Interlayer Exchange Coupling: RKKY Theory and Beyond

Abstract

1 INTRODUCTION The problem of exchange coupling between ferromagnetic transition-metal layers sep-arated by a non-magnetic metal spacer is currently attracting considerable attention. The impetus has been given by the discovery, for Fe/Cr/Fe and Co/Ru/Co systems, that the coupling oscillates periodically in sign and magnitude as a function of the non-magnetic spacer thickness 111. It has been subsequently found that oscillatory cou-pling occurs indeed for almost all transition and noble metals (as spacer material) [2]. The explaination of this remarkable phenomenon is a challenge to the theory and has motivated a number of theoretical investigations. The oscillatory behavior bears much resemblance with the one observed for Ruder-man-Kittel-Kasuya-Yosida (RKKY) interactions between magnetic impurities. Thus the RKKY interaction appears a good candidate for the mechanism of oscillatory in-terlayer coupling. However, when applied in its simplest version (i.e. making a free-electron approximation and assuming a uniform continuous spin distribution within the ferromagnetic layers) [3], the RKKY theory predicts a period A = X F / ~ M 1 monolayer (ML), which is much shorter than the experimental ones. It has been shown then that