  We have synthesized 20 nm thick films of tetragonally strained interstitial
Fe-Co-B alloys epitaxially grown on Au55Cu45 buffer layer. The strained axis is
perpendicular to the film plane and the corresponding lattice constant c is
enlarged with respect to the in-plane lattice parameter a. By adding the
interstitial boron with different concentrations 0, 4, and 10 at.% we were able
to stabilize the tetragonal strain in 20 nm Fe-Co films with different c/a
ratios of 1.013, 1.034 and 1.02, respectively. Using ferromagnetic resonance
(FMR) and x-ray magnetic circular dichroism (XMCD) we found that the orbital
magnetic moment increases with increasing the c/a ratio, pointing towards the
enhancement of spin-orbit coupling (SOC) at larger strain. Our results show
that careful doping of ferromagnetic films allows to control the SOC by
stabilizing anisotropic strain states. These findings are applicable in
material design for spintronics applications. We also discuss the influence of
B doping on the Fe-Co film microstructure, its magnetic properties and magnetic
relaxation.
