Magnetism and electronic structure of hcp Gd and the Gd(0001) surface

We investigate hcp Gd and the Gd(0001) surface on the basis of density functional theory. The localized 4f states of Gd, which represent a challenge for first-principles theory, are treated in four different models, employing consistently the full-potential linearized augmented plane-wave method. Our results support previous findings that within the local density approximation (LDA) or generalized gradient approximation (GGA) the itinerancy of the 4f states is overestimated. In particular, the large density of states at the Fermi energy due to the minority 4f electrons is unphysical, and our results show that this is the origin of the incorrect prediction of the antiferromagnetic ground state for hcp Gd by many LDA and GGA calculations. We show that different models of removing these states from the region close to the Fermi energy, for example the treatment of the 4f electrons as localized core electrons or by using the LDA + U formalism, lead to the prediction of the correct ferromagnetic ground state for the bulk and a ferromagnetically coupled (0001) surface layer. With these models ground-state properties such as the magnetic moment and structural parameters can be determined in good agreement with experiment. The energetic positions of the surface states of the Gd(0001) surface are compared with experimental data.