Crystal field parameters at the (0001) surface of rare-earth metals: anab initiostudy

The modifications of the crystal field parameters A lm at the (0001) surface of the rare-earth metals Tb, Dy, Ho, Er and Tm due to the surface-induced modifications of the charge density are investigated by the ab initio density functional electron theory. For the three outermost atomic layers the values of the parameters A 20 , A 40 , A 60 and A 66 which are nonzero for the hexagonal bulk are drastically different from the bulk values. The additional parameters A 43 and A 63 originating from the symmetry reduction at the surface are very large close to the surface. Altogether, the results show that for a thermodynamic calculation of the spin structure at the surface a simple broken-bond model does not suffice.