MAGNETISM IN COMPLEX ATOMIC STRUCTURES - GRAIN-BOUNDARIES IN NICKEL

We study the influence of structural disorder on the magnetic properti es of nanocrystalline nickel, in particular the Sigma 13 and Sigma 5 s pecial grain boundaries and the extreme case of a purely amorphous sam ple, for which experimental results are controversial. The configurati ons were minimized using molecular-dynamics simulations with embedded- atom method potentials. The electronic-structure calculations are perf ormed using the tight-binding linear muffin-tin orbital atomic-sphere- approximation approach. Our calculations reveal that the magnetic mome nt is rather insensitive to the amount of disorder present in the stru cture, varying by at most 20% at the special grain boundaries. These r esults correlate extremely well with recent observations in electropla ted 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 vo lume belongs to interfaces and is amorphous, the average magnetic mome nt 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 t he magnitude of the local moment and the average nearest-neighbor dist ance is observed.