Magnetically driven giant negative thermal expansion covering room temperature in Hf0.875Ta0·125Fe2

Abstract

Abstract Neutron diffraction is used to reveal the origin of the unusual thermal expansion properties of the itinerant-electron system Hf0.875Ta0.125Fe2. Hf0.875Ta0.125Fe2 shows temperature-induced magnetic transitions from ferromagnetic (FM) to antiferromagnetic (AFM) order and then to the paramagnetic (PM) state upon heating. The FM-AFM transition proceeds in a stepwise fashion, as a first-order phase transition, and is accompanied by an extremely anisotropic lattice collapse. The unit cell volume shrinks abruptly, but only in the basal plane; the dimension along the c-axis varies almost continuously. Hf0.875Ta0.125Fe2 exhibits a 0.37 % spontaneous volume contraction across the FM-AFM transition, where a giant negative thermal expansion (NTE), with a volumetric thermal expansion coefficient -74×10-6 K-1, is observed over a temperature interval of ΔT∼50 K around room temperature.