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 < <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>C</mml:mi></mml:mrow> </mml:msub> </mml:math> < 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:msubsup><mml:mrow><mml:mi>S</mml:mi></mml:mrow> <mml:mrow><mml:mi>M</mml:mi></mml:mrow> <mml:mrow><mml:mi>P</mml:mi> <mml:mi>k</mml:mi></mml:mrow> </mml:msubsup> </mml:math> = - 6.31 J kg^-1 K^-1, and an adiabatic temperature change, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>ad</mml:mi></mml:mrow> </mml:msub> </mml:math> = 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.