Neutron Diffraction Investigation of the Magnetic Order in MnI2

Neutron diffraction measurements were made on polycrystalline and single crystal samples of anhydrous Mn${\mathrm{I}}_2$ at sample temperatures from 298°K to 1.3°K. The results show an unusual type of antiferromagnetic order below the Néel temperature of 3.40°K for this hexagonal layer type crystal. The indicated magnetic structure is a helical type in which the moments within (307) planes are aligned ferromagnetically but rotate by $\frac{2\pi }{16}$ in successive (307) planes. This structure is supported both by intensity measurements of the antiferromagnetic reflections and by the domain transformations effected with an applied magnetic field.