  Magnetocaloric hydrogen liquefaction could be a "game-changer" for liquid
hydrogen industry. Although heavy rare-earth-based magnetocaloric materials
show strong magnetocaloric effects in the temperature range required by
hydrogen liquefaction (77 ~ 20 K), the high resource criticality of the heavy
rare-earth elements is a major obstacle for upscaling this emerging
liquefaction technology. In contrast, the higher abundances of the light
rare-earth elements make their alloys highly appealing for magnetocaloric
hydrogen liquefaction. Via a mean-field approach, it is demonstrated that
tuning the Curie temperature ($T_C$) of an idealized light rare-earth-based
magnetocaloric material towards lower cryogenic temperatures leads to larger
maximum magnetic and adiabatic temperature changes ($\Delta S_T$ and $\Delta
T_{ad}$). Especially in the vicinity of the condensation point of hydrogen (20
K), $\Delta S_T$ and $\Delta T_{ad}$ of the optimized light rare-earth-based
material are predicted to show significantly large values. Following the
mean-field approach and taking the chemical and physical similarities of the
light rare-earth elements into consideration, a method of designing light
rare-earth intermetallic compounds for hydrogen liquefaction is proposed:
tunning $T_C$ of a rare-earth alloy to approach 20 K by mixing light rare-earth
elements with different de Gennes factors. By mixing Nd and Pr in Laves phase
$(Nd,Pr)Al_2$, and Pr and Ce in Laves phase $(Pr,Ce)Al_2$, a fully light
rare-earth intermetallic series with large magnetocaloric effects covering the
temperature range required by hydrogen liquefaction is developed, demonstrating
a competitive maximum effect compared to the heavy rare-earth compound
$DyAl_2$.
