Magnetic Anisotropy of the Iron-Group Fluorides

The magnetic anisotropy of single crystals of Fe${\mathrm{F}}_2$ and Co${\mathrm{F}}_2$ has been measured over the temperature range 11 to 340°K. Like Mn${\mathrm{F}}_2$, the anhydrous difluorides of iron and cobalt have the tetragonal rutile crystal structure, and the anisotropy results are compared with those found by Stout and Griffel for Mn${\mathrm{F}}_2$. In all three compounds the magnetic susceptibility parallel to the crystallographic $c$ axis is at room temperatures greater than that in a direction perpendicular to this axis. Because of the orbital magnetic moment the high temperature anisotropy is much greater for Fe${\mathrm{F}}_2$ and Co${\mathrm{F}}_2$ than for Mn${\mathrm{F}}_2$. As the temperature is lowered, the anisotropy changes sign and at 11°K has large negative values, indicating that the susceptibility parallel to the $c$ axis becomes small. This indicates that at low temperatures there is an ordered antiferromagnetic arrangement with the elementary magnetic moments aligned parallel and antiparallel to the $c$ axis. Such a magnetic structure has been found from neutron diffraction measurements by Shull and Erickson. The anomalous behavior of the magnetic anisotropy, associated with the antiferromagnetic ordering, is in the case of Co${\mathrm{F}}_2$ spread out over a much wider temperature range than in the case of Mn${\mathrm{F}}_2$ and Fe${\mathrm{F}}_2$. At the lowest temperatures the anisotropy of Mn${\mathrm{F}}_2$ varies as $T^2$, whereas those of Fe${\mathrm{F}}_2$ and Co${\mathrm{F}}_2$ vary as the fourth power of the temperature.