Recent progress on cryogenic magnetocaloric materials

Cryogenic magnetocaloric materials (MCMs) attract growing attention for solid-state refrigeration technologies, especially in sub-Kelvin applications such as quantum computing and space exploration. Among various candidates, rare-earth-based compounds containing Gd3+ and Eu2+ ions exhibit remarkable magnetocaloric effects due to their large spin (J = S = 7/2 and L = 0), weak magnetic anisotropy, and tunable magnetic interactions. This review summarizes recent progress on the structure–property-performance of representative Gd3+/Eu2+-based compounds, including fluorides, phosphates, hydroxides, halides, and metal–organic frameworks. Key design principles are proposed for optimizing cryogenic MCMs, including high magnetic ion concentration, weak or single-ion magnetic coupling, and suppressed magnetic ordering. Finally, we also discuss spin supersolid materials Na2BaCo(PO4)2, which exhibits excellent adiabatic demagnetization cooling temperature below 100 mK, and look into the future of cryogenic magnetocaloric materials and the emerging quantum magnetic phenomena.