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

alpha ''-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 alpha ''-Fe16N2 nanocones. Magnetocrystalline anisotropy energy is suggested as the driving force for the anisotropic growth of alpha ''-Fe16N2@C nanocones because the easy magnetization direction of tetragonal alpha ''-Fe16N2 nanocrystals is along the c axis. The alpha ''-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 alpha ''-Fe16N2@C nanocones are similar to 200 nm and the diameters vary from similar to 10 nm on one end to similar to 40 nm on the other end. Carbon shells with a thickness of 2-3 nm protect alpha ''-Fe16N2 nanocones from oxidation in air atmosphere. The alpha ''-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.

Automated summary

A new one-pot microemulsion method was developed to synthesize shape anisotropic α''-Fe16N2@C nanocones at low temperatures, exhibiting high coercivity performance. The anisotropic growth of nanocones was driven by magnetocrystalline anisotropy energy and magnetostatic energy, resulting in a unique fan-like microstructure.