INTERLAYER COUPLING IN MAGNETIC MULTILAYERS - ANALOGY TO SUPEREXCHANGE PROCESSES IN INSULATORS

The mechanism of interlayer exchange coupling in transition metal sandwiches and multilayers is currently a topic of great interest, the challenge being to explain how in some cases the coupling is antiferromagnetic (across Cu and Cr, for example), in other cases it is ferromagnetic (e.g. Au, Pd, Ag, Mo,...) and possibly oscillates in sign for others (e.g. Ru). In this paper we investigate the magnetic energy terms which arise from a fourth order perturbation treatment of the hybridization between magnetic 3d states (e.g. Co) and the conduction electrons considered as a free electron band (e.g. Cu). The calculations yield terms which describe oscillating RKKY-type interactions and terms which describe antiferromagnetic coupling which occurs through processes which may be termed 'superexchange interactions through the conduction electron band', by analogy with superexchange in insulators. These latter antiferromagnetic terms may dominate the interlayer coupling.