Theory of Magnetism of NiF2

The magnetic properties of Ni${\mathrm{F}}_2$ are studied theoretically with the use of a spin Hamiltonian approach. It is shown that the anisotropy of the susceptibility above the Néel temperature indicates a spin arrangement below the Néel temperature in which all the spins are perpendicular to the $c$ axis and the angle between the two sublattice magnetizations is a little smaller than $\pi$, giving rise to a small net ferromagnetic moment along the $〈100〉$ direction. This model explains the torque data below the Néel temperature quantitatively. The spin wave frequencies and the magnetic resonance frequencies are calculated. There is a low-frequency branch whose lowest frequency ($k=0\mathrm{}$) corresponds to the anisotropy energy in the $\mathrm{ab}$ plane. The magnetic resonance is expected at around 170 kilomegacycles/second. A recent nuclear magnetic resonance measurement by Shulman seems to support the present model. The spin arrangement below the Néel temperature proposed by Erickson from his neutron diffraction data seems to be neither possible theoretically nor consistent with the other experimental data. Possible structures of domains and domain walls are discussed based on the same model.