Neutron-diffraction study of antiferromagnetic order in the magnetic superconductor ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C

We have carried out powder-neutron-diffraction studies of the antiferromagnetic order that develops below ${\mathit{T}}_{\mathit{N}}$=6.8 K in the magnetic superconductor (${\mathit{T}}_{\mathit{c}}$=11 K) ${\mathrm{ErNi}}_2$${\mathrm{B}}_2$C. The antiferromagnetic structure is associated with spins on the Er atoms, which order in a transversely polarized planar sinusoidal structure propagating along the a or b axis in equal domains, with the Er moments parallel to the b or a axis, respectively. Third and fifth harmonics of the sine wave are observed at low temperatures, indicating a squared-up sine wave, with a low-temperature amplitude of 7.8${\mathrm{\mu }}_{\mathit{B}}$/Er atom. There is also evidence of an induced moment of -0.35${\mathrm{\mu }}_{\mathit{B}}$ on the Ni ions. A similar study of the isostructural compound ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C revealed no magnetic peaks (μ<0.13${\mathrm{\mu }}_{\mathit{B}}$) developing down to 0.3 K for Q values less than 1.6 A${\mathrm{̊}}^{\mathrm{-}1}$.