Modulated antiferromagnetic structure on a plane triangular lattice
Abstract
It is well known that the ground state of a modulated magnetic structure has a specific continuous degeneracy associated with the uncertainty in the phase of the noncommensurate structure. On the other hand, the existence of continuous degeneracy in a two-dimensional spin system may lead, according to the Mermin-Wagner theory, to a violation of the long-range orientational order and thus cast a doubt on the very possibility of the existence of noncommensurate magnetic structures in two-dimensional systems. Such a situation is analyzed in the present work by considering the example of a modulated antiferromagnetic structure in a plane hexagonal lattice. It is shown that in this case the long-range orientational order is indeed broken by fluctuations. However, the magnetic symmetry is found to be lower than the symmetry of the actual paraphase but higher than the symmetry of the orientationally ordered magnetic state. The method of quasiaverages is used to propose a microscopic description of the statistical equilibrium state of the spin system, and the equations of motion are derived.