On the existence of long-range magnetic order in two-dimensional easy-plane magnets

A consistent phenomenological approach is used to show that a true long-ran ge order can exist in two-sublattice two-dimensional antiferromagnets (AFM) and ferrites closed to the compensation point. The effect is due to the lo ng-range component of dipole forces. A similar result was obtained earlier for ferromagnets by Maleev [Sov. Phys. JETP 43, 1240 (1976)], who suggested that the Mermin-Wagner theorem may not be valid for interactions decreasin g in proportion to 1/R-3 or more slowly. It is found that the effect exists in the case of magnets with completely identical sublattices (AFM) only du e to some types of the Dzyaloshinskii-Moriya interaction. For example, it i s observed for AFM with an even (in Turov's sense) principal axis and is ab sent otherwise. For a magnet with nonidentical sublattices, the effect can take place only for ferrites, i.e., for sublattices that are not compensate d in the exchange approximation. The effect of stabilization of long-range order disappears at the point of compensation of magnetic moment. If this p oint does not coincide with the point of compensation of spin angular momen tum, the intensities of fluctuations are nonmonotonic functions of temperat ure. The obtained estimates for the phase transition temperature are compar ed with experimental results. (C) 1998 American Institute of Physics. [S106 3-777X(98)00811-1].