MICROMAGNETISM AND THE MICROSTRUCTURE IN NANOCRYSTALLINE MATERIALS

Micromagnetic calculations using computational techniques are a pre-re quisite to treat complex magnetization processes in ensembles of ferro magnetic grains. Applying the finite element method spin structures of small spherical prismatic and platelet-like particles and the critica l fields for the reversal of magnetization have been determined as a f unction of grain size, isotropic and textured distribution of easy axe s and different magnetic properties of grain boundaries. Adapting the mesh sizes of finite elements to the gradients of the direction cosine s of the spontaneous magnetization, inhomogeneous spin distributions c ould be determined within the grains as well as within grain boundarie s. Numerical results have been obtained for nanocrystalline single pha se and composite materials taking into account exchange and dipolar co upling between grains and inhomogeneous magnetic material parameters w ithin the grain boundaries. The results of these investigations show c learly that large coercivities require exchange decoupling between the grains. Dipolar long-range magnetic stray fields reduce the coercivit y mainly for large grain sizes, whereas the remanence enhancing by exc hange coupling becomes effective for grain sizes of the order of twice the domain wall width of the hard-magnetic phase. For larger grain si zes the coercivity breaks down. From the model calculations general ru les for the development of optimized nanocrystalline materials with la rge remanences and large coercivities are derived.