Cryogenic magnetic properties and magnetocaloric performance in the RE(Fe0.25Co0.75)2H3 (RE = Ho and Er) laves phase compounds

Exploiting the magnetocaloric effect for low-temperature magnetic refrigeration presents a compelling green technology for cryogenic applications in space science and hydrogen liquefaction. In this work, we investigate the structural and magnetic properties of RE(Fe_0.25Co_0.75)₂H₃ compounds, where RE represents Ho and Er. These compounds crystallize in the cubic MgCu₂ (C15) structure with a lattice parameter of 7.62 Å for RE = Ho and 7.60 Å for RE = Er. Magnetization measurements as a function of temperature, performed in a 0.05 T magnetic field, reveal that both hydrides exhibit ferrimagnetic behaviour. The Curie temperatures (T_C) for Ho and Er hydrides were determined to be 124 K and 75 K, respectively. We studied the magnetic entropy change (−∆S_M) and refrigerant capacity (RC) of both samples by measuring isothermal magnetizations under various field changes between 0 and 7 Tesla. With increasing applied field, both −∆S_M and RC increase, and the temperature span of the −∆S_M versus T plots widens. Furthermore, from the −∆S_M (T) curves under an external magnetic field change of 5 T, we observed a maximum -ΔS_M of ∼ 3.11 and 4.41 Jkg^-1K^-1 for RE = Ho and Er, respectively. The corresponding temperature-averaged entropy change over a 10 K temperature span was TEC_10K ∼2.9 and ∼3.8 Jkg^-1K^-1 for RE = Ho and Er, respectively. These results indicate that Er(Fe_0.25Co_0.75)₂H₃ exhibits superior thermomagnetic properties compared to Ho(Fe_0.25Co_0.75)₂H₃ particularly for low-temperature magnetic cooling applications.