Ordered phase formation in Sm-Co1−xCux and Er-Co1−xCux alloy films prepared on Cr(100) single-crystal underlayer

•Sm(Co,Cu)5 and Er(Co,Cu)5 ordered alloy films are obtained on Cr(1 0 0) underlayers.•Er(Co,Cu)5 phase is stabilized by partial substitution of the Co sites with Cu atoms.•Control of the amount of Cu atom is important to stabilize the Er(Co,Cu)5 phase. Sm17(Co1−xCux)83 and Er17(Co1−xCux)83 (at. %, x = 0–1) alloy thin films are prepared on Cr(100) single-crystal underlayers at 500 °C by molecular beam epitaxy. The effect of Cu/Co composition on the ordered phase formation is investigated. Sm17(Co1−xCux)83(112¯0) films with hexagonal Sm(Co,Cu)5 (D2d) ordered phase grow epitaxially on the underlayers for all the compositional range of x = 0–1. On the contrary, the structure of Er17(Co1−xCux)83 film varies depending on Cu composition. Amorphous phase is formed in the Er17Co83 (x = 0) film. Er17(Co1−xCux)83(112¯0) epitaxial films with Er(Co,Cu)5 (D2d) ordered phase are obtained for the compositional range of x = 0.25–0.5. With further increasing the x value, the crystallographic phase varies to a phase other than D2d. Partial substitution of the Co sites in ErCo5 structure with Cu atoms and control of the amount of Cu atom are important to stabilize the Er(Co,Cu)5 phase.