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Several series of y-Fe203 nanoparticles dispersed in a polymer have been studied by means of various techniques. The variation of the relaxation time of the magnetic moment of the particle is in very good agreement with the NBel-Brown and our interparticle interaction models which lead to a reduced damping constant varying between 0.08 and 1. For non interacting particles, the main component of the anisotropy comes from the surface. For interacting particles, the interaction energy includes dipolar and surface induced contributions. The susceptibility in the superparamagnetic state shows a strong dependence of the results on the external shape of the sample and the direction of applied field, very well checked experimentally, which is well interpreted with the Onsager model.

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