[OPEN ACCESS] Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1-xO|Fe3-δO4 Core|Shell Nanocubes to Single-Phase Particles

ACS Nano, 2013

DOI: 10.1021/nn402487q

Erik Wetterskog , Cheuk-Wai Tai , Jekabs Grins , Lennart Bergström, and German Salazar-Alvarez

Abstract:

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe1-xO|Fe3-δO4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of anti-phase boundaries. Dark-field transmission electron microscopy and powder x-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe1-xO core is removed, but that the anti-phase boundaries remain. The anti-phase boundaries result in anomalous magnetic behavior, i.e., a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.