Superparamagnetic γ-Fe2O3

Static magnetization and Mössbauer measurements on ultra-fine crystallites (≈︁ 65 Å) of γ-Fe2O3 show them to be superparamagnetic at 296 °K with an average relaxation time of 2 × 10−9 s. The particle size distribution is derived from the magnetization curve which exhibits no hysteresis, and independently from the temperature variation of the ratio of remanence to saturation magnetization. Mössbauer spectra at 5 °K in an applied field of up to 50 kOe show a cation distribution of 1:(1.71 ± 0.05) for A and B sites, and a non-collinear spin arrangement. The low value of the magnetization, 59 e.m.u./g at 4.2 °K for 1/H = 0, is explained by assuming that the spins of cations, which lie in the surface layer, make random angles between 0 and 90° with the direction of the net moment. Allowance is also made for water adsorbed onto the particle surface. When a field is applied to the absorber at 296 °K the spectrum changes from a broad single peak into two broadened hyperfine patterns from A and B sites which are explained by the magnetic polarization and range of ordering temperature of the particle distribution.