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.

2. Pyrrhotite corresponds closely to the molecular formula ${\mathrm{Fe}}_7$${\mathrm{S}}_8$. The crystal structure contains holes: one of the eight sites which could be occupied by iron ions is vacant. Bertaut showed that at room temperature these holes form an ordered lattice such that the crystal unit of ${\mathrm{Fe}}_7$${\mathrm{S}}_8$ does not have the NiAs unit but one much more complicated, pseudohexagonal and slightly monoclinic, containing 8 ${\mathrm{Fe}}_7$${\mathrm{S}}_8$. This unit is such that the successive planes of iron perpendicular to the pseudo-hexagonal axis are not identical: only the odd-numbered planes contain holes. Because the two sublattices are crystallographically different, we have imperfect antiferromagnetism or ferrimagnetism, analogous to that of ferrites. A magnetic study by Benoit, at temperatures above the Curie point, seems to show that the ordered distribution of the holes disappears at 560°K. Finally, a thermomagnetic study by Pauthenet shows that the pseudo-hexagonal axis, which at ordinary temperature is a direction of very difficult magnetization, becomes progressively, at lower temperatures, a direction of easy magnetization.