57Fe Mössbauer and magnetic studies of ErFe12−xNbx
The structural and magnetic properties of ErFe12−xNbx compounds (x = 0.6, 0.7 and 0.8) have been investigated by x-ray diffraction, ac susceptibility and dc magnetization measurements and 57Fe Mössbauer spectroscopy. Refinements of the x-ray diffraction patterns show that the Nb atoms preferentially occupy the 8i sites; this can be understood in the terms of enthalpy effects and differences in the metallic radii. The average Fe–Fe distance at the different sites is found to behave as dFe−Fe(8i)> dFe−Fe(8j)> dFe−Fe(8f). The unit cell volume increases slightly with increasing Nb content, consistent with the larger radius of Nb compared with Fe. A spin reorientation from easy-axis at room temperature to easy-cone at low temperatures has been detected for all compounds. The spin reorientation temperatures Tsr in ErFe12−xNbx compounds remain essentially unchanged (Tsr~42–44 K) with increasing Nb concentration, whereas a significant decrease in Tsr (Tsr 1~236–204 K; Tsr 2~154–94 K) is obtained in DyFe12−xNbx from x = 0.6 to 0.8. This can be understood by taking the different crystal-field terms responsible for the spin reorientation in the two systems into account. We find that the spin-reorientation process is particularly sensitive to the sixth-order term B60O60 of the crystal field acting on the Er3+ ion, due to its large and positive value of γJ. 57Fe hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j and 8f sites from the Mössbauer spectra. The weighted average 57Fe hyperfine field values were found to follow a T2 dependence; this suggests that a single-particle excitation mechanism is responsible for reduction of the 3d-sublattice magnetization with increasing temperature.
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