Quadrupolar Hyperfine Anisotropy in Fe(NH4SO4)2 · 6H2O and Its Comparison with the Magnetic Susceptibility

Abstract

An experiment to measure the quadrupolar hyperfine anisotropy, that is, the intensity ratio of the members of the quadrupole doublet in the ${\mathrm{Fe}}^{57}$ Mössbauer spectrum for ferrous ammonium sulfate, is reported. Measurements were carried out at 300 and 4.2°K with thin slabs of single crystals. Analysis of the data yields results in accord with a positive value of the principal electric-field-gradient (EFG) tensor component $q$, and values of the asymmetry parameter $\eta$ of 0.7 at 300°K and 0.3 at 4.2°K. The orientation of the EFG axes of the two ${\mathrm{Fe}}^{2+}$ sites is also determined and related to the crystal axes. The quadrupolar data are also explained reasonably well in terms of a rhombic crystal field for the ${\mathrm{Fe}}^{2+}$ ion. The results are then used to explain in some detail the observed principal susceptibility data. It is explained why a tetragonal model works well in describing the magnetic susceptibility at temperatures above 20°K, even though it does not for the quadrupolar data. Finally, it is found that the local EFG and susceptibility axes do not coincide, presumably because the crystal-field symmetry is lower than rhombic, as a result of the dipole moments on the neighboring water molecules.