Magnetic alloy-C core-shell nanoparticles with tunable elemental components are highly expected for their controllable magnetic, catalytic, and electromagnetic properties, and high air stability. Here we present a simple one-step metal organic chemical vapor deposition for the synthesis of CoNi-C core-shell nanoparticles with tunable Co/Ni molar ratio. The as-synthesized nanoparticles have core sizes of 5-50 nm, shell thicknesses of similar to 5 nm, and tunable Co/Ni molar ratio determined by the molar ratio of Co(acac)(3)/Ni(acac)(2) used in the precursor. The core-shell nanoparticles are ver...