THEORY OF THE SYSTEMATIC VARIATION OF THE EXCHANGE COUPLING STRENGTH IN MAGNETIC MULTILAYERS

The amplitude of the exchange coupling between Co or Fe layers separat ed by different nonmagnetic transition metal spacers is observed to de crease exponentially with decreasing number of electrons per atom in t he spacer. An earlier theory based on size quantization of magnetic ca rriers in the spacer layer is generalized to show that the systematic variation of the coupling strength across the transition metal series is due to a misalignment between the ferromagnet and spacer bands whic h increases with increasing number of holes in the spacer. A general p hysical picture of the effect based on conservation laws and phase spa ce availability is presented. Tight-binding calculations confirm the v alidity of the band misalignment theory. It is also shown that the sys tematic variation of the coupling strength cannot be explained by simp le RKKY type theories.