High-field magnetic and magnetocaloric properties of pseudo-binary Er1−xHoxNi2 (x = 0.25–0.75) solid solutions

Hydrogen is quickly becoming a desired type of fuel, however, the energy and cost required for liquefaction using today’s cooling technology is excessively high. Magnetic cooling based on the magnetocaloric effect is an energy-efficient and environmentally friendly alternative to commonly used vapor compression, but improvements in refrigerants are crucial for this technology to succeed. Polycrystalline Er _1−xHo _xNi ₂ (x = 0.25, 0.5, 0.75) Laves-phase solid solutions obtained by the arc-melting method have been investigated due to their potential for low-temperature refrigerants. Er _0.75Ho _0.25Ni ₂ and Er _0.5Ho _0.5Ni ₂ crystallize in cubic Laves phase superstructure (space group F-43 m), while Er _0.25Ho _0.75Ni ₂, similarly to the initial ErNi ₂ and HoNi ₂ binary compounds, crystallizes with the formation of the regular cubic C15 structure (space group Fd-3 m). To evaluate how the structure affects magnetic and magnetocaloric properties, studies in a wide magnetic field range, up to 14 T, were conducted. Measurements show all samples obey the second-order magnetic phase transition from ferromagnetic to paramagnetic state, and their Curie temperatures increase with increasing Ho content from 8 K for Er _0.75Ho _0.25Ni ₂ to 12.3 K for Er _0.25Ho _0.75Ni ₂ . At higher temperatures, all solid solutions are Curie-Weiss paramagnets. However, it has been observed that the formation of the superstructure with lower translational symmetry seems to affect magnetic moments and magnetic entropy values. Er _0.25Ho _0.75Ni ₂ , with the regular cubic C15 structure, showed the highest entropy changes of 43.5 J/kgK around 12 K, while Er _0.75Ho _0.25Ni ₂ and Er _0.5Ho _0.5Ni ₂ , with cubic superstructure, provided ∼30 % lower results of 30.3 and 35.1 J/kgK, around 8 and 10 K, respectively, for magnetic field change of 14 T. Nevertheless, relatively large and reversible values of magnetic entropy prove that these compounds can be promising candidates for magnetic cooling operating within the low-temperature range needed for hydrogen liquefaction.