Crystal electric field level scheme leading to giant magnetocaloric effect for hydrogen liquefaction

In recent years, magnetic refrigeration has attracted considerable attention for hydrogen liquefaction. Most materials used for magnetic refrigeration contain heavy rare earth ions with complex crystalline electric field energy splittings, whose effect on the magnetic entropy change ΔS_M has not been systematically studied. In particular, the theoretical upper limits of ∣ΔS_M∣ for general heavy earth cases are unknown. Here, we show that the crystalline electric field level schemes result in a large ΔS_M for general heavy rare earth cases. We provide a specific example of the magnetic refrigeration material HoB₂ using inelastic neutron scattering experiments combined with mean-field calculations with crystal field splitting and exchange interactions. The relationship between ΔS_M and crystal field parameters presented in this study can be useful for developing compounds with a large ∣ΔS_M∣ and advancing the design of magnetic refrigeration materials.