Helical and cyloidal magnetic orderings in noncentrosymme­tric EuTGe3 (T=Ir, Ni, Rh)

Abstract

In noncentrosymmetric magnetic materials, various types of nontrivial magnetic structures are realized as a result of competition among symmetric magnetic exchange interaction, Dzyaloshinskii-Moriya–type antisymmetric exchange interaction, and Zeeman energy in external magnetic fields, especially in Gd and Eu compounds with weak crystal-field anisotropy. In many cases, they are non-collinear or non-coplanar structures associated with incommensurate spiral ordering. In this talk, I focus on the tetragonal EuTGe₃ family without an inversion center but with mirror planes including the c axis (space group I4mm). From our recent studies on EuIrGe₃, EuNiGe₃, and EuRhGe₃, using circularly polarized resonant x-ray diffraction to investigate magnetic helicities, it was clarified that each compound exhibits distinctive ordering phenomena reflecting competing interactions. In EuIrGe₃, successive transitions take place from sinusoidal (m∥c) to cycloidal (m∥ac or bc) structures with a tiny reorientation of the propagation vector from q=(0 0 0.792) to (0.017 0 0.792). Each of the four cycloidal domains has its own helicity. In EuNiGe₃, single-q helical ordering at zero field with q=(0.26 0.053 0) transforms into a triple-q distorted-triangular skyrmion-lattice state in a magnetic field. Surprisingly, the original helicity at zero field is reversed to form a skyrmion lattice with unified helicity. In EuRhGe₃, in contrast, the helical order propagating along the c axis is free from the antisymmetric interaction.

[1] T. Matsumura et al., J. Phys. Soc. Jpn. 91, 073703 (2022). (arXiv:2206.06596)
[2] K. Kurauchi et al., J. Phys. Soc. Jpn. 92, 083701 (2023). (arXiv:2306.12669)
[3] T. Matsumura et al., J. Phys. Soc. Jpn. 93, 074705 (2024). (Open Select)

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