MODULATED ANTIFERROMAGNETIC STRUCTURE ON A PLANAR TRIANGULAR LATTICE

It is common knowledge that the ground state of the modulated magnetic structure possesses a specific continuous degeneracy caused by the uncertainty of the incommensurate structure phase. On the other hand, according to the Mermin-Wagner theorem, this continuous degeneracy leads to a frustration of the two-dimensional orientational long-range spin order at any finite temperature and this fact provokes some suspicion concerning the existence of incommensurate planar magnetic structures. Such a situation is analyzed here for a modulated antiferromagnetic structure on a planar traiangular lattice. In this case fluctuations destroy the long-range orientational spin order. Nevertheless, the magnetic symmetry proves to be lower as compared to the genuine paraphase, but, at the same time, higher than the symmetry of the orientationally ordered magnetic state. In the framework of the method of quasiaverages the microscopic description of the equalibrium state and derivation of the motion equations for the modulated planar antiferromagnetic structure are proposed.