Random dipolar ferromagnetism in Co∕CoO core-shell cluster assemblies observed by electron holography

The formation of large-scale ferromagnetic domains has been detected in quasi-three-dimensional random assemblies of monodisperse 6nm Co∕CoO core-shell clusters by electron holography. This is primarily attributed to the magnetic dipole interactions between these clusters, because the exchange coupling between neighboring Co cores is effectively interrupted by antiferromagnetic CoO surface layers. The magnetic field-cooled effect has been also detected by in-situ electron microscope experiment. The field-induced alignment of magnetic moments during the cooling experiment is ascribable to the exchange interactions between Co cores and CoO shells.