Specific features of the states of cobalt fluoride in the vicinity of the critical field

The state of cobalt fluoride in the vicinity of the critical value $H_{c}$ of a longitudinal magnetic field $\mathbf H$, in which the magnetic subsystem of a CoF$_2$ crystal with a strong Dzyaloshinskii interaction is transformed from the antiferromagnetic phase into the canted phase, has been investigated taking into account the increasing number of experimental studies related to the use of cobalt fluoride. It has been found that, despite the unusually high magnetic anisotropy of the crystal, the state of the magnetic subsystem at $H=H_{c}$ is extremely sensitive to a small deviation of the vector $\mathbf H$ from the $C_4$ axis. Another feature is that the high sensitivity disappears with an increase or decrease in the magnetic field by only a few thousandths of $H_{c}$. The results of the investigations performed in this work are applicable to magnetically ordered crystals FeF$_{3}$ and Cu$_{2}$OSeO$_{3}$, which, as well as the CoF$_2$ crystals, are characterized by a strong Dzyaloshinskii interaction and a significant magnetic anisotropy. The revealed anomaly in the reduction of the effective magnetic anisotropy is of interest in connection with numerous attempts to decrease the magnetic anisotropy in crystals with giant magnetostriction, which are necessary for the use as sensors and vibrators.