Chapter 11 - Uncovering New Magnetic Phenomena in Metal Clusters

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Abstract The study of metal-containing clusters in isolated form offers the opportunity to observe the emergence of novel magnetic behavior atom-by-atom, in the size range that bridges atomic and bulk properties. In this contribution, the results of Stern–Gerlach molecular beam deflection studies of bare and adsorbate-covered transition metal clusters will be presented. Clusters of the normally ferromagnetic transition metals, Fe n , Co n , and Ni n deflect strongly toward high magnetic fields, consistent with superparamagnetic behavior as is displayed by larger ferromagnetic particles. Magnetic moments obtained by Curie Law analysis show that their mean magnetic moments per atom are larger than those of the corresponding bulk solids because of the combined effects of enhanced spin polarization and unquenched orbital contributions. Atomic and molecular adsorbates can significantly influence the magnetism of small ferromagnetic clusters. In the case of Ni n , a single oxygen atom or CO molecule can reduce the moment of the underlying cluster by several μ b . In the case of Fe n , chemisorbed hydrogen is found to strongly enhance the magnetic moments for certain cluster sizes. Clusters of normally nonmagnetic transition elements also can display unexpected magnetic behavior. Bare manganese clusters (Mn 5–99 ) exhibit spatial deflections or broadening of magnitudes far in excess of those expected on the basis of the susceptibility of bulk manganese, indicating that clusters in this size range are magnetically ordered. The magnitude of the magnetic moments, interpreted in light of recent density functional theory calculations, suggest that Mn clusters in this size range are molecular ferrimagnets. Magnetic deflection results for clusters composed of the Group IIIB metals, Sc n , Y n , and La n , indicate that the majority of clusters in the n = 5─20 range are magnetically ordered. Several Group IIIB clusters stand out as bona fide high-spin molecular magnets: Sc 13 (6.0 ± 0.2μ b ), Y 8 (5.2 ± 0.1μ b ), Y 13 (9.6 ± 0.1μ b ), and La 6 (4.8 ± 0.2μ b ).