An internal hyperfine field of 62.4 T in ferromagnetically ordered alpha-iron(II) octaethyl-tetraazaporphyrin

We report on the magnetic properties of the molecular solids derived from s quare-planar, neutral iron(II) octaethyltetraazaporphyrin, FeOETAP. This co mpound exists as two polymorphs, alpha and beta. de SQUID and ac susceptome try measurements (1-1000 Hz) show that the alpha polymorph orders as a soft molecular ferromagnet below T-Curie =2.8 K, whereas the beta form, with th e same number of unpaired spins and a similar g value, exhibits zero-field splitting to a non-magnetic ground state. Fe-57-Mossbauer spectroscopy indi cates that alpha-FeOETAP exhibits a remarkably large internal hyperfine fie ld, H-int of 62.4 T. This, to our knowledge, is the highest value of H-int ever reported for Fe, regardless of its spin state.