Abstract

Adiabatic demagnetization refrigeration (ADR) based on the magnetocaloric effect (MCE) is a promising technique to achieve cryogenic temperature. However, magnetic entropy change (ΔSM), the driving force of ADR, remains far below theoretical −ΔSM = nRln(2J + 1)/MW 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 −ΔSM ∼ 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 −ΔSM ∼ 36.8 J·kg–1·K–1. Further, direct quasi-adiabatic demagnetization measurements demonstrate that its large −ΔSM 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.