Solubility extension and phase formation in gas-condensed Co–W nanoclusters
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Co–W alloy clusters with extended solubility of W in hcp Co were produced by inert-gas condensation. The structural state of the as-deposited Co–W clusters was found to be critically dependent on processing parameters such as the cooling scheme and sputtering power. For the water-cooled clusters, the mean size and percent crystalline were strongly dependent on sputtering power, while the percent crystalline of the liquid nitrogen-cooled clusters was not as affected by the sputtering power. At low sputtering powers, the water-cooled clusters were predominantly amorphous, but became increasingly more crystalline as the sputtering power increased. The predominant crystalline phase was hcp Co(W), but high-resolution transmission electron microscopy revealed that very small and very large clusters contained fcc and Co3W structures, respectively. For liquid nitrogen cooling the clusters were predominantly amorphous regardless of sputtering power, although at the highest sputtering power a small percentage of the clusters were crystalline. The magnetic properties were dependent on cooling schemes, sputtering power, and temperature, with the highest coercivity of 893 Oe obtained at 10 K for water-cooled clusters sputtered at 150 W. The magnetocrystalline anisotropy of the water-cooled sample increased with increasing sputtering power, with the highest anisotropy of 3.9 × 106 ergs/cm3 recorded for clusters sputtered at 150 W. For liquid nitrogen-cooled samples, the anisotropy was approximately constant for all sputtering powers.
KeywordsNanomagnetics Cobalt alloys Tungsten alloys Metal clusters Nanofabrication Nanostructured materials Sputter deposition Coercive force Transmission electron microscopy X-ray diffraction
This work was supported by the Department of Energy-Energy Efficiency and Renewable Energy, Vehicles Technology Office, PEEM program, under Contract No. DE-AC02-07CH11358 for the operation of Ames Laboratory (USDOE) and sub-contract no. SC-10-343 to the University of Nebraska-Lincoln.
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