NO2-assisted molecular-beam epitaxy of Fe3O4, Fe3-delta O4, and gamma-Fe2O3 thin films on MgO(100)

We report on the molecular beam epitaxial growth of single-crystalline, sto ichiometric Fe3O4 and gamma-Fe2O3 films on MgO(100), using NO2 as the oxidi zing agent. Mossbauer spectroscopy on Fe-57 probe layers is used to determi ne accurately the stoichiometry of the films. It is found that also all int ermediate nonstoichiometric Fe3-deltaO4 phases can be obtained. The formati on of the metastable compound gamma-Fe2O3 clearly demonstrates the large ox idizing power of NO2. Although the shape anisotropy dictates that the zero- field magnetization direction should lie entirely in the plane of the film, this is never observed. Stoichiometric Fe3O4 has large out-of-plane compon ents and only in the case of highly oxidized Fe3-deltaO4 does the magnetiza tion approach the frlm plane. Upon further oxidation to stoichiometric gamm a-Fe2O3, however, it rotates back, and finally becomes almost completely pe rpendicular to the plane of the film. Furthermore, in the case of (near-) s toichiometric Fe3O4, the magnetizations of the A and B sublattices are not completely coupled antiparallel. On average, the magnetization of the B sit e ions is 4 degrees closer to the film plane than the magnetization of the A site ions. All the as-grown films exhibit a (root 2 x root 2)R45 degrees surface reconstruction, independent of the stoichiometry. Using simple elec trostatic considerations, we propose three possible surface terminations: a half-filled A layer, a B layer with oxygen vacancies and a B layer with hy droxyl groups. Upon annealing, the (root 2 x root 2)R45 degrees reconstruct ion irreversibly transforms to a 3 x 1 reconstruction, caused by Mg outdiff usion from the substrate. Strong reflection high-energy electron diffractio n intensity oscillations give direct, unambiguous evidence that Fe3O4 has a two-dimensional layer-by-layer growth mode over the entire temperature ran ge studied, i.e., from 273 to 723 K, guaranteeing atomically flat surfaces and interfaces in multilayer structures. The largest oscillations are obtai ned on ex situ cleaved, UHV-annealed MgO(100) substrates, or on in situ ann ealed Fe3O4/MgO(100) films. Deposition above similar to 700 K is accompanie d by rapid Mg outdiffusion. [S0163-1829(99)01539-8].