Mössbauer measurements on spinel-structure iron oxide nanoparticles

Mössbauer spectra were measured for ⁵⁷Fe in two samples of iron oxide nanoparticles made from iron oleate. One sample was prepared in paste form with a particle diameter of 15 ± 1 nm, a superparamagnetic blocking temperature T_B ≈ 300 K, and a deduced magnetic anisotropy of K ≈ 0.52 x 10⁴ J/m³. The other was prepared as a powder with a particle size of 7.5 ± 1.0 nm and TB ≈ 120 K. At low temperatures their spectra were identical but unlike that of bulk Fe₃O₄. Both samples were concluded to have oxidized from Fe₃O₄ to γ-Fe₂O₃. The powder sample also contained about 30% of α-Fe₂O₃ impurity which was not superparamagnetic but had a reduced Morin temperature.