An Internal Hyperfine Field of 62.4 T in Ferromagnetically Ordered α-Iron(II) Octaethyl-Tetraazaporphyrin

Abstract
We report on the magnetic properties of the molecular solids derived from square-planar, neutral iron(II) octaethyltetraazaporphyrin, FeOETAP. This compound exists as two polymorphs, α and β. dc SQUID and ac susceptometry measurements (1–1000 Hz) show that the α polymorph orders as a soft molecular ferromagnet below TCurie = 2.8 K, whereas the β form, with the same number of unpaired spins and a similar g value, exhibits zero-field splitting to a non-magnetic ground state. 57Fe-Mössbauer spectroscopy indicates that α-FeOETAP exhibits a remarkably large internal hyperfine field, Hint, of 62.4 T. This, to our knowledge, is the highest value of Hint ever reported for Fe, regardless of its spin state.