Orbital Magnetism in Low Dimensional Systems: Surfaces, Thin Films and Clusters

Abstract

The local and average orbital magnetic moments (L) of several low-dimensional systems are determined in the framework of a self-consistent tight-binding theory. For transition metal surfaces, results for the local orbital magnetic moments L _δ(i) at different layers i and magnetization directions 8 are discussed. It is shown that L _δ(i) is significantly enhanced at the surface atoms as compared to the corresponding bulk moment L _δ(bulk). (L) depends strongly on the local coordination number and is generally larger the more open the surface is and decreases abruptly as we move from the uppermost layer (i = 1) to the second layer (i = 2). After some oscillations, convergence to bulk values is reached. The orbital moments at pure surfaces are compared with results for deposited films by considering Co on Pd(111) as a representative example. The role of L on the magneto-anisotropic behavior of thin Co/Pd(lll) films is also discussed. For Ni clusters, a remarkable enhancement of the average orbital moment per atom is observed, which for the very small sizes can be up to an order of magnitude larger than the corresponding bulk value [e.g., L(Ni₇) ≃ 0.5 µ _B while L(Ni-bulk) ≃ 0.05 µ _B]. The relation between the enhancement of the local orbital moments L(i) and the reduction of local coordination number is discussed, in particular by comparison with results for TM surfaces. The transition from atomic to bulk-like behavior is determined. For large clusters (N = 100-150 atoms) we observe bulk-like quenching at inner atoms and enhanced local moments at the cluster surface. The importance of orbital contributions to the total moments μ _N is quantified. In addition, (L_N) amounts to 20-35% of µN and is therefore crucial for comparing theoretical results with experiment