Collective magnetic behavior of biocompatible systems of maghemite particles coated with functional polymer shells

• Core–shell γ-Fe2O3/polymer particles were prepared using new surface active oligoperoxides.
• Grafting of particle shells with functional polymers, suitable for biomedical applications.
• Magnetic properties of nanocomposites are dominated by dipolar interactions.
• On cooling magnetostatic interactions lead to a spin-glass-like state.
• The most diluted maghemite particles display superparamagnetism at 300 K.

Three series of core–shell maghemite nanoparticles were prepared by a template synthesis using surface active oligoperoxides and further surface initiated grafting functional polymers, forming shell suitable for biomedical applications. Because the polymer shells prevent exchange coupling between maghemite particles, the overall magnetic properties of the samples studied are dominated by dipolar interparticle interactions. Only the sample with the highest polymer fraction displays superparamagnetic relaxation phenomena close to the room temperature. On cooling, the magnetostatic interactions lead to a disordered collective magnetic state that should be described in terms of a spin-glass phenomenology. This collective freezing cannot however be considered as a generic spin-glass phase transition at a well-defined temperature but rather as freezing to a metastable glass-like state of locally correlated structural domains (clusters) without a long-range order. A quasi static spin ordering is only achieved at temperatures much below the freezing temperature.