Dynamic origin of conical helix magnetization textures stabilized by Dzyaloshinskii-Moriya interaction

Abstract

Since the beginning of the century, the possibility of having chiral spin textures in magnetic materials has been the subject of intense scientific interest. Chiral spin textures have been observed experimentally and described theoretically, bearing potential applications associated with their topological nature. This work theoretically explores the formation of chiral magnetic order in ultrathin magnetic films, where the antisymmetric Dzyaloshinskii-Moriya interaction induces a conical helix magnetization. By minimizing the internal energy of the helix, a simple model predicts the nucleation field, the pitch vector, and the cone angle that characterize the ground-state magnetization texture. It is further demonstrated that the formation of the helical order is connected with the spin waves excited close to the instability of the field-polarized state. Namely, when an in-plane magnetic field is reduced from saturation, a second-order phase transition arises when the spin-wave frequency approaches zero at a critical point where the conical helix nucleates. Interestingly, the wave vector at which the frequency becomes zero matches the pitch vector of the conical helix texture. Thus the instability point of the magnonic excitations is associated with the spin texture, as if the softened spin-wave modes crystallize in the chiral magnetic film. A critical competition among the magnetostatic and the anisotropy field is also found which influences the orbit described by a dynamic magnetization, changing it from circular to elliptical.