Observation of a Paramagnetic 57Fe Mössbauer Spectrum due to an Isotropic Magnetic Hyperfine Interaction

We have observed for the first time a clean paramagnetic 57Fe Mossbauer spectrum due to an isotropic magnetic hyperfine interaction. It was obtained by ion implantation of 57Co in an argon matrix at 4.2 K and is due to 57Fe+(3d7) in a magnetically isotropic Γ6 or Γ7 Kramers state. The hyperfine coupling constant of the free 57Fe+(3d7) ion in the nuclear ground state is deduced to be A = (5.0 ± 0.3), (7.9 ± 0.5) or (11.1 ± 0.6) MHz, depending on whether the Fe+(3d7) is in a J = 9/2, 7/2 or 5/2 state, respectively.