A review of magnetic behaviour of different nanocrystalline systems is presented in this work. Particular emphasis is given to the magnetic properties of the particle shell and grain boundaries, generally different to these of the core, and responsible for phenomena as inter-phase exchange penetration, Curie temperature enhancement and magnetic coupling. Disorder at the surface can be due to competing interactions with different sign originated from broken bonds or topological disorder (grain boundaries), random surface anisotropy, surface magnetostriction, compositional gradients and in general to the enhanced gradient of different properties at the surface.
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