Nanostructured heterogeneous systems consisting of antiferromagnetic core and ferromagnetic shell are very interesting due to proximity effects, in particular, the exchange bias. The report presents an overview of the latest results of such bi-magnetic systems studied by neutron diffraction. The oxide systems MnO/Mn3O4 and FeO/Fe3O4, where a shell is synthesized by oxidation of a core, and a new system, where a core is a mesoporous replica of Co3O4 with a gyroidal symmetry with "nanowires" coated with Fe3O4 are considered.
The stable magnetic order in a shell and a core, which persists up to room temperature, much above the magnetic ordering temperatures of core and shell, was discovered. This phenomenon was attributed to proximity effect, when the net ferromagnetic moment at interface biases the magnetic counterparts. Other new phenomena, in particular the unusual magnetic domain fragmentation near the magnetic transition and a considerable enhancement of Néel temperature of the MnO core were found.