Quantum well states in Fe/Nb(001) multilayers: First principles study

We present a first-principles study to understand the phenomena of interlayer exchange coupling in Fe/Nb multilayers using the linearized-muffin-tin-orbitals method within the generalized gradient approximation. We find that the exchange coupling oscillates with both short and long periodicities, which have been examined in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) model as well as the quantum well (QW) model. We have investigated the behavior of the exchange coupling by artificially varying moments of Fe atoms in ferromagnetic layers. For a small moment of Fe, the coupling shows bilinearity in the magnetic moments, implying its RKKY character. However, at higher moments close to that of bulk Fe, the saturation of long-period oscillations is in accordance with the QW model. Quantum well dispersions around the Fermi level demonstrate that the majority-spin bands contribute largely to the formation of quantum well states, which we analyze quantitatively by making use of the phase accumulation model. Our analysis indicates that the quantum well model gives a better description of the oscillatory behavior of the exchange coupling in Fe/Nb multilayers.