Large magnetocaloric effect near liquid hydrogen temperatures in Er1-xTmxGa materials

Abstract

Low-temperature magnetocaloric materials are of great importance for potential applications of gas liquefaction such as nitrogen, hydrogen and helium for their low liquidation temperatures (∼4 K for helium, ∼20 K for hydrogen and ∼77 K for nitrogen respectively), of which the working temperature, the maximal magnetic entropy change ((-ΔS_M)_max) , the maximal adiabatic temperature change ((ΔT_ad)_max), and the temperature average entropy change (TEC) are the key assessment parameters. Herein, we designed and synthesized Er_1-xTm_xGa series compounds based on the optimization of the spin quantum number (Spin) with their magnetic ordering temperature successfully adjusted from 31.0 K to 15.0 K, which covers the liquid hydrogen temperature range. Particularly, Er_0.8Tm_0.2Ga shows outstanding (-ΔS_M)_max, TEC(20), and (ΔT_ad)_max values of 13.6 J/kg K , 10.1 J/kg K, and 4.3 K under the field change of 0-2 T, respectively, which are increased by 32.0 %, 36.4 %, and 48.2 % compared with the parent ErGa compound. It should be noted that the refrigerant capacity (RC) of Er_0.8Tm_0.2Ga is not only larger than ErGa but also larger than TmGa. Furthermore, neutron powder diffraction (NPD) was employed on Er_0.8Tm_0.2Ga to reveal the physical mechanism of its enhanced magnetocaloric effect (MCE). It is found that for Er_0.8Tm_0.2Ga the more pronounced order-to-disorder transition than the spin reorientation (SR) transition, the characteristic second order phase transition, and the existence of the short-range magnetic ordering above the magnetic ordering temperature should be jointly responsible for its large magnetocaloric effect.