State of iron atoms in SiO2 center dot xFe(2)O(3) gels

Mossbauer studies are made of Fe-57 nuclei in dry SiO2. xFe(2)O(3) gels at low temperatures in external magnetic fields. It is shown that the state of the iron atoms in the gels depends significantly on their concentration. I n dry gels with an iron content exceeding 0.01Fe(2)O(3), the Fe atoms belon g to finely dispersed particles of the oxide with maximum sizes of similar to 80 Angstrom in a superparamagnetic state. For lower iron contents, the F e atoms are in the form of a highly diluted paramagnetic impurity of trival ent ions in a diamagnetic matrix. It is assumed that clusters develop from a small number of iron and oxygen ions dispersed in the gel matrix. Here sp in-lattice relaxation of the magnetic moments of the iron atoms is observed . (C) 1999 American Institute of Physics. [S1063-7834(99)01608-1].