Direct experimental study of the exchange spring formation process

The remagnetization of a soft ferromagnetic film exchange coupled with a high-coercivity ferromagnetic film is studied by a magneto-optic imaging technique. If the magnetic field is antiparallel to the macroscopic unidirectional in-plane anisotropy, the soft layer reverses via the formation of exchange springs consisting of subdomains with opposite spin twistings. However, if the field is instead rotated in-plane, remagnetization initially proceeds via formation of a single uniform exchange spring. Then, at a critical angle, the spring incoherently untwists, leading again to subdomains with opposite chirality. These phenomena are attributed to the influence of inhomogeneity in the unidirectional magnetic anisotropy.