Influence of Cap-Layer on the Structure of FePt Alloy Thin Film Formed on VN and VC Underlayers

10-nm-thick FePt alloy films without and with MgO, VC, and VN cap-layers of 2 nm thickness are prepared on MgO, VC, and VN single-crystal underlayers of (001) orientation by using a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. The surface energies of MgO and vanadium compounds (VC, VN) are respectively lower and higher than that of FePt alloy. The influences of combination of underlayer and cap-layer materials on the surface flatness, the lattice deformation, the degree of L10 ordering, and the magnetic anisotropy are investigated. FePt(001) single-crystal films without and with MgO, VC, and VN(001) single-crystal cap-layers grow epitaxially on all the underlayers. The in-plane lattice of FePt film is expanded in accommodation of lattice mismatch with underlayer and/or cap-layer. The lattice deformation aligns the c-axis perpendicular to the substrate surface and enhances L10 ordering. A higher order degree and a stronger perpendicular magnetic anisotropy are obtained by using a combination of VN underlayer and MgO cap-layer. The present study shows that employment of underlayer and cap-layer materials whose surface energies are respectively higher and lower than that of FePt alloy is effective in enhancing order degree and magnetic anisotropy.