We report on a systematic analysis of x-ray photoelectron spectroscopy ͑XPS͒ core-and valence-level spectra of clean and well-characterized iron oxide films, i.

We show that linear polarized x-ray-absorption spectroscopy can be used to measure the temperature and thickness dependence of magnetic moments in NiO thin films. We demonstrate that both the long-range order and the nearest-neighbor spin-spin correlations can be revealed. NiO (100) films with thicknesses of 5, 10, and 20 monolayers epitaxially grown on MgO (100) are studied. The Neel temperature is found to be strongly reduced from the bulk value even for the 20 monolayer film

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We report on the molecular beam epitaxial growth of single-crystalline, stoichiometric Fe 3 O 4 and ␥-Fe 2 O 3 films on MgO͑100͒, using NO 2 as the oxidizing agent. Mössbauer spectroscopy on 57 Fe probe layers is used to determine accurately the stoichiometry of the films. It is found that also all intermediate nonstoichiometric Fe 3Ϫ␦ O 4 phases can be obtained. The formation of the metastable compound ␥-Fe 2 O 3 clearly demonstrates the large oxidizing power of NO 2 . 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 Fe 3 O 4 has large out-of-plane components and only in the case of highly oxidized Fe 3Ϫ␦ O 4 does the magnetization approach the film plane. Upon further oxidation to stoichiometric ␥-Fe 2 O 3 , however, it rotates back, and finally becomes almost completely perpendicular to the plane of the film. Furthermore, in the case of ͑near-͒ stoichiometric Fe 3 O 4 , the magnetizations of the A and B sublattices are not completely coupled antiparallel. On average, the magnetization of the B site ions is 4°closer to the film plane than the magnetization of the A site ions. All the as-grown films exhibit a (ͱ2ϫͱ2)R45°surface reconstruction, independent of the stoichiometry. Using simple electrostatic considerations, we propose three possible surface terminations: a half-filled A layer, a B layer with oxygen vacancies and a B layer with hydroxyl groups. Upon annealing, the (ͱ2ϫͱ2)R45°reconstruction irreversibly transforms to a 3ϫ1 reconstruction, caused by Mg outdiffusion from the substrate. Strong reflection high-energy electron diffraction intensity oscillations give direct, unambiguous evidence that Fe 3 O 4 has a two-dimensional layer-by-layer growth mode over the entire temperature range studied, i.e., from 273 to 723 K, guaranteeing atomically flat surfaces and interfaces in multilayer structures. The largest oscillations are obtained on ex situ cleaved, UHV-annealed MgO͑100͒ substrates, or on in situ annealed Fe 3 O 4 /MgO(100) films. Deposition above ϳ700 K is accompanied by rapid Mg outdiffusion.