Stability considerations and dynamic effects in twisted magnetic structures

We theoretically study the creation of a helical-like structure in a soft magnetic material. This forms when a slowly rotating magnetic field is applied, in plane, to a very soft, low-exchange film (50-1000 atomic layers of Cu-doped Permalloy), which is magnetically pinned at the bottom. Both a self-consistent local mean-field theory as well as a spin dynamic approach (implementing the Landau-Lifshitz equation) are employed to find the stable twisted structures. Results from the two methods match well. We find that multiple full rotations of the magnetization---or ``twists''---are possible in these films. At some point an instability occurs in the system at a critical applied field angle, preventing additional full rotations, and often causing one or more twists to be annihilated. The magnetic structure that forms depends on the thickness of the film, the strength of the applied field, and the exchange coupling strength. Analytic energy minimization calculations give an accurate estimate for how many twists will fit into a given film. We study the energy stored and released by the twisted structure, dynamic effects such as the unwinding frequency if the applied field is removed, and potential applications including energy storage This model presents a substantial advancement compared to previous similar studies, as it only fixes one end of the system and applies the rotating magnetic field to the entire structure, enabling a reduction in the strength of the applied field.