Structural phase transition of chiral magnet DyNi3Ga9 at low temperatures

DyNi$_3$Ga$_9$ is a chiral magnetic material with a space group of $R$32 ${[1]}$, which shows helical magnetic order below $T_N$ = 10 K and canted antiferromagnetic(AFM) order below $T_N$' = 8 K. In addition, transverse elastic modulus $C_{66}$ shows a significant softening toward $T_N$${[2]}$, indicating that a ferroquadrupole order also takes place at $T_N$ .
　 The transition from the helical to the canted AFM structure at $T_N$' is considered to be associated with the underlying ferroquadrupole order. The main interest of this material is in the relation of these successive transitions with the magnetic and quadruupolar exchange interaction, Dzyaloshinskii-Moriya interaction, and the crystal field anisotropy. However, the structural distortion expected from the anomaly in $C_{66}$ has not yet been observed.
　To study the structural transition, we have performed a precise measurement of the lattice parameter at BL-3A of the Photon Factory in KEK. The temperature dependence of the lattice parameter shows an anomaly at $T_N$ and the (6, 0, 0) fundamental reflection splits below $T_N$'. Based on these results, we analyze the crystal field anisotropy and the change in the magnetic structure below $T_N$' by assuming a monoclinic distortion.
The results of magnetic structure and crystal distortion analysis will be presented.
${[1]}$H. Ninomiya $ et. al.$, J. Phys. Soc. Jpn. $\bf 86$, 124704 (2017).
${[2]}$I. Ishii $ et. al.$, J. Phys. Soc. Jpn. $\bf 87$, 013602 (2018).