Bergmann et al. [Phys. Rev. B 77, 054414 (2008)] present an analytical theory explaining the behavior of ferromagnetic cobalt nanowires with perpendicular anisotropy. This theory, which predicts a sinusoidal variation of the magnetization along the long axis of the wire, depends upon an assumption that "the magnetization is constant within a cross section of the wire." In this Comment we use micromagnetic modeling to show that this assumption does not hold in any relevant setting. For very thin wires, we show that a uniform magnetization configuration is the lowest energy state, which is consistent with some of the larger exchange stiffness results from Bergmann et al. For thicker wires, such as those in the referenced experimental systems, the micromagnetic simulations produce magnetization patterns containing vortices. Across all wire thickness, the sinusoidal configuration has higher energy density than either the uniform configuration or the vortex configuration, and is therefore not attained. The micromagnetic simulations explain not only the periodic magnetization patterns observed in experiments, but also the occasional absence (or disappearance) of periodic structures as described in the literature.
Citation: Physical Review B
Pub Type: Journals
micromagnetics, nanowires, perpendicular anisotropy