A Record-High Cryogenic Magnetocaloric Effect Discovered\nin EuCl<sub>2</sub> Compound

Adiabatic demagnetization refrigeration (ADR) based on\nthe magnetocaloric\neffect (MCE) is a promising technique to achieve cryogenic temperature.\nHowever, magnetic entropy change (Δ<i>S</i><sub>M</sub>), the driving force of ADR, remains far below theoretical −Δ<i>S</i><sub>M</sub> = <i>nR</i>ln(2<i>J</i> + 1)/<i>M</i><sub>W</sub> for most magnetic refrigerants.\nHere, we report giant MCE in orthorhombic EuCl<sub>2</sub>, where\na ferromagnetic ground state with excellent single-ion behavior of\nEu<sup>2+</sup> and free spins has been demonstrated by combining <i>ab initio</i> calculations with Brillouin function analysis\nand magnetic measurements. Consequently, a record-high −Δ<i>S</i><sub>M</sub> ∼ 74.6 J·kg<sup>–1</sup>·K<sup>–1</sup> (1.8 K) at 5 T was experimentally achieved,\napproaching 96% of the theoretical limit (77.5 J·kg<sup>–1</sup>·K<sup>–1</sup>). At a lower field of 1 T, EuCl<sub>2</sub> also achieves the highest-ever record of −Δ<i>S</i><sub>M</sub> ∼ 36.8 J·kg<sup>–1</sup>·K<sup>–1</sup>. Further, direct quasi-adiabatic demagnetization\nmeasurements demonstrate that its large −Δ<i>S</i><sub>M</sub> allows EuCl<sub>2</sub> to maintain a long holding time\nat sub-Kelvin temperature (∼346 mK), surpassing all previously\nreported materials. These superior magnetocaloric performances position\nEuCl<sub>2</sub> as an attractive cryogenic refrigerant.