Exchange bias behavior of monodisperse Fe3O4/γ-Fe2O3 core/shell nanoparticles

We have carried out systematic studies on well-characterized monodisperse Fe3O4/γ-Fe2O3 core/shell nanoparticles of 2–30 nm having a very narrow size distribution and possessing a uniquely mono-layer of surface γ-Fe2O3. This unique core-shell structure, probably having a disordered magnetic surface state, leads us to three key observations of unusual magnetic properties: i) a very large magnetic exchange anisotropy reaching over 7 × 106 erg/cm3 for the smaller particles, ii) exchange bias behavior in the magnetization data of the core/shell Fe3O4/γ-Fe2O3 nanoparticles, and iii) the temperature dependence of the coercive field following an unusual exponential behavior.

► We report on the extensive studies of Fe3O4 nanoparticles in core-shell structure. ► We discover very large magnetic exchange anisotropy due to core-shell coupling. ► Exchange bias behavior is found in the magnetization data. ► Our data show an unusual temperature dependence of the coercive field.