  We report exchange bias (EB) effect in the Au-Fe3O4 composite nanoparticle
system, where one or more Fe3O4 nanoparticles are attached to an Au seed
particle forming dimer and cluster morphologies, with the clusters showing much
stronger EB in comparison with the dimers. The EB effect develops due to the
presence of stress in the Au-Fe3O4 interface which leads to the generation of
highly disordered, anisotropic surface spins in the Fe3O4 particle. The EB
effect is lost with the removal of the interfacial stress. Our atomistic
Monte-Carlo studies are in excellent agreement with the experimental results.
These results show a new path towards tuning EB in nanostructures, namely
controllably creating interfacial stress, and open up the possibility of tuning
the anisotropic properties of biocompatible nanoparticles via a controllable
exchange coupling mechanism.
