Mössbauer Study of Electronic Spin-Flip Processes in NH4 Fe(SO4)2 ·12H2O

Relaxation-broadened Mössbauer spectra of ferric alum have been obtained in the temperature range 85-250 K, with 0-5-kG applied magnetic field. In fields above a few kG the spectra are only influenced by hyperfine fields fluctuating parallel to the applied-field direction. In low applied fields the spectra are also influenced by components of the hyperfine field fluctuating perpendicular to the applied-field direction, thereby inducing transitions between the nuclear sublevels.

In the case of high applied fields, a simple model permits the determination of the spin-correlation time. In 5-kG applied fields, the spin-correlation time was found to decrease from 1.6±0.15 nsec at 85 K to 0.50±0.05 nsec at 245 K due to spin-lattice interactions. It is shown that these results cannot be compared with the values obtained by high-frequency susceptibility measurements.