Structures and room-temperature magnetic properties of entropy-modulated Gd₂Co₁₇ based intermetallic compounds

Abstract

<sec>The entropy-modulated material has been a hot topic due to its unique design concept and excellent properties. However, previous studies of entropy-modulated materials mainly focused on the alloys with simple face-centered cubic, or body-centered cubic, or hexagonal close-packed structures. In this work, the design concept of entropy-modulation is introduced into Gd₂Co₁₇ based intermetallic compound, and the effect of high configuration entropy on the structural stabilization and room-temperature magnetic properties of Gd₂Co₁₇ based intermetallic compound are studied systematically.</sec><sec>The samples are prepared by vacuum Arc melting technology in an ultrahigh-purity Ar atmosphere and followed by annealing at 1000 ℃ for 8 days and finally by quenching in cool water. The fine powders are prepared by grinding the ingots in an agate mortar. The powder XRD and SEM-EDS are used to check the crystal structures and chemical compositions. To study the magnetic properties, the column-like samples are prepared by mixing the fine powder and epoxy with a weight ratio of 1∶1, and then aligned under an applied field of 1 T at room temperature.</sec><sec>The high configuration entropy is found to play an important role in the structural stabilization and magnetic properties of Gd₂Co₁₇ based medium- and high-entropy intermetallic compounds. The XRD patterns and Rietveld structural refinement results confirm that all the samples are single-phase. The structure depends on the effective atomic radius <i>R</i>_<i>A</i>, the structure of entropized Gd₂Co₁₇ based intermetallics can be stabilized into rhombohedral Th₂Zn₁₇-type with <i>R</i>_A> 1.416 or hexagonal Th₂Ni₁₇-type with <i>R</i>_{A<i> </i>}< 1.4105. According to thermodynamic calculations of entropized Gd₂Co₁₇ intermeatllics, the atomic radius difference Δ<i>r</i> ranges from 0.55% to 1.81%, and the mixing enthalpy <inline-formula><tex-math id="M3">\begin{document}$ \Delta {{\boldsymbol{H}}}_{{\rm{m}}{\rm{i}}{\rm{x}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20221995_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20221995_M3.png"/></alternatives></inline-formula> is corresponding to 0 for the rare earth site, –4 to –1 kJ/mol for the transition metal site, and –8.54 to –5.13 kJ/mol between rare earth and transition metal sites. It is suggested that all the thermodynamic parameters meet the criteria for the formation of single-phase medium- and high-entropy intermetallic compounds. The configuration entropy changes from 0.69R to 1.39R. The room temperature magnetic properties are significantly improved by the modulation of entropized design at rare earth and transition metal sublattices. The entropization enhances the saturation moments of all samples, which can be explained with a modified magnetic valence model. The value of <inline-formula><tex-math id="M4">\begin{document}${N}_{{\rm{sp}}}^{\uparrow }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20221995_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20221995_M4.png"/></alternatives></inline-formula> (the number of the electrons in the unpolarized sp conduction bands) increases from 0.3 to 0.4 after entropization, the indirect interaction between rare earth and transition metal sublattice is increased, the spin moment of s conducting electron as a medium of two sublattices is enhanced, and the magnetic moment is increased. The entropization also induces magnetic anisotropy to transform from basal plane to easy axis for the entropized design at transition metal sublattice and the coercivity of rare earth to increase.</sec>