Achievement of zero thermal expansion in Mn1-<i>x</i>Co<i>x</i>B alloys via weakening magnetoelastic coupling

Abstract

Zero thermal expansion (ZTE) alloys show great potential for high-precision instruments due to their thermal stability and metallic properties. In this study, we revisit the ferromagnetic MnB alloy, which exhibits a significant axial negative thermal expansion (NTE) during its magnetic transition. By employing isostructural alloying, the Co-doped Mn1-xCoxB retains the orthorhombic structure, while the NTE gradually weakens, leading to the single-phase ZTE along longitudinal direction with a linear thermal expansion coefficient of −3.9 × 10−7 K−1 for the Mn0.6Co0.4B composition. Additionally, the alloy demonstrates an ultrawide ZTE of 8.1 × 10−7 K−1 along the transverse direction. Direct experimental evidence indicates that the weakening of magnetoelastic coupling and microscopic lattice orientation contribute to the formation of this unique anisotropic ZTE. Furthermore, density functional theory calculation shows that when introducing Co atom, the weak Co–Co interaction and decrease in Mn–Co interaction simultaneously bring about a decrease in the Curie temperature and the correlated working temperature range for thermal expansion.