Spin-Spin Relaxation and Karyagin-Gol'danskii Effect in Fe${\mathrm{Cl}}_3$·6${\mathrm{H}}_2$O

The Mössbauer spectrum of Fe${\mathrm{Cl}}_3$·6 ${\mathrm{H}}_2$O at room temperature is an anomalously asymmetric quadrupole pattern, which is exceptionally sensitive to external magnetic fields. The asymmetry of the spectrum is diminished both when an external magnetic field is applied and when the temperature is decreased. Qualitatively, the experimental results can be explained by a combination of a temperature-and magnetic-field-dependent spin-spin relaxation and the Karyagin-Gol'danskii effect. This implies that the zero-field splitting is about 20°K between the lowest-lying Kramers doublet, found to be the $|\pm \frac{1}{2}〉$, and the other doublets. The fluctuating magnetic field at the Fe sites is found to be directed along the axis of crystal-field symmetry.