Spin freezing in maghemite nanoparticle systems

The static and dynamical properties of two powder samples of gamma -Fe2O3 n anoparticles, with average diameter D = 2.7 and 4.6 nm have been investigat ed by AC and DC susceptibility measurements. The results provide evidence o f a collective dynamical freezing due to dipole interactions, at a size-dep endent temperature (T = 95 K for D = 2.7 nm; 130 K for D = 4.6 nm). Between 50 and 60 K, a change of magnetic regime occurs for both samples induced b y increasing surface effects with decreasing temperature. (C) 2001 Elsevier Science B.V. All rights reserved.