Preparation of (111)-oriented epitaxial Fe3-xO4 films on alpha-Al2O3 (0001) substrates by coating-pyrolysis process using postepitaxial topotaxy via (0001)-oriented alpha-Fe2O3

摘要

We have succeeded in preparing the (111)-oriented epitaxial films of magnetite (Fe3O4) and maghemite (gamma-Fe2O3), or Fe(3-x)O4, on the C-planes of sapphire [alpha-Al2O3(0001)] by a coating-pyrolysis (CP) process using postepitaxial topotaxy (PET). In this set of film and substrate, not only the structure type is different, i.e., the cubic spinel type versus hexagonal corundum type, but the lattice mismatch is large, i.e., 8.0%. The PET process consists of one epitaxial growth and one or more topotactic reaction steps. In the present work, a (0001)-oriented epitaxial film of hematite (alpha-Fe2O3) [corundum-type iron (111) oxide] was first formed on an alpha-Al2O3(0001) substrate by CP process. Second, alpha-Fe2O3 was reduced to Fe3O4 in a gas flow of an argon-hydrogen gas mixture with a hydrogen partial pressure of 0.023 atm. Finally, the Fe3O4 was oxidized in air to gamma-Fe2O3, or Fe3-xO4. Pole-figure analysis exhibited that both the products, Fe304 and gamma-Fe2O3, were (111)-oriented and epitaxially grown on the substrate surface. The crystallographic relationship is Fe3O4, gamma-Fe2O3 (111) lpha-Al2O3 (0001) and Fe3O4, gamma-Fe2O3 [1 (1) over bar0]lpha-Al2O3 <1 (1) over bar 00>, <(1) over bar 100>. This implies that alpha-Fe2O3(0001) has been topotactically converted to Fe3O4O(111) and gamma-Fe2O3(111). Magnetic properties and surface morphology of the films were also investigated using SQUID and atomic force microscopy, respectively. (C) 2002 Elsevier Science. [References: 33]