Abstract
The more interesting features in magnetism of systems of nanoparticles are reviewed. Tailoring of soft and hard magnetic materials as well as basic studies on magnetic interactions are discussed. 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 such as interphase exchange penetration, Curie temperature enhancement and magnetic coupling. The magnetic behaviour of different nanocrystalline systems has been described. Spin disorder has been found to be a general trend for the magnetic ground state of the outer shell of magnetic particles. Disorder at the surface can be due to competing interactions with different signs originating 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. The spin-glass-like ground state of the surface only affects the macroscopic properties in nanocrystalline samples for which the ratio between the number of atoms at the interface and the number of atoms in the core can be enormous, actually as large as 30%.
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