Stoichiometry–anisotropy connections in epitaxial L10 FePt(001) films

The order parameters and anisotropy constants of a series of epitaxial, L1{sub o} FePt films with compositions in the range of 45-55 at% Fe and nominal thicknesses of 50 nm have been characterized. The films were made by cosputtering the elements onto single crystal MgO(001) substrates. The substrates were coated with 1 nm Pt/1 nm Fe bilayer seeds prior to alloy deposition. Both the bilayer seed and the alloy film were deposited at 620 C. Lattice and order parameters were obtained by x-ray diffraction. Film compositions and thicknesses were determined by Rutherford Backscattering Spectrometry, and room-temperature magnetocrystalline anisotropies were determined with a torque magnetometer. It is found that the order parameter has a maximum for the film composition closest to the equiatomic composition, whereas the magnetocrystalline anisotropy increases as the Fe content is increased from below to slightly above the equiatomic composition. These results imply that non-stoichiometric FePt compositions, with a slight excess of Fe, may in fact be preferred for applications that require high anisotropy.