Experimental and theoretical analysis of magnetocaloric behavior ofDy1−xErxNi2intermetallics(x=0.25,0.5,0.75)and their composites for low-temperature refrigerators performing an Ericsson cycle

Abstract

In the present work, the structural, magnetic, and thermodynamic properties of ${\mathrm{Dy}}_{1\ensuremath{-}x}{\mathrm{Er}}_{x}{\mathrm{Ni}}_{2}$ $(x=0.25,\phantom{\rule{0.16em}{0ex}}0.5,\phantom{\rule{0.16em}{0ex}}0.75)$ Laves-phase intermetallics, being continuous solid solutions in the ${\mathrm{DyNi}}_{2}\text{\ensuremath{-}}{\mathrm{ErNi}}_{2}$ system, synthesized by arc melting, are experimentally studied and analyzed in accordance with mutual substitution within the rare-earth sublattice. The existence of Laves-phase superstructure (space group $F\overline{4}3m$) in the substituted ${\mathrm{Dy}}_{1\ensuremath{-}x}{\mathrm{Er}}_{x}{\mathrm{Ni}}_{2}$ compounds was found by x-ray diffraction analysis. Low-temperature magnetic studies showed the ferromagnetic behavior of the compounds; their Curie temperatures correlate with the erbium content; namely, they decrease from 16.8 K for ${\mathrm{Dy}}_{0.75}{\mathrm{Er}}_{0.25}{\mathrm{Ni}}_{2}$ to 10.0 K for ${\mathrm{Dy}}_{0.25}{\mathrm{Er}}_{0.75}{\mathrm{Ni}}_{2}$. Heat capacity measurements performed in magnetic fields of 1 and 2 T allowed us to estimate the isothermal magnetic entropy and adiabatic temperature changes for ${\mathrm{Dy}}_{1\ensuremath{-}x}{\mathrm{Er}}_{x}{\mathrm{Ni}}_{2}$. Independently, a theoretical analysis was performed for a polycrystalline sample in the frame of a model Hamiltonian taking into account the Zeeman exchange interaction and crystal electric field anisotropy and was expanded to experimental results. Composites with optimum proportions of the individual ${\mathrm{Dy}}_{0.75}{\mathrm{Er}}_{0.25}{\mathrm{Ni}}_{2}$, ${\mathrm{Dy}}_{0.5}{\mathrm{Er}}_{0.5}{\mathrm{Ni}}_{2}$, and ${\mathrm{Dy}}_{0.25}{\mathrm{Er}}_{0.75}{\mathrm{Ni}}_{2}$ components were theoretically determined. The results indicate that the proposed composites are good candidates to be used as the refrigerant material in a magnetic refrigerator performing an Ericsson cycle at low temperatures. The relative cooling power RCP, refrigerant capacity RC, temperature averaged entropy change TEC, and the changes of emitted or absorbed quantity of heat \ensuremath{\Delta}Q were calculated for investigated samples. Direct measurements of the adiabatic temperature change in the vicinity of the magnetic transition temperature were carried out in magnetic fields up to 14 T. The regularities found are discussed in terms of Landau's theory of second-order magnetic phase transitions.