MAGNETIC EXCHANGE IN RARE-EARTH-METAL SUPERLATTICES

A realistic tight-binding model for calculating the Ruderman-Kittel-Kasuya-Yosida exchange interaction in superlattices made of magnetic rare-earth-metal and nonmagnetic yttrium is presented. The energy bands, Fermi surfaces, and generalized susceptibilities of Gd and Y are fitted with the relativistic augmented-plane-wave results. The s-f exchange interaction is also taken into account. The model is used to calculate the nonlocal susceptibility of (Gd)M(Y)N superlattices. It is found that the nonlocal susceptibility chi(r1, r2) between two Gd monolayers located at r1 and r2 in two neighboring superlattice unit cells shows a positive maximum at N = 6 and a negative minimum at N = 10 for the (Gd)10(Y)N superlattices, in agreement with the experimental observation of magnetic ordering. It is also found that the behavior of the nonlocal susceptibility depends not only on the number of Y monolayers, but also on the number of Gd monolayers in a superlattice.