14-09U

Exact solutions of the ground-state problem for the classical Heisenberg model with nearest-neighbor interactions on two- and three-dimensional lattices composed of zigzag (triangular) ladders are obtained in a very simple way, with the use of a cluster method. It is shown how the geometrical frustration due to the presence of triangles as structural units leads to the emergence of incommensurate spiral orderings and their collinear limits. These orderings are determined by a single angle (along with the signs of the interactions between neighboring spins); therefore, the term "single-angle spiral orderings" is proposed.