Magneto-optical study of exchange spring degradation

Magnetic structures of SmCo/Fe bilayers during remagnetization and after multiple field cycling are studied magneto-optically (MO). Rotation of the remanent magnetization in the soft Fe layer away from the easy axis is found after large enough remagnetizing fields. This is indicative of the appearance of biquadratic coupling between the layers. It is explained by the fluctuation exchange mechanism due to the field induced magnetic inhomogeneity of the hard layer. Such an inhomogeneity revealed in the MO patterns also explains the degradation of the bias field in exchange springs.