Magnetic coupling and spin disorder in Co and Fe nanocrystalline ferromagnets

The magnetic properties of two different nanocrystalline systems are described. The magnetic properties strongly depend on the exchange coupling between different phases that take place across the interface. For the first case considered here the magnetic ground state of the interfaces corresponds to a speromagnetic structure. Disorder at the grain boundaries and interfaces can be due to competing interactions with different signs originating from broken bonds or topological disorder, random surface anisotropy, surface magnetostriction, compositional gradients and, in general, to the enhanced gradient of different properties at the interfaces. The speromagnetic ground state of the interfaces evolves with temperature towards either a paramagnetic or a ferromagnetic configuration depending on the exchange and anisotropy of the phases. For the second case the interface is paramagnetic.