Morphology and magnetism of Fen clusters (n=1-9) supported on a Pd(001) substrate

We present a detailed ab initio density-functional study of the morphology and magnetic and electronic properties of small Fe-n (n = 1-6,9) clusters and Fe monolayers supported on a Pd(001) substrate. Our focus is on the variation of the cluster moments with the size and shape of the cluster and on the induced magnetization of the substrate. For the smallest clusters, the magnetic moments of the Fe atoms are strongly enhanced compared to bulk Fe. With increasing size of the cluster, the moment decreases linearly with the number of Fe nearest neighbors. The magnetic moment of the adsorbed cluster induces a substantial magnetization of the substrate. The induced magnetic moments are largest on the Pd atoms binding to the cluster atoms, increasing with the number of Fe neighbors. The induced magnetic polarization is quite long ranged, and the enhanced magnetic moments of the Fe atoms and the induced magnetization of the substrate add up to a "giant effective moment" per Fe atom ranging between 7.6 mu(B) for an isolated adatom and 4.6 mu(B) for a nine-atom cluster. Relaxation of the cluster structure reduces the Fe moments but enhances the induced moments. Orbital moments induced by spin-orbit coupling have been determined via self-consistent relativistic calculations for an isolated Fe adatom. Due to the strong hybridization, the orbital moment on the Fe atom remains rather modest, but a much higher ratio of orbital and spin moments is found for the Pd atoms of the substrate. Relativistic effects also lead to a substantial anisotropy of the induced spin and orbital moments, which makes a significant contribution to the magnetic anisotropy energy.