Fabrication, Structure, and Magnetism of Transition Metal and Oxide Nanoclusters

Nanoclusters display unusual properties due to the high surface-to-volume atom ratios. Our ability to fabricate nanoclusters with various sizes, structures, compositions and morphologies with a cluster-deposition system provides unique ways to investigate several nanoscale phenomena. In this work, we have investigated the structural and magnetic properties of various transition-metal and oxide clusters. One example is bimetallic MnAu nanoclusters. Annealing induces size dependences for the lattice parameters, tetragonal-distortion ratios, composition, as well as the morphologies. The size dependence for lattice parameters agrees well with density-functional-theory calculations. The size-dependent composition and formation dynamics are in good agreement with previous thermodynamics calculations for similar bimetallic nanoclusters. One especially interesting issue is the formation of L10 MnAu-fcc Mn core-shell clusters. After annealing, with core-shell formation the average magnetic moment is significantly enhanced from 0.17 µB/Mn for the L10 MnAu clusters to 2.1 µB/Mn. The origin of this high magnetic moment is discussed in terms of previous theoretical and experimental work on nanoscale ferromagnetic and ferrimagnetic Mn structures. This size dependence results from Ostwald ripening at high temperature and likely exists for all bimetallic nanoparticles with different mobilities for the two constituent elements. Another class of interesting material is dilute magnetic oxides, the room-temperature ferromagnetism of which has been suggested to be defect-related, especially involving oxygen vacancies. For TiO clusters, both experimental and theoretical work is carried to investigate the role of hydroxyl ions for the ferromagnetism. By utilizing the water dissociative adsorption and creation of hydroxyls at oxygen vacancies, we have shown that the magnetic moment increases initially linearly with increasing exposure time in moisture. Reducing the humidity level or destroying oxygen vacancies by oxygen annealing induces an exponential decay for the magnetic moment. Besides reproducing the magnetic moment creation upon hydroxyl adsorption, density-functional-theory calculations also reveal crucial information regarding the contribution of two types of nonequivalent hydroxyls, the long-term stability, and location of the magnetic moment