Phase diagrams and uniaxial magnetic anisotropy of Fe-Co-X and Fe-Co-V-X (X = B, C, N, O) films

Abstract The increasing demand for advanced magnetic materials in various technological applications requires a comprehensive understanding of the factors influencing the properties of Fe-Co alloys. This study aimed to achieve a stable body-centered tetragonal (bct) Fe-Co structure and to improve the magnetic properties, such as the uniaxial magnetic anisotropy constant ( K u ), for advanced applications. The phase diagrams revealing the crystal structures and magnetic properties of two types of films, namely, V-free Fe-Co-X and Fe-Co-V-X containing 10 at.% V, were investigated in this study by systematically changing the additive element X (X = B, C, N, O) and the Fe:Co composition ratio. The addition of X in any composition range to the V-free Fe-Co film did not result in the formation of an ideal bct structure with an axial ratio ( c / a ) of 1.2, only forming crystal structures close to the cubic system of body-centered cubic ( c / a = 1.0) or face-centered cubic ( c / a = 1.4) structures. However, an ideal bct ( c / a ≈ 1.2) structure was formed in the Fe-Co-V-X film, where X = C or N, in the specific composition ratio range of Fe 70 Co 30 ‒Fe 40 Co 60 , resulting in a high K u of 10 6 ‒10 7 erg/cm 3 . The addition of a large amount of X = B induced an amorphous structure, and the addition of a large amount of X = O formed a ferrite-based compound.