Domain-wall excitations in helical phase of spin chain with competing exchange interactions

The classical Heisenberg spin chain with competing exchange interactions of ferro-(F) and antiferromagnetic (AF) types has been considered. This model describes qualitatively properties of the edge-sharing cuprates. The model id characterized by the frustration parameter which is a ratio of the AF- and F-interactions. The ground state of the model is either ferromagnetic or singlet with helical spin correlations in dependence of the frustration parameter. The main attention is given to the study of excited states in the helical phase. These states are domain walls separating the regions with opposite chiralities. It is shown that these excitations are gapped and their energy scales the temperature region in which the phase transition from the helical to the ferromagnetic phase takes place. The calculated energies of domain-walls excitations are used for the determination of the Lifshitz boundary on the phase diagram.