Iron nanoparticles with tunable tetragonal structure and magnetic properties

Abstract

Body-centered cubic (bcc) Fe is known as a typical soft magnetic material with high-saturation magnetization $({M}_{s})$ and low magnetocrystalline anisotropy. However, first-principles calculations demonstrate that body-centered tetragonal (bct) Fe has higher magnetocrystalline anisotropy than bcc Fe and comparable ${M}_{s}$. 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, $(2.65\ifmmode\pm\else\textpm\fi{}0.67)\ifmmode\times\else\texttimes\fi{}1{0}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{J}/{\mathrm{m}}^{3}\phantom{\rule{4pt}{0ex}}\mathrm{[}(21.2\ifmmode\pm\else\textpm\fi{}5.3)\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{eV}/\mathrm{atom}]$, is observed, which is around seven times higher than that of bcc Fe $4.8\ifmmode\times\else\texttimes\fi{}1{0}^{4}\phantom{\rule{0.28em}{0ex}}\mathrm{J}/{\mathrm{m}}^{3}\phantom{\rule{4pt}{0ex}}(3.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{eV}/\mathrm{atom})$. The bct Fe NPs sample has coercivity of $3.22\ifmmode\times\else\texttimes\fi{}1{0}^{5}$ A/m at 5 K and $1.04\ifmmode\times\else\texttimes\fi{}1{0}^{5}$ 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 $(1.6\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}1{0}^{6}$ A/m $(2.2\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{atom})$, comparable to that of bcc Fe $1.7\ifmmode\times\else\texttimes\fi{}1{0}^{6}$ A/m $(2.2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{atom})$.