form a longitudinal cycloid structure, also adopts a helical structure with the same helicity as that of other components. This change indicates that the energy gain obtained by unifying the helicity dominates the inherent antisymmetric interactions.

It is intriguing that the preference to form a triangular lattice in the framework of the tetragonal crystal results in the distortion of the triangular skyrmion lattice. This highlights the diversity in the formation of magnetic skyrmion lattices.

(Written by T. Matsumura on behalf of all the authors)

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