Anomalies in the magnetic susceptibility in the second-order phase transitions beyond the Curie point

Taking into account the inexhaustible interest in studying the peculiarities of physical properties in the neighborhood of phase transitions and the growth of experimental investigations of cobalt fluoride, we have studied the peculiarities of magnetic susceptibility in the vicinity of the critical field $H_C$ at which cobalt fluoride performs the second-order phase transition from the antiferromagnetic phase to the angular phase. It is discovered that in the magnetic field $\mathbf{H}\parallel C_{4}$, the magnetic susceptibility becomes infinite at $H\to H_C$. It is shown that as the magnetic field direction deviates from the $C_4$ axis, the magnetic susceptibility in the critical field $H_C$ proves to be finite. It is also shown that the change in the magnetic susceptibility with the change in the magnetic field considerably decreases at extremely insignificant deviations of the field H from the $C_4$ axis. Since the calculations are performed in terms of the Landau theory of phase transitions, we pay attention to the similarity and difference between the obtained results and those in the vicinity of the Curie point obtained by using the Landau theory of phase transitions.