Quantum Effects at a Spin-Flop Transition in the Antiferromagnetic Topological Insulator MnBi$_2$Te$_4$

It is shown that the experimentally detected features in the low-temperature behavior of the magnetization in an external magnetic field perpendicular to the layers of manganese ions of the topological antiferromagnet MnBi$_2$Te$_4$ are due to quantum effects induced by the off-diagonal nature of the trigonal component of the crystal field. In this case, the anomalous increase in the magnetization of the material before the spin-flop transition, as well as after it in the phase of "collapsed" sublattices, is explained by the suppression of contributions from quantum effects. The comparison of the results of the theoretical analysis with experimental data has made it possible to refine the parameters of the effective spin model of MnBi$_2$Te$_4$ and to establish the important role of the noted trigonal component.