Energy Functional dependence of exchange coupling and magnetic properties of Fe/Nb multilayers

We present an $\it{ab}$ $\it{initio}$ calculation of the exchange coupling for Fe/Nb multilayers using the self-consistent full-potential linearized augmented-plane wave (FLAPW) method. The exchange correlation potential has been treated in the local spin density approximation (LSDA) as well as generalized gradient approximation (GGA). We find that for the LSDA as well as the GGA the exchange coupling oscillates with a period of 6.0 \AA of Nb spacer thickness which is close to the experimental value in the preasymptotic region. This is also close to the earlier calculated period (i.e. 4.5 \AA) by augmented spherical wave (ASW) method. The LSDA shows antiferromagnetic coupling for 2 and 5 Nb monolayers (ML) but the GGA shows the ferromagnetic coupling for all Nb spacer layers. The period of oscillation is found to be in good agreement with the period calculated using the Ruderman-Kittel-Kasuya-Yosida (RKKY) and quantum well (QW) models. The magnetic moment of Fe is found to be higher in the GGA than the LSDA. Fe magnetic moment also shows strong oscillations as a function of the spacer layer thickness, in agreement with the experimental results. We find that the GGA results show better agreement with the experiment than the LSDA results.