Mössbauer studies on highly dilute ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in solid ammonia are reported. The hyperfine parameters of the paramagnetic reaction product ${\mathrm{FeNH}}_3$ point to a nearly atomic configuration of iron [Ar]3$d^7$4s. The electronic spin relaxation slows down rapidly under application of an external magnetic field. The field dependence of the magnetic hyperfine patterns indicates a strong axial magnetic anisotropy.

• Received 3 October 1988

DOI:https://doi.org/10.1103/PhysRevB.39.6403