The magnetic behavior of iron oxide passivated iron nanoparticles

Iron oxide passivated iron nanoparticles were synthesized in an inert gas condensation system. The nanoparticles were single domain but not superparamagnetic. Varying amounts of oxide passivation of the nanoparticles were achieved. The oxide-passivated particles exhibit an exchange bias when cooled below a blocking temperature characteristic of the thickness of the oxide layer present. It was found that the exchange bias and blocking temperature both increase with oxide thickness with the blocking temperature in all cases being much lower than the Néel temperature for Fe oxides. We find that the oxide shell-core spin interaction leaves its imprint on the low field dc and ac magnetizations as well. Furthermore, below a characteristic freezing temperature a sharp increase in the field cooled magnetic moment of the samples is evidenced and is suggestive of a spin freezing process at the surface of the particles.