Normal and inverse magnetocaloric effects in structurally disordered Laves phase Y1-xGdxCo2 (0 ≤ x ≤ 1) compounds

Abstract

Magnetic and magnetocaloric properties of Y1-xGdxCo2 compounds, where x = 0.2, 0.4, 0.6, 0.8 and 1.0, were investigated experimentally and theoretically. Crystal structures were characterized by X-ray diffraction (Rietveld analysis) and investigated samples possess the MgCu2-type single phase with Fd-3m space group. Melt-spinning process introduced a chemical and topological disorder, which directly affected the magnetic properties. Refrigerant capacity (RC), strictly connected to the full width at half maximum δTFWHM of the ΔSM(T) curve and the maximum of magnetic entropy changes ΔSMpk(T,ΔH), increases from 29 to 148 J/kg with replacement of Y by Gd atoms from x = 0.2 to x = 0.8. RC and δTFWHM indicate the presence of disorder. Temperature dependences of magnetic entropy change ΔSM(T,ΔH) and RC were measured in as-quenched and annealed state for Y0.4Gd0.6Co2. This particular composition was chosen for detailed investigation mainly due to its Curie point (TC = 282 K), which is close to the room temperature. After isothermal annealing (τa = 60 min, Ta = 700°C) RC decreased from 122 to 104 J/kg, which clearly indicates the homogenization of the heat treated sample. Furthermore, observed inverse magnetocaloric effect is associated with the presence of antiferromagnetically coupled Gd and Co magnetic moments. The phase transition temperature increases with increasing Gd content from 74 to 407 K for Y0.8Gd0.2Co2 and GdCo2, respectively. Within the FPLO-LDA DFT method the non-magnetic ground state for YCo2 and the magnetic ground state for GdCo2 are predicted in agreement with experiment. The dependence of calculated total and species-resolved magnetic moments on Gd concentration reasonably agrees with available experimental data.