Crystal Orientation, Order Degree, and Surface Roughness of FePd-Alloy Film Formed on MgO(001) Substrate

FePd alloy films of 1 to 500 nm thicknesses are prepared on MgO(001) single-crystal substrates by employing a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. The influences of film thickness on the crystallographic orientation, the degree of L1₀ ordering, the magnetic property, and the surface roughness are systematically investigated. FePd(001) single-crystal films with disordered structure grow epitaxially on the substrates at 200 °C. The in-plane lattice is expanded to accommodate the lattice misfit between the film and the substrate (-9%). The distortion is decreased with increasing the thickness. When the films are annealed at 600 °C, transformation to L1₀ phase occurs. The films thinner than 40 nm consist of L1₀(001) variant with the c-axis normal to the substrate surface, where the transformation along the perpendicular direction is preferentially taking place. On the contrary, the films thicker than 100 nm involve L1₀(100) and L1₀(010) variants whose c-axes lie in the film plane and are rotated around the film normal by 90° of each other. The volume ratio of L1₀(001) variant decreases to 0.22, as the thickness increases up to 500 nm. The order degrees of films thinner than 40 nm are 0.6-0.7. With further increasing the thickness, the order degree decreases. The lattice deformation in the disordered FePd film before annealing is effective in enhancing L1₀ ordering and aligning the c-axis perpendicular to the substrate surface. The easy magnetization direction varies from perpendicular to in-plane, as the thickness increases from 100 to 500 nm. The magnetic anisotropy is affected by the crystal orientation and the order degree. The films thinner than 2 nm have island-like surfaces, where condensation of deposited atoms on substrate is taking place during the annealing process. When- the thickness is increased beyond 10 nm, a flat surface is kept before and after annealing.