Methods for preparing magnetic nanoparticles comprising metal, metal carbide, metal nitride, metal sulfide, metal phosphide, metal oxide or a mixture thereof are disclosed. Methods for preparing magnetic nanoparticles having a core comprising metal, metal carbide, metal nitride, metal sulfide, metal phosphide, or a mixture thereof and a metal oxide shell are also disclosed. The methods comprise the solution-phase decomposition of a precursor at elevated temperature then exposure of the reaction mixture to an oxidizing medium, such as air.