Influence of the phenomena occurring at the soft/hard interface on the coercivity behavior in exchange-spring magnets

L10L10-ordered FePd thin films were grown onto MgO-(100) monocrystalline substrates at 450 and 650 °C by means of a molecular beam epitaxy system. Fe/FePd bi-layers were grown by covering the L10L10-FePd hard films with Fe layers of different thickness. For a soft/hard thickness ratio up to 1, these bi-layers show behavior typical of a single phase hard magnet while, for a higher thickness ratio, they behave as exchange-coupled magnets characterized by two critical fields. The bi-layers’ coercivity decreases as a function of the deposited Fe when it is controlled by the domain wall pinning mechanism, while it initially increases and then decreases, when the nucleation of reversed domains is the predominant coercivity mechanism. These different behaviors are due to the phenomena occurring at the soft/hard interface and in particular to the degree of Fe/FePd intermixing and to the composition of the thin interfacial layer.

► Thin Fe/FePd bilayers were grown by means of MBE onto MgO single crystals.
► The hard films show different coercivity mechanisms depending on growth temperature.
► The magnetic behaviors of the bilayers depend on these hard phase mechanisms.
► The coercivity as a function of deposited Fe is influenced by interface phenomena.