OXIDATION OF NANOMETER-SIZED IRON PARTICLES

The evolution of the oxidation of ultrafine (5 nm diameter) alpha-iron particles in ambient air has been studied using Mossbauer spectroscop y and electron microscopy. A 1-2 nm thick oxide layer was found to app ear almost immediately, whereafter the oxidation proceeded rather slow ly. The rate of oxidation can be understood from the Caberra-Mott mode l of oxidation of metal surfaces. The oxide formed consists of a mixtu re of Fe3O4 and gamma-Fe2O3, but with the magnetic properties signific antly modified due to the finite size of the oxide crystallites, e.g. the magnetic hyperfine fields are somewhat smaller than for the bulk F e3O4 and gamma-Fe2O3, and a very strong spin-canting was revealed. A V erwey transition was found to occur between 12 and 80 K. The Debye tem perature of the oxide layer was found to be about 185 K for the thinne st observed oxide layer, increasing to about 215 K after exposure of t he a-iron particles to air for one week.