  In order to convert the well-known Fe-Co-B alloy from a soft to a hard
magnet, we propose tetragonal strain by interstitial boron. Density functional
theory reveals that when B atoms occupy octahedral interstitial sites, the bcc
Fe-Co lattice is strained spontaneously. Such highly distorted Fe-Co is
predicted to reach a strong magnetocrystalline anisotropy which may compete
with shape anisotropy. Probing this theoretical suggestion experimentally,
epitaxial films are examined. A spontaneous strain up to 5 % lattice distortion
is obtained for B contents up to 4 at%, which leads to uniaxial anisotropy
constants exceeding 0.5 MJ/m^3. However, a further addition of B results in a
partial amorphization, which degrades both anisotropy and magnetization.
