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

Abstract

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 <$T_C$ < 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, $\mathrm{\Delta }{S}_M^{Pk}$ = − 6.31 J kg⁻¹ K⁻¹, and an adiabatic temperature change, $\mathrm{\Delta }{T}_{\mathrm{ad}}$ = 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.

The magnetocaloric effect could allow for more efficient refrigeration compared to traditional vapour compression, however, it requires materials with large magnetocaloric coefficients, and these typically require rare-earths. Herein, Levinsky et al. demonstrate a large magnetocaloric effect in a rare-earth free layered coordination polymer.