Strong perpendicular uniaxial magnetic anisotropy in tetragonal Fe_0.5Co_0.5 films of artificially ordered B2 state

We studied the relationship between the magnetic anisotropy properties and crystal structure of an artificial B2 state of Fe_0.5Co_0.5 epitaxial films on a Rh(001) seed layer with a tetragonal distortion structure, which exhibited large perpendicular uniaxial magnetic anisotropy over a wide range of thicknesses and c/a ratios at room temperature. We obtained the bulk anisotropy energy of the Fe_0.5Co_0.5 films, which reached 2.5 × 10⁷ erg/cm³ at c/a=1.20. The magnetization easy axis of the Fe_0.5Co_0.5 films transferred from out-of-plane to in-plane when the thickness was increased from 24 to 30 monolayers. This is due to the lattice relaxation caused by the increase of the total thickness. We observed that the cubic anisotropy component around the in-plane of the films was much smaller than the perpendicular effective magnetic anisotropy, which demonstrates that our films exhibited the uniaxial anisotropy property. Furthermore, we found the bulk anisotropy values of our artificial B2-state Fe_0.5Co_0.5 to be larger than those of the A2 state under the same lattice distortion. Thus, we suggest that tuning the degree of crystal order of FeCo alloys is an effective way to experimentally enhance their perpendicular uniaxial magnetic anisotropy.