Giant magnetocaloric effect in a rare-earth-free layered coordination polymer at liquid hydrogen temperatures

Magnetic refrigeration, which utilizes the magnetocaloric effect, can provide a viable alternative to the ubiquitous vapor compression or Joule-Thompson expansion methods of refrigeration. For applications such as hydrogen gas liquefaction, the development of magnetocaloric materials that perform well in moderate magnetic fields without using rare-earth elements is highly desirable. Here we present a thorough investigation of the structural and magnetocaloric properties of a novel layered organic-inorganic hybrid coordination polymer Co₄(OH)₆(SO₄)₂[enH₂] (enH₂ = ethylenediammonium). Heat capacity, magnetometry and direct adiabatic temperature change measurements using pulsed magnetic fields reveal a field-dependent ferromagnetic second-order phase transition at 10 K < \({T}_{C}\) $T_C$  < 15 K. Near the hydrogen liquefaction temperature and in a magnetic field change of 1 T, a large maximum value of the magnetic entropy change, \(\Delta {S}_{M}^{{Pk}}\) $\mathrm{\Delta }{S}_M^{Pk}$  = − 6.31 J kg⁻¹ K⁻¹, and an adiabatic temperature change, \(\Delta {T}_{{{\rm{ad}}}}\) $\mathrm{\Delta }{T}_{\mathrm{ad}}$  = 1.98 K, are observed. These values are exceptional for rare-earth-free materials and competitive with many rare-earth-containing alloys that have been proposed for magnetic cooling around the hydrogen liquefaction range.