Crystal field parameters at the (0001) surface of rare-earth metals:: an ab initio study

The modifications of the crystal field parameters A,. 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.