Helical ordering of spin trimers in a distorted kagome lattice of GD₃Ru₄Al₁₂ studied by resonant X-ray diffraction

Successive magnetic phase transitions at T₁ = 17.5 K and T₂ = 18.5 K in Gd₃Ru₄Al₁₂, with a distorted kagome lattice of Gd ions, is studied using resonant X-ray diffraction with polarization analysis. It has been suggested that in this compound the S = 7=2 spins on the nearest-neighbor Gd-triangle form a ferromagnetic trimer and the Gd lattice can be effectively considered as an antiferromagnetic triangular lattice of S = 21=2 spin trimers [S. Nakamura et al., Phys. Rev. B 98, 054410 (2018)]. We show that the magnetic order in this system is described by an incommensurate wave vector q ð0:27; 0; 0Þ, which varies slightly with temperature. In the low temperature phase below T₁, the experimental results are well explained by considering that the spin trimers form a helical order with both the c-axis and c-plane components. In the intermediate phase above T₁, the c-axis component vanishes, resulting in a sinusoical structure within the c-plane. The sinusoidal-helical transition at T₁ can be regarded as an ordering of chiral degree of freedom, which is degenerate in the intermediate phase.