Competition between helimagnetic and ferroquadrupolar orderings in a monoaxial chiral magnet DyNi${}_3$Ga${}_9$ studied by resonant x-ray diffraction

Successive phase transitions in a rare-earth monoaxial chiral magnet DyNi${}_3$Ga${}_9$ have been investigated by resonant x-ray diffraction. Magnetic dipole and electric quadrupole degrees of freedom arising from the large angular moment of $J=15\mathrm{/}2$, in combination with the symmetric and antisymmetric exchange interactions and the crystal field anisotropy, give rise to competing ordered phases. We show that the antiferromagnetically coupled Dy moments in the $ab$-plane form an incommensurate helimagnetic order with $q\sim(0, 0, 0.43)$justbelow$T_{$\text{N}$}=10$ K, which further exhibits successive first-order transitions to the commensurate helimagnetic order with $q=(0,0,0.5)$ at $T_{\text{N}}^{\mathrm{\prime }}=9.0$ K, and to the canted antiferromagnetic order with $q=(0,0,0)$ at $T_{\text{N}}^{\mathrm{\prime }\mathrm{\prime }}=8.5$ K, both with large coexistence regions. The relation of the magnetic helicity and the crystal chirality in DyNi${}_3$Ga${}_9$ is also uniquely determined. Splitting of the $(6,0,0)$ Bragg peak is observed below $T_{\text{N}}^{\mathrm{\prime }\mathrm{\prime }}$, reflecting the lattice distortion due to the ferroquadrupole order. In the canted antiferromagnetic phase, a spin-flop transition takes place at 5 K when the temperature is swept in a weak magnetic field. We discuss these transitions from the viewpoint of competing energies described above.