Phase-transition-induced magneto-elastic coupling and negative thermal expansion in (Hf,Ta)Fe_1.98 Laves phase

The effects of Ta substitution on the phase transition have been studied in Hf_1-xTa_xFe_1.98 (x = 0.125, 0.13, 0.135) alloys with kagome-type lattice. Increased Ta substitution leads into the reduced metamagnetic transition temperature (T_t). The nature of such phase transition is also changed from second-order type to the weak first-order one, suggested by the gradually observed hysteresis. The magnetic entropy change (ΔS) is meanwhile enhanced from 3.04 J kg⁻¹ K⁻¹ to 3.4 J kg⁻¹ K⁻¹. Such enhanced magnetocaloric effects are strongly correlated with the enhanced magneto-elastic effect, which is further confirmed by the maximized saturation magnetostriction (λ^s_//) of ~1700 ppm near the T_t of Hf_0.865Ta_0.135Fe_1.98 alloy. The concurrence of maximized ΔS^max and λ^s_// suggests the strong magneto-elastic coupling during phase transition and the high linear co-efficiency of negative thermal expansion can be triggered in Fe-deficient Hf_0.865Ta_0.135Fe_1.98 alloy.