Large magnetocaloric effect and negative thermal expansion of Mn-Ni-Si-Fe-Co-Ge high-entropy alloys

Magnetocaloric high-entropy alloys (HEAs) with a vast compositional space have attracted significant interest for some time due to their great potential in multifunctional application. However, magnetocaloric HEAs generally exhibit modest magnet-entropy change as their undergo second-order phase transition. This work focuses on designing MM`X (M, M′ = transition metals, X = main group elements) HEAs undergoing magnerostructural transition to achieve great magnetocaloric response. For this purpose, Mn <sub>x</sub>Fe <sub>1-x</sub>Ni <sub>0.6</sub>Co <sub>0.4</sub>Si <sub>0.62</sub>Ge <sub>0.38</sub> ( x = 0.43, 0.45, 0.48, 0.50) with intriguingly crystallographic texture are successfully synthesized and their magnetocaloric effect and negative thermal expansion (NTE) are investigated. The substitution of Fe for Mn reduces the transition temperature ( T <sub>t</sub>) so that weakens the first-order transition of MM`X HEAs. Besides, Mn _0.55Fe _0.45Ni _0.6Co _0.4Si _0.62Ge _0.38 undergoing first-order transition from Ni ₂In-paramagnetic-austenitic phase to TiNiSi-ferromagnetic-martensitic phase exhibits a great magnetic entropy change of 16 J∙kg ⁻¹K ⁻¹ at 206 K for 2 T. Moreover, an obviously NTE is observed through phase transition with liner NTE coefficient of −6.77×10 ⁻⁶ K ⁻¹. In addition, Mn _0.55Fe _0.5Ni _0.6Co _0.4Si _0.62Ge _0.38 displays an intriguing NTE with liner NTE coefficient of −1401.7×10 ⁻⁶ K ⁻¹ (220–270 K) along the direction parallel to field in a temperature range from 220 K to 270 K, which is much higher than typical NTE materials. Therefore, the present work demonstrates that MM`X HEAs exhibit potential to achieve some magnetocaloric and NTE multifunctional application.