Surface Effects in Superparamagnetic Magnetite Particles∗

We have studied the properties of unshielded and shielded (hybrid) nanosized spherical magnetite particles with diameter 10 ± 2 nm, which are superparamagnetic at room temperature, through magnetometry, X-ray powder diffraction, magnetic force microscopy imaging and Mössbauer spectroscopy. The unshielded material was prepared by co-precipitation either in air or in inert atmosphere and part of it was shielded subsequently by β-cyclodextrin (βC42H70O35). The studies indicated that in the unshielded particles there is a surface layer with a depth of 3 nm wherein an exponential rise in the number of vacancies is observed in the octahedral sublattice ([B]-sites), so that the particle surface is highly defective and could be represented by the general formula (Fe)A[Fe3+ 5x Fe 2.5+ 2−6x x]BO4, where x = 0–0.3 and denotes vacancies. In contrast, the hybrid particles, being protected from oxidation, are structurally close to bulk magnetite so that the surface magnetic effects could be understood as due to the chaotic orientation of the magnetic moments at the magnetic–nonmagnetic material interface. An empirical model for a spherical single domain particle was developed in attempting to describe the evolution of structural defects in the surface layer. PACS codes: 75.75.-c; 71.30+h