Critical behavior and magnetocaloric effect in Laves-phase Tb1−xHoxNi2 (x = 0.25–0.75) solid solutions and composites

The critical behavior around ferromagnetic–paramagnetic phase transition of polycrystalline Tb1−xHoxNi2 solid solutions (x = 0.25, 0.5, 0.75) was investigated using magnetization measurements in a magnetic field range of 0–5 T. The critical exponents obtained using the Kouvel–Fisher modified Arrott plot methods and the Widom scaling relation are consistent. The tricritical and 3D-Ising models adequately describe the samples, likely due to Ho substitution affecting the critical parameters. The spin interactions indicate long-range character in all studied compositions. Critical parameter analysis shows that the magnetic transition temperature decreases with the increasing Ho content, from 29.3 K for Tb0.75Ho0.25Ni2 to 17.1 K for Tb0.25Ho0.75Ni2. For the Tb0.5Ho0.5Ni2, direct measurements of adiabatic temperature change near the Curie temperature were performed in magnetic fields up to 14 T. The maximum ΔTad reaches ∼11 K near 26.3 K for μ0ΔH = 14 T. These results are compared with TbNi2 and HoNi2 and analyzed within the Landau theory of second-order phase transitions. To demonstrate application potential, composite materials based on Tb1−xHoxNi2 (x = 0.25–0.75) were proposed. Optimal molar ratios of Tb0.75Ho0.25Ni2, Tb0.5Ho0.5Ni2, and Tb0.25Ho0.75Ni2 were theoretically determined. The composites exhibit a nearly constant calculated magnetic entropy changes, ∼3.4 J/kg K for μ0ΔH = 1 T and ∼ 6.3 J/kg K for μ0ΔH = 2 T over a wide temperature range of 18–30 K. These results indicate that the proposed composites are promising candidates for use as refrigerants in low-temperature magnetic refrigerators.