Tetragonal distortion and perpendicular magnetic anisotropy of FeCo layer in the FePt/ FeCo and FePt/Cu/FeCo films

The combination of the large uniaxial magnetocrystalline anisotropy (7×10⁷ erg/cm³) of L1₀ ordered FePt and the large M_s of FeCo is quite attractive for the practical application to high density magnetic recording media and for developing innovative permanent magnets. Moreover, it is predicted by the first-principle calculations that the tetragonal distortion (c/a~1.25) yields a large uniaxial MAE (700-800 μeV/atom) in FeCo alloys. There are two factors which are important to understand the magnetization process of FePt/FeCo bilayer films: one is the uniaxial magnetic anisotropy induced by a tetragonal distortion at the interface between FePt and FeCo layers and the other is an exchange coupling between two layers. In the present work, the tetragonal distortion in FeCo at the interface between FePt and FeCo layers is observed by XRD and TEM analysis, and the uniaxial magnetic anisotropy is evaluated from the magnetization process of FePt/FeCo and FePt/Cu/FeCo films. Correlation between the tetragonal distortion and magnetic anisotropy is studied.