Surface and Internal Spin Canting in γ-Fe₂O₃ Nanoparticles

TLDR

Structural and magnetic properties of γ‑Fe₂O₃ have been studied in isometric nanoparticles sized 3–14 nm with a narrow size distribution. The study proposes a model explaining the magnetic properties of γ‑Fe₂O₃ nanoparticles by incorporating both surface and order–disorder effects. The model attributes the size‑dependent saturation magnetization to surface spin canting and, for disordered samples, to internal spin canting arising from cation vacancy disorder. Cation vacancy ordering appears in particles larger than 5 nm, while smaller particles exhibit disordered vacancies, and magnetic properties—including saturation magnetization and coercivity—vary strongly with crystallite size, with surface spin canting driving a linear decrease in magnetization for ordered samples and internal spin canting further reducing magnetization in disordered ones, and coercivity decreasing at room temperature but increasing to over 3000 Oe at low temperatures as size shrinks.

Abstract

Structural and magnetic properties of γ-Fe2O3 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.