Exchange bias effect in Au-Fe3O4 dumbbell nanoparticles induced by the charge transfer from gold

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2015-08, Vol.92 (5)
• Main Authors: Feygenson, Mikhail, Bauer, John C, Gai, Zheng, Marques, Carlos, Aronson, Meigan C., Teng, Xiaowei, Su, Dong, Stanic, Vesna, Urban, Volker S, Kevin, Beyer, Dai, Sheng
• Format: Article
• Language: English
• Subjects: NANOSCIENCE AND NANOTECHNOLOGY
• ISSN: 1098-0121; 1550-235X

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 wüstite 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.