Transition from an incommensurate spin density wave to a commensurate magnetic order in a triangular lattice compound Ho₂PdAl₆Ge₄

Rare-earth (RE) intermetallics on a triangular lattice are promising candidates for generating interesting magnetic phases due to the complex interplay between Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction and geometrical frustration. Here we report the exotic magnetic structure of a layered compound Ho₂PdAl₆Ge₄ with triangular lanthanide nets. Magnetization and heat capacity measurements in zero magnetic field reveal two magnetic phase transitions at T_N1 = 10.8 K and T_N2 = 6.0 K. Neutron powder diffraction demonstrates a commensurate antiferromagnetic phase with k₁ = (0, 0, 1.5) below T_N2. With increasing temperature, another incommensurate vector appears and therefore, the magnetic structure of the intermediate state is identified as an unusual incommensurate spin density wave with two propagation vectors k₁ = (0, 0, 1.5) and k₂ = (0.0492, 0.0492, 1.5). The magnetic moments in the intermediate state rotate continuously and form an unusual S-shaped wave arrangement in the ab plane, sharing similarities with typical cycloid and helix magnetic orders. These results identify Ho₂PdAl₆Ge₄ as a candidate for exploring field-induced topological magnetic phases such as skyrmions, opening the way for further investigations on the family of RE₂PdAl₆Ge₄ materials.