Magnetism and Hyperfine Parameters in Iron Rich $\hbox {Gd}_2\hbox {Fe}_{17-x}\hbox {Si}_x$ Intermetallics

Abstract

$\hbox {Gd}_2\hbox {Fe}_{17-x}\hbox {Si}_x$ ($x = 0.25$, 0.5 and 1) samples were synthesized by arc melting and annealed at 1073 K for 1 week. X-ray diffraction analysis by the Rietveld method has shown that these materials crystallize in the rhombohedral $\hbox {Th}_2\hbox {Zn}_{17}$-type structure (space group $R\bar{3}m$). The Curie temperature increases with Si content x, whereas the unit-cell parameters decrease slightly. The temperature dependence of magnetization data revealed that $\hbox {Gd}_2\hbox {Fe}_{17-x}\hbox {Si}_x$ exhibits a second-order ferromagnetic to paramagnetic phase transition in the vicinity of the Curie temperature. Exchange coupling parameters of R–R, M–M and R–M (R—rare earth, M—transition metal) have been determined from M(T) magnetization curves based on the mean field theory calculation. The magnetic entropy change $\Delta S_\mathrm{M}$ and the relative cooling power were estimated from isothermal magnetization curves for all samples. In the proximity of ${T}_\mathrm{C}$ and in an applied field of 1.56 T, $\Delta S_\mathrm{M}$ reached a maximum values of 1.38, 1.67 and 3.07 J/kg K for x = 0.25, 0.5 and 1, respectively. We have calculated the magnetic moment per Fe atom from magnetization measurements at 293 K up to 17 kOe, and it decreases with Si content. These results are verified by the Mössbauer spectrometry measurements obtained at the same temperature. The Mössbauer spectra analysis is based on the correlation between the Wigner–Seitz volume and the isomer-shift evolution of each specific site 6c, 9d, 18f, and 18h of the $R\bar{3}$m structure. For all Si concentrations, the magnitude of the hyperfine fields are ${H_{\rm HF}}\{6c\} > {H_{\rm HF}}\{9d\} > {H_{\rm HF}}\{18f\} > {H_{\rm HF}}\{18h\}$. The mean hyperfine field decreases with the Si content.