EFFECT OF PRESSURE ON MAGNETIC STATES OF EASY-PLANE ANTIFERROMAGNETS

The phase II-P-beta diagram of an easy-plane strongly anisotropic anti ferromagnet is studied. It is shown that in such a system a realizatio n of the phase with the nonmagnetic order parameter, the so-called qua druple phase, is possible. The realization of this phase is caused by several purely quantum effects, moreover the mechanism of its appearan ce differs in principle from that of the antiferromagnet or ferromagne t. The possibility of the disappearance of the angular phase (the so-c alled quadruple ferromagnetic phase) is also shown.