A Record-High Cryogenic Magnetocaloric Effect Discovered in EuCl₂ Compound

Adiabatic demagnetization refrigeration (ADR) based on the magnetocaloric effect (MCE) is a promising technique to achieve cryogenic temperature. However, magnetic entropy change (ΔS_M), the driving force of ADR, remains far below theoretical -ΔS_M = nRln(2J + 1)/M_W for most magnetic refrigerants. Here, we report giant MCE in orthorhombic EuCl₂, where a ferromagnetic ground state with excellent single-ion behavior of Eu²⁺ and free spins has been demonstrated by combining ab initio calculations with Brillouin function analysis and magnetic measurements. Consequently, a record-high -ΔS_M ∼ 74.6 J·kg^-1·K^-1 (1.8 K) at 5 T was experimentally achieved, approaching 96% of the theoretical limit (77.5 J·kg^-1·K^-1). At a lower field of 1 T, EuCl₂ also achieves the highest-ever record of -ΔS_M ∼ 36.8 J·kg^-1·K^-1. Further, direct quasi-adiabatic demagnetization measurements demonstrate that its large -ΔS_M allows EuCl₂ to maintain a long holding time at sub-Kelvin temperature (∼346 mK), surpassing all previously reported materials. These superior magnetocaloric performances position EuCl₂ as an attractive cryogenic refrigerant.