The effect of surface spin disorder on the magnetism of γ-Fe2O3 nanoparticle dispersions

The nanomagnetism of monodisperse 7 nm gamma-Fe2O3 nanoparticles exhibits unique features due to a significant amount of surface spin disorder. To correctly characterize the superparamagnetism of a dilute dispersion requires including the effects of the magnetic anisotropy and a shell of disordered spins surrounding the ordered core. The nanoparticle shell's disordered spin structure is exchange coupled to that of the ordered core. This enables an exchange bias loop shift, Hex, when the nanoparticle dispersion is field cooled. The surface spin disorder also leads to an unusual exponential-like decrease of the nanoparticle's total saturation magnetization with increasing temperature.