G. Uimin et A. Pimpinelli, HELIMAGNETISM IN XY MODELS - DOMAIN-WALLS, FRUSTRATIONS, FRACTIONAL VORTICES, AND PHASE-TRANSITIONS, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 49(2), 1994, pp. 1123-1135
We consider in this paper a model for two-dimensional XY helimagnets,
which was proposed in Pimpinelli et al. [J. Phys. Condens. Matter 3, 4
693 (1991)]. In this model the usual spin waves and vortices coexist w
ith chirality degrees of freedom in such a way that the degeneracy spa
ce of the order parameter is O(2)XZ2. In Pimpinelli et al. the low-tem
perature behavior of the model was addressed, and an infinite sequence
of first-order phase transitions was shown to take place, due to the
spin-wave coupling between domains with opposite chirality. Here we in
vestigate the same model at higher temperatures, where vortices in the
spin system and kinks on domain walls are thermally excited. The kink
s in domain walls are associated with noninteger (fractional) vortices
. This is presumably a common feature also in conventional helimagnets
, where the Z2 chirality is due to the twofold handedness of helices.
We also discuss the applicability of this scenario to the helimagnetic
compound BaCO2(AsO4)2.