Orbital magnetism in transition-metal nanostructures

In atoms, Hund's rules predict the maximum orbital angular moment L compatible with maximum spin multiplicity, while in transition- metal (TM) solids, electron delocalization and band formation result in an almost complete quenching of <L>. Such intrinsic differences between atomic and bulk behaviors are characteristic of systems developing itinerant-electron magnetism. Consequently, the orbital contributions to the magnetism of nanostructures - in the way from the atom to the solid - should be at the origin of novel size dependent phenomena that are important both from a fundamental standpoint and in view of applications.

For example, systematic self-consistent studies of magnetic TM clusters have demonstrated the strong dependence of L as a function of size, geometry, and composition including the crossover from atomic to bulklike behavior. A remarkable enhancement of <L> is found, which demonstrates the importance of orbital polarizations, in agreement with recent measurements. For the smallest sizes (N ? 10), <L> can represent 20­­-40% of the total magnetization and is therefore crucial for the comparison between theory and experiment. Large clusters (N ? 150) show nearly bulklike quenching of L at the interior but retain a considerable surface enhancement. In addition, the anisotropy <?L> of the orbital moments shows oscillations as a function of d-band filling that correlate well with previously observed oscillations of the magnetic anisotropy energy (MAE). One concludes that the size- and environment-dependent enhancement of <L> is important for all magnetic TM nanostructures.

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