Crystal Electric Field Level Scheme Leading to Giant Magnetocaloric Effect for Hydrogen Liquefaction

Abstract

In recent years, magnetic refrigeration (MR) in cryogenic temperature range has attracted much attention especially for the purpose of hydrogen liquefaction. Most materials used for the MR contain heavy rare earth ions with complex crystal electric field (CEF) energy level scheme. Since energy scales of CEF level scheme of the heavy rare earth ions and system temperature (several tens of kelvin) are comparable to each other, CEF level schemes in zero magnetic field strongly affect the magnetocaloric effect (MCE) when applying magnetic field to MR materials. Here, providing a specific example of the MR material HoB₂, which shows a giant MCE around the hydrogen condensation point 20 K, we discuss the relationship between the MCE and crystal field level scheme, which was determined by the inelastic neutron scattering experiment in HoB₂. We also extend the discussion to general heavy rare earth cases, by using mean-field calculations with crystal field splitting and exchange interactions. The relationship between ΔS_M and crystal field parameters presented here can be useful for developing compounds with a large magnetocaloric effect and advancing the design of magnetic refrigeration materials.