Abstract
We have studied the origin of the exchange bias effect in the Au-Fe3O4 dumbbell nanoparticles
in two samples with different sizes of the Au seed nanoparticles (4.1 and 2.7 nm) and same size
of Fe3O4 nanoparticles (9.8 nm). The magnetization, small-angle neutron scattering, synchrotron
x-ray diffraction and scanning transmission electron microscope measurements determined the antiferromagnetic
FeO w¨ustite phase within Fe3O4 nanoparticles, originating at the interface with the
Au nanoparticles. The interface between antiferromagnetic FeO and ferrimagnetic Fe3O4 is giving
rise to the exchange bias effect. The strength of the exchange bias fields depends on the interfacial
area and lattice mismatch between both phases. We propose that the charge transfer from the Au
nanoparticles is responsible for a partial reduction of the Fe3O4 into FeO phase at the interface with
Au nanoparticles. The Au-O bonds are formed across the interface to accommodate an excess of
oxygen released during the reduction of magnetite.
