Neutron diffraction study of antiferromagnetic ErNi₃Ga₉ in magnetic fields

We report specific heat, magnetization, magnetoresistance, and neutron diffraction measurements of single crystals of ErNi₃Ga₉. This compound crystalizes in a chiral structure with space group R32. The erbium ions form a two–dimensional honeycomb structure. ErNi₃Ga₉ displays antiferromagnetic order below 6.4 K. We determined that the magnetic structure is slightly amplitude–modulated as well as antiferromagnetic with q=(0,0,0.5). The magnetic properties are described by an Ising–like model in which the magnetic moment is always along the c–axis owing to the large uniaxial anisotropy caused by the crystalline electric field effect in the low temperature region. When the magnetic field is applied along the c–axis, a metamagnetic transition is observed around 12 kOe at 2 K. ErNi₃Ga₉ possesses crystal chirality, but the antisymmetric magnetic interaction, the so-called Dzyaloshinskii–Moriya (DM) interaction, does not contribute to the magnetic structure, because the magnetic moments are parallel to the DM–vector.