Switching behavior of Fe-Pt/Ni-Fe exchange-spring films studied by resonant soft-x-ray magneto-optical Kerr effect

We have studied the magnetic switching behavior of exchange-coupled Fe55Pt45/Ni80Fe20 films. On top of magnetically hard Fe55Pt45 films having coercive fields up to 10 kOe were deposited soft magnetic NiFe films of different thickness values to form exchange-spring magnet structures. Magnetometry measurements exhibit the loop shapes characteristic of the exchange coupling between hard and soft magnetic phases: a reversible switching of the soft layer at low fields and an irreversible switching of the hard layer at higher fields. To investigate the switching behavior in more detail we prepared samples with 20-Angstrom Co layers either deposited on top of the soft magnetic NiFe film or at the NiFe-FePt interface. Soft-x-ray magneto-optical Kerr effect measurements performed at the Co L-edge resonance allows us to use the Co layer as a local probe of the reversal process. The element specific Kerr loops reveal that the reversible twist in the soft layer is not pinned rigidly at the interface but rather propagates significantly into the hard magnetic layer. As a consequence the reversible magnetization is not only stored in the soft layer, as often assumed. Additionally the major loops do not exhibit any well-defined switching field of the hard layer. Instead the results indicate that the irreversible switching of the hard phase evolves continuously with increasing external field in this materials system.