Iron nanoparticles with tunable tetragonal structure and magnetic properties

Body-centered cubic (bcc) Fe is known as a typical soft magnetic material with high-saturation magnetization (Ms) and low magnetocrystalline anisotropy. However, first-principles calculations demonstrate that body-centered tetragonal (bct) Fe has higher magnetocrystalline anisotropy than bcc Fe and comparable Ms. In this work, bct Fe nanoparticles (NPs) were successfully fabricated by a gas-phase condensation method for the first time. The bct Fe phase is confirmed by the x-ray diffraction pattern and diffraction images of transmission electron microscopy. An increased magnetocrystalline anisotropy of bct Fe, (2.65±0.67)×105J/m3[(21.2±5.3)μeV/atom], is observed, which is around seven times higher than that of bcc Fe 4.8×104J/m3(3.5μeV/atom). The bct Fe NPs sample has coercivity of 3.22×105 A/m at 5 K and 1.04×105 A/m at 300 K, which are much higher than that of bcc Fe NPs. In addition, the saturation magnetization at 5 K is estimated to be (1.6±0.4)×106 A/m (2.2±0.5μB/atom), comparable to that of bcc Fe 1.7×106 A/m (2.2μB/atom).