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, $(2.65±{}0.67)×{}1{0}⁵{{0.28em}{0ex}}J/{m}³{{4pt}{0ex}}[(21.2±{}5.3){{0.16em}{0ex}}{μ}eV/atom]$, is observed, which is around seven times higher than that of bcc Fe $4.8×{}1{0}⁴{{0.28em}{0ex}}J/{m}³{{4pt}{0ex}}(3.5{{0.16em}{0ex}}{μ}eV/atom)$. The bct Fe NPs sample has coercivity of $3.22×{}1{0}⁵$ A/m at 5 K and $1.04×{}1{0}⁵$ 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 $(1.6±{}0.4)×{}1{0}⁶$ A/m $(2.2±{}0.5{{0.16em}{0ex}}{{μ}}B/atom)$, comparable to that of bcc Fe $1.7×{}1{0}⁶$ A/m $(2.2{{0.16em}{0ex}}{{μ}}B/atom)$.