Competition between helimagnetic and ferroquadrupolar orderings in a monoaxial chiral magnet DyNi3Ga9 studied by resonant x-ray diffraction

Successive phase transitions in a rare-earth monoaxial chiral magnet DyNi 3 Ga9 have been investigated by resonant x-ray diffraction. Magnetic dipole and electric quadrupole degrees of freedom arising from the large angular moment of J = 15/2, 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 c-plane form an incommensurate helimagnetic order with q ∼ (0, 0, 0.43) just below TN = 10 K, which further exhibits successive first-order transitions to the commensurate helimagnetic order with q = (0, 0, 0.5) at TN′ = 9 K, and to the canted antiferromagnetic order with q = (0, 0, 0) at TN′′ = 8.5 K. The relation of the magnetic helicity and the crystal chirality in DyNi3 Ga9 is also uniquely determined. Splitting of the (6, 0, 0) Bragg peak is observed below TN′′ , 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.