Antiferromagnetism and Antiferromagnetic Resonance in a Rhombic Crystal at $T=0\mathrm{}$

Neél's phenomenological theory of antiferromagnetism at zero temperature has been worked out for a three-dimensional case. When the external magnetic field comes near to a certain hyperbola in the field space lying in the plane through the favored and the least favored direction of magnetization, an anomaly occurs. This anomaly consists of a rapid turn-over of the two opposed directions of magnetization into the direction perpendicular to the plane just mentioned. At much higher fields the two opposed directions of magnetization will deviate towards the field, and finally antiferromagnetism will go over into paramagnetic saturation.

Following the general method of Kittel, Ubbink has investigated antiferromagnetic resonance. At not too high frequencies two resonance fields will be flanking the hyperbola in field space. At higher frequencies one resonance field will decrease and vanish, while the other one will approach the field of paramagnetic resonance.