Anisotropic magnetic diffuse scattering in an easy-plane type antiferromagnet ErNi₂Ge₂

We report on neutron scattering studies of a rare earth intermetallic compound ErNi₂Ge₂. Polarized neutron scattering experiments revealed that the magnetic ordered moment m lies in ab-plane. Taking account of a lack of the third higher harmonic reflection, ErNi₂Ge₂ is considered to have a helical magnetic structure. The magnetic scattering profiles along the [100]*- and the [110]*-directions are well described by the sum of Gaussian and modified-Lorentzian terms, even far below TN, indicating that short-range orders coexist with a long-range order. Interestingly, the modified-Lorentzian-type diffuse scattering is not present in the profiles along the [001]*-direction. The anisotropy of the diffuse scattering suggests that the short-range-order consists of one dimensional long-range helices along the c-axis.