Uniaxial magnetic anisotropy of rhombohedral CoCO$_3$ crystals at $T$ = 0 K

A method for calculating the contribution of exchange interaction to uniaxial anisotropy with the use of $g'$ factors has been worked out using CoCO$_3$ crystals as an example. The calculated contribution of dipole-dipole interactions to the anisotropy of CoCO$_3$ is 0.93 cm$^{-1}$. The sum of the contributions to the anisotropy constant of CoCO$_3$ with the inclusion of the dipole-dipole interactions is 36.1 cm$^{-1}$.