  We analyze the interplay between disorder and band structure in current
perpendicular to the planes (CPP) giant magnetoresistance (GMR). We consider
finite magnetic multilayers attached to pure crystalline leads, described by a
tight-binding simple cubic two-band model (s-d). Several models of disorder are
considered, including random on-site potentials, lattice distortions,
impurities, vacancies, and cross-section fluctuations. Magneto-transport
properties are calculated in the zero-temperature zero-bias limit, within the
Landauer-Buttiker formalism. Using a very efficient numerical scattering
technique, we are able to perform simulations, over large length scales, and to
investigate spin-transport in the ballistic, diffusive and localized regimes,
as well as the crossover between them. The competition between disorder-induced
mean free path reduction and disorder-induced spin asymmetry enhancement of the
conductance highlights several different regimes of GMR.
