The determination of spin and orbital magnetic moments from the free atom to the bulk phase is an intriguing challenge for nanoscience, in particular, since most magnetic recording materials are based on nanostructures. We present temperature-dependent x-ray magnetic circular dichroism measurements of free Co clusters (N=8-22) from which the intrinsic spin and orbital magnetic moments of noninteracting magnetic nanoparticles have been deduced. An exceptionally strong enhancement of the orbital moment is verified for free magnetic clusters which is 4-6 times larger than the bulk value. Our temperature-dependent measurements reveal that the spin orientation along the external magnetic field is nearly saturated at ~20 K and 7 T, while the orbital orientation is clearly not.

We present size dependent spin and orbital magnetic moments of cobalt (Con (+), 8 ≤ n ≤ 22), iron (Fen (+), 7 ≤ n ≤ 17), and nickel cluster (Nin (+), 7 ≤ n ≤ 17) cations as obtained by X-ray magnetic circular dichroism (XMCD) spectroscopy of isolated clusters in the gas phase. The spin and orbital magnetic moments range between the corresponding atomic and bulk values in all three cases. We compare our findings to previous XMCD data, Stern-Gerlach data, and computational results. We discuss the application of scaling laws to the size dependent evolution of the spin and orbital magnetic moments per atom in the clusters. We find a spin scaling law "per cluster diameter," ∼n(-1/3), that interpolates between known atomic and bulk values. In remarkable contrast, the orbital moments do likewise only if the atomic asymptote is exempt. A concept of "primary" and "secondary" (induced) orbital moments is invoked for interpretation.

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Bi-metallic nanoalloys of mixed 3d-4d or 3d-5d elements are promising candidates for technological applications. The large magnetic moment of the 3d materials in combination with a high spin-orbit coupling of the 4d or 5d materials give rise to a material with a large magnetic moment and a strong magnetic anisotropy, making them ideally suitable in for example magnetic storage devices. Especially for clusters, which already have a higher magnetic moment compared to the bulk, these alloys can profit from the cooperative role of alloying and size reduction in order to obtain magnetically stable materials with a large magnetic moment. Here, the influence of doping of small cobalt clusters on the spin and orbital magnetic moment has been studied for the cations [Co(8-14)Au](+) and [Co(10-14)Rh](+). Compared to the undoped pure cobalt [Co(N)](+) clusters we find a significant increase in the spin moment for specific Co(N-1)Au(+) clusters and a very strong increase in the orbital moment for some Co(N-1)Rh(+) clusters, with more than doubling for Co12Rh(+). This result shows that substitutional doping of a 3d metal with even just one atom of a 4d or 5d metal can lead to dramatic changes in both spin and orbital moment, opening up the route to novel applications.

Reactions under single collision conditions with benzene C(6)H(6) and with benzene-d(6) C(6)D(6) of size selected cationic cobalt clusters Co(n)(+) and of anionic cobalt clusters Co(n)(-) in the cluster size range n = 3-28 revealed that dehydrogenation by cationic clusters is sparse, whereas it is ubiquitous in reactions by anionic clusters. Kinetic isotope effects (KIE) in total reaction rates are inverse and, in part, large. Dehydrogenation isotope effects (DIE) are normal. A multistep model of adsorption and stepwise dehydrogenation from the precursor adsorbate unravels a possible origin of the inverse KIE: Single step C-H bond activation is swift (no KIE in forward direction) and largely reversible (normal KIE backward) whereas H/D tunneling is likely to contribute (backward). DFT calculations of the structures and energetics along the reaction path in [Co(13)C(6)H(6)](+) lend support to the proposed multistep model. The observed effects on rates and KIEs of cluster charges and of cluster sizes are noted to elucidate further.