  The spin structure of exchange-coupled MnBi:Co-Fe bilayers is investigated by
X-ray magnetic circular dichroism (XMCD), polarized neutron reflectometry
(PNR), and micromagnetic simu-lations. The purpose of the present research is
two-fold. First, the current search for new permanent-magnet materials includes
hard-soft nanocomposites, and the analysis of coercivity mechanisms in these
structures is an important aspect of this quest. Second, topological
micro-magnetic structures such as skyrmions have recently become of intense
fundamental and applied research, for example in the context of spin-based
electronics. We find that the magnetization reversal of the MnBi:Co-Fe bilayer
structure involves a curling-type twisting of the magnetization in the film
plane. This curling in the exchange-coupled hard-soft magnetic bilayers is
reminiscent of chiral spin structures known as bobbers and, in fact,
establishes a new type of skyrmionic spin structure.
