High magnetic anisotropy of strained epitaxial Fe-Co-X films — Buffer induced distortion versus spontaneous strain

Permanent magnets are a key issue for energy generation and conversion. Since the abundance of the commonly used rare earth based alloys has been questioned, possible alternatives are in focus of research. Fe-Co has been suggested to fulfil the requirements: It exhibits one of the highest magnetic moments and may have a strong magnetocrystalline anisotropy, if its lattice is strained tetragonally [1]. In experimental studies of binary Fe-Co [2,3], the strain was commonly induced via coherent growth of epitaxial films on a suitable buffer. The latter, e.g. Au-Cu [4], provided the required in plane lattice parameters. However, this concept does not allow the preparation of strained Fe-Co with significant film thickness, since the induced strain relaxes within the first 4 nm of grown film. Films of higher thickness exhibit the unstrained cubic equilibrium state without considerable magnetic anisotropy. The lattice relaxation was observed with Reflection High Energy Electron Diffraction (RHEED), which was performed in situ, i.e. during film growth [5]. The c/a ratio as measure of lattice strain is depicted in Fig. 1 (open squares).