Surface-related properties of γ-Fe2O3 nanoparticles

Surface/interface effects on static magnetic properties of γ-Fe2O3 nanoparticles of average diameter 3–10 nm, have been investigated by magnetisation measurements (0–55 kOe, 4.2–300 K) as a function of the field and temperature and by in-field (60 kOe) Mössbauer spectroscopy experiments. The comparison of the results between particle dispersions with different concentrations in polyvinylic alcohol and phosphate-coated particles provides evidence for the interplay between surface anisotropy, intraparticle exchange coupling between surface and core, and interparticle interactions. Enhanced local surface anisotropy and surface spin disorder at low temperature have been found in both systems. In the dispersions, the existence of a canted surface spin structure below ≈85 K is evidenced by in-field Mössbauer experiments and by magnetisation measurements showing a strong increase of the high-field magnetisation with decreasing temperature as the field increases, more and more important with decreasing particle size. In the phosphate-coated particles, consisting of a γ-Fe2O3 core surrounded by a disordered phosphated iron shell, the surface spins are paramagnetic down to ≈30 K, below which they freeze in a speromagnetic-like structure.