Evolution of anisotropy in bcc Fe distorted by interstitial boron

Abstract

The evolution of magnetic anisotropy in bcc Fe as a function of interstitial boron atoms was investigated in thin films grown by molecular beam epitaxy. The thermodynamic nonequilibrium conditions during film growth allowed one to stabilize an interstitial boron content of about $14\phantom{\rule{0.28em}{0ex}}\mathrm{at}.\phantom{\rule{0.16em}{0ex}}%$ accompanied by lattice tetragonalization. The $c/a$ ratio scaled linearly with the boron content up to a maximum value of 1.05 at $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ substrate growth temperature, with a room-temperature magnetization of. In contrast to nitrogen interstitials, the magnetic easy axis remained in-plane with an anisotropy of approximately $\ensuremath{-}5.1\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.28em}{0ex}}\mathrm{erg}/{\mathrm{cm}}^{3}$. Density functional theory calculations using the measured lattice parameters confirm this value and show that boron local ordering indeed favors in-plane magnetization. Given the increased temperature stability of boron interstitials as compared to nitrogen interstitials, this study will help to find possible ways to manipulate boron interstitials into a more favorable local order.