Observation of a very large internal hyperfine field (62.4 T) in the ferromagnetically ordered state of the S=1 alpha-iron(II) octaethyltetraazaporphyrin

The origins of the extraordinarily large internal hyperfine field (62.4 T) for the three-dimensional (weak) ferromagnetically ordered ground state of alpha-Fe(OETAP) are discussed semiquantitatively in terms of existing physi cal theory. In particular, the classical Fermi-contact contribution to the internal field is found to be highly enhanced by a very large orbital contr ibution and a significant dipolar term of the same sign. A rationale for th e unexpected ordering of this S = 1 non-Kramers system is also presented.