Magnetic order in honeycomb layered U2Pt6Ga15 studied by resonant x-ray and neutron scattering

Antiferromagnetic (AFM) order of U₂Pt₆Ga₁₅ with the ordering temperature TN = 26 K was investigated by resonant x-ray scattering and neutron diffraction on single crystals. This compound possesses a unique crystal structure in which uranium ions form honeycomb layers and then stack along the c axis with a slight offset, which gives rise to a stacking disorder. The AFM order can be described with the propagation vector of q=(1/6,1/6,0) in the hexagonal notation. The ordered magnetic moments orient perpendicular to the honeycomb layers, indicating a collinear spin structure consistent with Ising-like anisotropy. The magnetic reflections are found to be broadened along c*, indicating that the stacking disorder results in anisotropic correlation lengths. The semiquantitative analysis of neutron diffraction intensity, combined with group theory considerations based on the crystallographic symmetry, suggests a zigzag-type magnetic structure for the AFM ground state, in which the AFM coupling runs perpendicular to the stacking offset, characterized as q=(1,0,0)ₒᵣₜₕ. The realization of the zigzag magnetic structure implies the presence of frustrating intralayer exchange interactions involving both ferromagnetic first-neighbor and AFM second- and third-neighbor interactions in this compound.