Tetragonally distorted structure and uniaxial magnetic anisotropy of Fe_100-xCo_x/Rh/MgO epitaxial films

The uniaxial magnetic anisotropy of tetragonally distorted Fe_100-xCo_x epitaxial films prepared using Rh buffer on a MgO substrate was investigated. The structural relation of body-centered cubic FeCo (002)[1 1 0] //Rh(0 0 2)[1 0 0] //MgO(0 0 1)[1 0 0] was maintained up to 20 nm FeCo thickness. The uniaxial magnetic anisotropy constant (K _u1) of the Fe₅₀Co₅₀ films reached a maximum of 1.6 × 10⁷ erg cm^-3 when the c/a ratio was 1.15-1.25. The composition dependence of K _u1 of Fe_100-xCo_x (t _FeCo = 1.5 nm) was at a maximum at Fe₅₀Co₅₀. These results agree qualitatively with the theoretical value for FeCo films with a tetragonal distortion (c/a = 1.25) and without B2 chemical ordering. Additional uniaxial magnetic anisotropy observed at c/a 1.1 is attributable to the B2 ordering, in addition to the tetragonal distortion.