Structure, Magnetism, and Tunable Negative Thermal Expansion in (Hf,Nb)Fe2 Alloys

Abstract

Negative thermal expansion (NTE) is one of intriguing properties for solid compounds. Here we report structure and tunable negative thermal expansion in the (Hf1-xNbx)Fe2 magnetic alloys. The NTE mechanism has been investigated by a combined analysis of neutron powder diffraction, synchrotron X-ray diffraction, and macroscopic magnetic measurements. Direct experimental evidence shows that magnetovolume effect is the root of NTE of ferromagnetic phase, while the paramagnetic phase responds to the general positive thermal expansion (PTE). Especially, by adjusting the amount of Nb chemical substitution in (Hf1-xNbx)Fe2, the coexistence of NTE ferromagnetic and PTE paramagnetic phases can tune the large NTE (x = 0.05, αv = -23.13 × 10-6 K-1, 323~398 K) to the relatively low thermal expansion (x = 0.15, αv = -8.28×10-6 K-1, 173~323 K). The present study reveals the direct link between NTE and magnetic structure, and shows that thermal expansion could be tuned by the coexistence of magnetic and paramagnetic phases.