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 (Δ S M ), the driving force of ADR, remains far below theoretical −Δ S M = nR ln(2 J + 1)/ M W for most magnetic refrigerants. Here, we report giant MCE in orthorhombic EuCl 2, where a ferromagnetic ground state with excellent single-ion behavior of Eu 2+ and free spins has been demonstrated by combining ab initio calculations with Brillouin function analysis and magnetic measurements. Consequently, a record-high −Δ S 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, EuCl 2 also achieves the highest-ever record of −Δ S M ∼ 36.8 J·kg –1 ·K –1 . Further, direct quasi-adiabatic demagnetization measurements demonstrate that its large −Δ S M allows EuCl 2 to maintain a long holding time at sub-Kelvin temperature (∼346 mK), surpassing all previously reported materials. These superior magnetocaloric performances position EuCl 2 as an attractive cryogenic refrigerant.