The magnetic properties of NiF2

Measurements have been made of the magnetic properties of a single crystal of NiF 2 , both above and below its Néel temperature. Below T N the magnetic moment along the appropriate crystal axis contains two contributions. One of these is associated with χ ⊥ of an antiferromagnet and is linear with field and independent of temperature. The other contribution approaches a saturation moment which is temperature dependent and is ferromagnetic in nature. Immediately above T N the paramagnetic susceptibility along the same axis becomes very large as T N is approached, which is unusual in a substance undergoing a transition to the antiferromagnetic state but can be understood in the light of the weak ferromagnetic properties of NiF 2 below T N . These and previous results show that in the ordered state the spins are almost antiparallel in a plane perpendicular to the tetragonal axis, but are tilted away from the antiparallel position in such a way that the crystal acquires a ferromagnetic moment in the same plane perpendicular to the axis of antiferromagnetic ordering.