Giant low-field magnetocaloric effect in ferromagnetically ordered Er_1–xTm_xAl₂ (0 ≤ x ≤ 1) compounds

Magnetocaloric material is the key working substance for magnetic refrigerant technology, for which the low-field and low-temperature magnetocaloric effect (MCE) performance is of great importance for practical applications at low temperatures. Here, a giant low-field magnetocaloric effect in ferromagnetically ordered Er_1–xTm_xAl₂ (0 ≤ x ≤ 1) compounds was reported, and the magnetic structure was characterized based on low-temperature neutron powder diffraction. With increasing Tm content from 0 to 1, the Curie temperature of Er_1–xTm_xAl₂ (0 ≤ x ≤ 1) compounds decreases from 16.0 K to 3.6 K. For Er_0.7Tm_0.3Al₂ compound, it showed the largest low-field magnetic entropy change (–ΔS_M) with the peak value of 17.2 and 25.7 J/(kg K) for 0–1 T and 0–2 T, respectively. The (–ΔS_M)_max up to 17.2 J/(kg K) of Er_0.7Tm_0.3Al₂ compound for 0–1 T is the largest among the intermetallic magnetocaloric materials ever reported at temperatures below 20 K. The peak value of adiabatic temperature change (ΔT_ad)_max was determined as 4.13 K and 6.87 K for 0–1 T and 0–2 T, respectively. The characteristic of second-order magnetic transitions was confirmed on basis of Arrott plots, the quantitative criterion of exponent n, rescaled universal curves, and the mean-field theory criterion. The outstanding low-field MCE performance with low working temperatures indicates that Er_1–xTm_xAl₂ (0 ≤ x ≤ 1) compounds are promising candidates for magnetic cooling materials at liquid hydrogen and liquid helium temperatures.