In an aspect, a method of making a composite core-shell nanoparticle comprises forming a nanoparticle core comprising nickel oxide or iron oxide via thermal decomposition of a nickel complex or an iron complex; and forming an oxide shell over the core, the oxide shell comprising nickel, iron or a mixture thereof. In another aspect, a method of making composite nanoparticles comprises providing a mixture comprising nickel complex and iron complex; and thermally decomposing the nickel and iron complexes to provide the composite nanoparticles comprising (Ni,Fe)O.sub.x alloy. In yet another aspect, a composition comprises composite nanoparticles, the composite nanoparticles including a nickel oxide core and oxide shell, the oxide shell comprising a mixture of nickel and iron
