Magnetism in complex atomic structures: Grain boundaries in nickel

We study the influence of structural disorder on the magnetic properties of nanocrystalline nickel, in particular the Sigma 13 and Sigma 5 special grain boundaries and the extreme case of a purely amorphous sample, for which experimental results are controversial. The configurations were minimized using molecular-dynamics simulations with embedded-atom method potentials. The electronic-structure calculations are performed using the tight-binding linear muffin-tin orbital atomic-sphere-approximation approach. Our calculations reveal that the magnetic moment is rather insensitive to the amount of disorder present in the structure, varying by at most 20% at the special grain boundaries. These results correlate extremely well with recent observations in electroplated nickel that the magnetic moment depends very little on grain size, down to about 10 nm, i.e., is not critically determined by the amount of matter in the grain boundaries. Even in the limit where all the volume belongs to interfaces and is amorphous, the average magnetic moment is reduced by only about 15%. The local moments in amorphous nickel vary between 0.4 mu(B) and 06 mu(B), and a weak correlation between the magnitude of the local moment and the average nearest-neighbor distance is observed.