Surface and Internal Spin Canting in γ-Fe₂O₃ Nanoparticles

Structural and magnetic properties of γ-Fe 2 O 3 have been studied in isometric nanoparticles ranging from 3 to 14 nm with a narrow particle size distribution. Cation vacancy order is observed for particles larger than 5 nm in diameter giving rise to a cubic superstructure, while for the smallest particles these vacancies are disordered. All magnetic properties measured showed a strong dependence on the average crystallite size. For the ordered samples, saturation magnetization was found to decrease linearly with decreasing crystallite size due to a surface spin canting effect. However, a stronger decrease was observed in the disordered samples, suggesting that also an internal spin canting (cation vacancy order−disorder) has to be taken into account to explain the magnetic properties of nanoparticles. The room-temperature coercive field decreases with decreasing crystallite size; however at low temperatures, the coercivity increases as the size decreases, reaching values larger than 3000 Oe. A model to explain the magnetic properties of these particles considering both surface and order−disorder effects is proposed.