Low-Temperature Magnetothermodynamics Performance of Tb_1-xEr_xNi₂ Laves-Phases Compounds for Designing Composite Refrigerants

In this paper, the results of heat capacity measurements performed on the polycrystalline Tb_1-xEr_xNi₂ intermetallic compounds with x = 0.25, 0.5 and 0.75 are presented. The Debye temperatures and lattice contributions as well as the magnetic part of the heat capacity were determined and analyzed. The heat capacity measurements reveal that the substitution of Tb atoms for Er atoms leads to a linear reduction of the Curie temperatures in the investigated compounds. The ordering temperatures decrease from 28.3 K for Tb_0.25Er_0.75Ni₂ to 12.9 K for Tb_0.75Er_0.25Ni₂. Heat capacity measurements enabled us to calculate with good approximation the isothermal magnetic entropy ΔS_mag and adiabatic temperature changes ΔT_ad for Tb_1-xEr_xNi₂, for the magnetic field value equal to 1 T and 2 T. The optimal molar ratios of individual Tb_0.75Er_0.25Ni₂, Tb_0.5Er_0.5Ni₂ and Tb_0.25Er_0.75Ni₂ components in the final composite were theoretically determined. According to the obtained results, the investigated composites make promising candidates that can find their application as an active body in a magnetic refrigerator performing an Ericsson cycle at low temperatures. Moreover, for the Tb_0.5Er_0.5Ni₂ compound, direct measurements of adiabatic temperature change in the vicinity of the Curie temperature in the magnetic field up to 14 T were performed. The obtained high-field results are compared to the data for the parent TbNi₂ and ErNi₂ compounds, and their magnetocaloric properties near the Curie temperature are analyzed in the framework of the Landau theory for the second-order phase transitions.