Anisotropic Growth and Magnetic Properties of α″-Fe₁₆N₂@C Nanocones

α″-Fe₁₆N₂ nanomaterials with a shape anisotropy for high coercivity performance are of interest in potential applications such as rare-earth-free permanent magnets, which are difficult to synthesize in situ anisotropic growth. Here, we develop a new and facile one-pot microemulsion method with Fe(CO)₅ as the iron source and tetraethylenepentamine (TEPA) as the N/C source at low synthesis temperatures to fabricate carbon-coated tetragonal α″-Fe₁₆N₂ nanocones. Magnetocrystalline anisotropy energy is suggested as the driving force for the anisotropic growth of α″-Fe₁₆N₂@C nanocones because the easy magnetization direction of tetragonal α″-Fe₁₆N₂ nanocrystals is along the c axis. The α″-Fe₁₆N₂@C nanocones agglomerate to form a fan-like microstructure, in which the thin ends of nanocones direct to its center, due to the magnetostatic energy. The lengths of α″-Fe₁₆N₂@C nanocones are ~200 nm and the diameters vary from ~10 nm on one end to ~40 nm on the other end. Carbon shells with a thickness of 2-3 nm protect α″-Fe₁₆N₂ nanocones from oxidation in air atmosphere. The α″-Fe₁₆N₂@C nanocones synthesized at 433 K show a room-temperature saturation magnetization of 82.6 emu/g and a coercive force of 320 Oe.