The relationship between interlayer spacing and magnetic ordering in gadolinium

We report on the application of the local spin-density approximation (LSDA) and the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) within the linear muffin-tin orbital method in both the atomic sphere approximation (LMTO-ASA) and in the full-potential (FP-LMTO) method to the description of the magnetic coupling within bulk Gd. Using the LMTO-ASA approach to the band-structure problem it is found that, at the experimental lattice parameters, the PBE approximation predicts the experimentally observed ferromagnetic (FM) ground state whereas the LSDA does not. Moreover the nature of the magnetic coupling between successive layers is found to be dependent on the interlayer separation - in particular a reduction of the interlayer spacing will lead to an increased tendency towards FM coupling between successive layers and, conversely, increase of the interlayer spacing will lead to antiferromagnetic (AFM) coupling between layers being energetically favourable. A similar interdependence between the interlayer spacing and the magnetic coupling is also observed from calculations using the FP-LMTO method. These observations are used to analyse the nature of the magnetic coupling of the Gd(0001) surface to the underlying FM bulk.