Neutron Diffraction Studies of Antiferromagnetism in Manganous Fluoride and Some Isomorphous Compounds

Neutron diffraction studies of Mn${\mathrm{F}}_2$, Fe${\mathrm{F}}_2$, Co${\mathrm{F}}_2$, and Ni${\mathrm{F}}_2$ are reported. At low temperatures each compound showed the superlattice structure characteristic of antiferromagnetism. The data are accounted for with a magnetic cell of the same dimension as the chemical unit cell; the moments of the magnetic ions at the lattice corners being antiparallel to the moments of the body centered ions. The alignment of the magnetic moments is along the tetragonal axis in all cases except Ni${\mathrm{F}}_2$ where the alignment direction makes an angle of about 10° relative to the $c$ axis. The intensity of the magnetic scattering as a function of sample temperature follows a Brillouin curve of the form suggested by Van Vleck. Extrapolating the saturation data to zero peak intensity gives the Néel temperatures for threshold antiferromagnetism. These are Mn${\mathrm{F}}_2$-75°K; Fe${\mathrm{F}}_2$-90°K; Co${\mathrm{F}}_2$-50°K; Ni${\mathrm{F}}_2$-83°K.