Surface magnetism of α-FeO(OH) and β-FeO(OH) by Mössbauer spectroscopy

Fine particles of α-FeO(OH) and β-FeO(OH) were prepared using 56 Fe isotope and the surfaces of the particles were coated with extremely thin 57 Fe(III) layers. Mossbauer results show that the Fe(III) ions in the top surface layer are involved in the magnetic order and the surface hyperfine fields rapidly decrease with increasing temperature. The exchange fields at surface sites, estimated from the temperature dependences of the hyperfine fields, are smaller than the bulk values in both cases. The decrease of exchange fields at the surface sites corresponds to the reduced number of neighboring magnetic ions at each site.