Some New Results on Antiferromagnetism and Ferromagnetism

1. The study of the thermomagnetic properties of a natural single crystal of rhomobohedral iron sesquioxide, ${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$, by Pauthenet, confirms the hypothesis of the superposition of a fundamental antiferromagnetism and a parasitic ferromagnetism. Below 260°K the direction of antiferromagnetism coincides with the ternary axis; above 260°K this direction is situated in the plane perpendicular to the ternary axis, with complete freedom of orientation in this plane. The parasitic ferromagnetism seems to be formed of two parts, one isotropic, the other anisotropic. The latter, tightly coupled with the direction of antiferromagnetism, can be observed only above 260°K and in a direction perpendicular to the ternary axis. The corresponding spontaneous magnetization may arise from the local imperfections in the compensation of the two constituent sublattices of the antiferromagnetism, and it is the freedom of orientation of the antiferromagnetism in the basic plane which permits observation of this spontaneous magnetization.