The isotropic magnetic moment of a free atom is shown to develop giant magnetic anisotropy energy due to symmetry reduction at an atomically ordered surface. Single cobalt atoms deposited onto platinum (111) are found to have a magnetic anisotropy energy of 9 millielectron volts per atom arising from the combination of unquenched orbital moments (1.1 Bohr magnetons) and strong spin-orbit coupling induced by the platinum substrate. By assembling cobalt nanoparticles containing up to 40 atoms, the magnetic anisotropy energy is further shown to be dependent on single-atom coordination changes. These results confirm theoretical predictions and are of fundamental value to understanding how magnetic anisotropy develops in finite-sized magnetic particles.Permanent magnets play a key role in technology and industry, ranging from the generation and distribution of electrical power to communication devices and the processing of information (1, 2). The property that makes them so useful is the magnetic anisotropy energy (MAE), which describes the tendency of the magnetization to align along specific spatial directions rather than randomly fluctuate over time. The MAE determines the stability of the magnetization in bulk as well as nanoparticle systems. Extensive studies on ferromagnetic bulk materials and thin films have highlighted the MAE dependence on crystal symmetry and atomic composition (1). Whereas the exchange interaction among electron spins is purely isotropic, the orbital magnetization, via the spinorbit interaction, connects the spin magnetization to the atomic structure of a magnetic material, hence giving rise to magnetic anisotropy (3, 4). With respect to bulk solids, surfacesupported nanoparticles offer additional degrees of freedom to tune the MAE by ad hoc modifications of the particle size, shape, and coupling with the substrate, making nanosized systems attractive for basic investigations as well as for miniaturized data-storage applications (5, 6). However, fundamental points remain unclear: how the MAE evolves from single atoms to finite-size magnetic particles, how it correlates to the atomic magnetic moments, and how both depend on the details of the atomic coordination.Free transition metal (TM) atoms possess large spin (S) and orbital (L) magnetic moments according to Hund's rules. Conversely, the survival of localized magnetic moments on TM impurities dissolved in nonmagnetic metal hosts is a long-debated problem in solid-state physics (7,8). Electron delocalization and crystal field effects compete with the intra-atomic Coulomb interactions, responsible for Hund's rules, causing a substantial or total decrease of S and quenching of L. Theoretical calculations, however, predict such effects to be strongly reduced at surfaces owing to the decreased coordination of TM impurities, with implications also for the appearance of substantial magnetic anisotropy (9). The determination of S and particularly of L and the MAE for isolated surface adatoms, however, has been hampered by the lack of experiment...
