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, MnxFe1-xNi0.6Co0.4Si0.62Ge0.38 (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-t) so that weakens the first-order transition of MM'X HEAs. Besides, Mn0.55Fe0.45Ni0.6Co0.4Si0.62Ge0.38 undergoing first-order transition from Ni2In-paramagnetic-austenitic phase to TiNiSi-ferromagnetic-martensitic phase exhibits a great magnetic entropy change of 16 J center dot kg(-1)K(-1) at 206 K for 2 T. Moreover, an obviously NTE is observed through phase transition with liner NTE coefficient of -6.77x10(-6) K-1. In addition, Mn0.55Fe0.5Ni0.6Co0.4Si0.62Ge0.38 displays an intriguing NTE with liner NTE coefficient of -1401.7x10(-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.