Phase transition modulation and wide temperature interval zero thermal expansion in Dy(Co_1−xFe_x)₂ compounds

Abstract: Wide temperature interval zero thermal expansion (ZTE) materials have important applications in advanced technology and precision manufacturing. In this study, Fe doping was used to achieve a near room temperature magnetic phase transition and ZTE in Dy(Co_1−xFe_x)₂ compounds. The observed coefficient of thermal expansion α_l = 5.8 × 10^–7 K⁻¹ (5–305 K) is superior to most other ZTE materials. The combination of the positive contribution to thermal expansion from lattice anharmonic vibrations and the negative contribution from magnetic coupling leads to a wide temperature interval ZTE in the ferrimagnetic (FIM) state, which can be quantitatively described by the spontaneous volume magnetostriction parameter ω_s. First-principles calculations reveal the electron distribution and transfer properties from FIM state to the paramagnetic state in DyCo₂ system and explain the change in magnetic moment after Fe doping. The increase of the exchange coupling parameter J_eff determined that the Fe doping increases the phase transition temperature T_C by introducing additional exchange interactions. Moreover, evidences such as the Arrott plots, critical exponent n and coupling coefficient b demonstrated that Fe doping causes the transition from a first-order phase transition to a second-order phase transition with a smoother transition interval, which is conducive to broadening the ZTE interval. This work reveals the change in the electron transfer properties after magnetic transition and elucidates the ZTE mechanism from the combined effects of magnetic coupling and lattice anharmonic vibrations, which provides promising methods for exploring ZTE.