We have studied the origin of the exchange bias effect in the ${\text{Au-Fe}}_3{\mathrm{O}}_4$ dumbbell nanoparticles in two samples with different sizes of the Au seed nanoparticles (4.1 and 2.7 nm) and same size of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles (9.8 nm). The magnetization, small-angle neutron-scattering, synchrotron x-ray diffraction, and scanning transmission electron microscope measurements determined the antiferromagnetic FeO wüstite phase within ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles, originating at the interface with the Au nanoparticles. The interface between antiferromagnetic FeO and ferrimagnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ 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 ${\mathrm{Fe}}_3{\mathrm{O}}_4$ into the FeO phase at the interface with Au nanoparticles. The Au-O bonds are formed, presumably across the interface to accommodate an excess of oxygen released during the reduction of magnetite.

• Received 4 May 2015
• Revised 20 July 2015

DOI:https://doi.org/10.1103/PhysRevB.92.054416