Enhanced magnetocaloric and cold storage properties in HoCu_2-xNi_x (x=0.05–0.5) compounds for hydrogen liquefaction

Based on first-principle calculations and experiments, the electronic structure, magnetism, magnetocaloric effect and cold storage properties of HoCu_2-xNi_x (x=0.05–0.5) compounds have been investigated. The theoretical calculations revealed that the magnetic interactions of the HoCu_2-xNi_x compounds were gradually transformed to ferromagnetic states when the Ni element was added above 0.0625. In addition, the Ni element leads to a decreasing Fermi energy level, which results in a significant enhancement of orbital hybridization around the Fermi surface, which may be responsible for the magnetic jump. The magnetic phase transition temperatures of the HoCu_1.75Ni_0.25 and HoCu_1.5Ni_0.5 compounds are 14.4 K and 26.0 K, respectively. The ferromagnetic state transition is responsible for the excellent magnetocaloric effect of the HoCu_2-xNi_x compounds under low magnetic fields. The maximum magnetic entropy changes of the HoCu_1.75Ni_0.25 and HoCu_1.5Ni_0.5 compounds were 14.7 J/kg K and 11.5 J/kg under varying magnetic fields from 0 to 2 T. In addition, the Ni element make the HoCu₂ compound exhibit more excellent cold storage capability. The peak of specific heat capacities of HoCu_1.75Ni_0.25 and HoCu_1.5Ni_0.5 compounds were 0.6 J/cm⁻³K⁻¹ and 0.9 J/cm⁻³K⁻¹ under 0 T. The present results indicate that HoCu_2-xNi_x (x=0.05–0.5) compounds have promising applications in cryogenic cold storage and magnetic refrigeration.