Magnetic order in γ-Fe2O3 nanoparticles: a XMCD study

Spin-canting in spinel nanoparticles of maghemite has multiple origins, among which surface effects, finite-size effects or chemical disorder effects. XMCD at the Fe L2,3 edges allows to separate the contributions of the magnetic moments of Fe3+ ions in tetrahedral and octahedral sites of γ-Fe2O3. We investigate three powders of γ-Fe2O3 synthetized via aqueous precipitation: particles of average diameter 2.7, 8 nm and particles of average diameter 8 nm coated with phosphoric acid. The relative contributions of the spins of the FeTd3+ and the FeOh3+ ions are observed, varying the external magnetic field. Under high magnetic fields, a reduction of the magnetic contribution of the FeOh3+ ions occurs for 8 nm phosphate-coated particles by comparison with the uncoated ones. A similar reduction appears at lower magnetic fields for the small 2.7 nm particles.