Magnetocaloric effect of nonstoichiometric La 1−x Fe 11.4+x Si 1.6 alloys with first-order and second-order magnetic transitions

Variation from a first-order magnetoelastic transition (FOMT) to a second-order magnetic transition (SOMT) could be controlled by finely adjusting the Fe/La ratio in the non-stoichiometric La1−xFe11.4+xSi1.6 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys. Particularly, a nearly stoichiometric ratio of NaZn13-type phase was obtained with an addition of excess 0.15 at% Fe. When x value increased from 0 to 0.20, the Curie temperature (TC) increased from 198.6 to 216.6 K due to lattice shrinkage and the magnetic entropy change (−ΔSM) decreased from 18.4 to 8.0 J/(kg·K) (at 0−2 T) and from 22.5 to 13.8 J/(kg·K) (at 0−5 T), which may be ascribed to the change from the itinerant-electron metamagnetism (IEM) to second order magnetic transition. Thus their effective refrigeration capacities declined from 347.6 to 245.9 J kg−1 under 0–5 T, which values were comparable to the Gd5Si2Ge1.9Fe0.1. The Fe/La ratio in NaZn13-phase was possibly dominant to induce the transition from first to second order among La1−xFe11.4+xSi1.6 alloys. The thermal hysteresis decreased by 87% to 0.3 K and magnetic hysteresis losses reduced by 90% to 1.4 J kg−1 under 0–5 T. Near the transition border, the alloys with x = 0.15 produced a giant magnetocaloric effect of about 11.8 J/(kg·K) under 0–2 T at TC = 205.6 K. The magnetic hysteresis was 3.01 J kg−1 and the thermal hysteresis was 1.9 K. The material near the border of transition could be suitable for future magnetic refrigeration application.