57Fe Mössbauer andmagnetic studies of ErFe12−xNbx

Abstract

The structural and magnetic properties ofErFe12−xNbx compounds(x = 0.6, 0.7and 0.8) have been investigated by x-ray diffraction, ac susceptibility and dc magnetization measurementsand 57Fe Mössbauer spectroscopy. Refinements of the x-ray diffraction patternsshow that the Nb atoms preferentially occupy the 8i sites; this can beunderstood in the terms of enthalpy effects and differences in the metallicradii. The average Fe–Fe distance at the different sites is found to behave asdFe−Fe(8i)>dFe−Fe(8j)>dFe−Fe(8f).The unit cell volume increases slightly with increasing Nb content, consistent with the larger radiusof Nb compared with Fe. A spin reorientation from easy-axis at room temperature to easy-coneat low temperatures has been detected for all compounds. The spin reorientation temperaturesTsr inErFe12−xNbx compounds remainessentially unchanged (Tsr∼42–44 K) with increasing Nb concentration, whereas a significant decrease inTsr (Tsr 1∼236–204 K;Tsr 2∼154–94 K) isobtained in DyFe12−xNbx from x = 0.6 to 0.8. This can be understood by taking the different crystal-field terms responsiblefor the spin reorientation in the two systems into account. We find that thespin-reorientation process is particularly sensitive to the sixth-order termB60O60 of the crystalfield acting on the Er3+ ion, due to its large and positive value ofγJ.57Fe hyperfine interaction parameters and magnetic moments values have been determinedfor the 8i, 8j and 8f sites from the Mössbauer spectra. The weighted average57Fe hyperfine field valueswere found to follow a T2 dependence; this suggests that a single-particle excitation mechanism is responsiblefor reduction of the 3d-sublattice magnetization with increasing temperature.