Formation and transformation of vortex structures in soft ferromagnetic ellipsoids

Magnetization processes in ideally soft ferromagnetic ellipsoids are studied numerically using the equations of micromagnetism. The ellipsoids have semiaxis in the ratios b/a=c/b=Φ, where Φ is the golden ratio for which (c-b)/a=1. For a≫20 nm, the magnetization processes include the formation and transformation of a vortex core. As vortex core motion takes place with little change in exchange energy, the response of the total magnetization to the applied fields is close to that of an ideally soft magnetic material as long as the vortex is mobile. At critical fields, the vortex core changes between states where it is free to move and those where it is immobilized by symmetry. Complex transformations at critical fields are observed.