![]() ![]() In this paper we analyze the effect of annealing and oxygen exposure on an epitaxial Fe 3O 4 film grown on a Fe-p(1 × 1)O surface. In magnetic tunnel junctions, often there is a thin Fe yO x layer at the interface between the Fe electrodes and the MgO barrier, which can affect the device transport characteristics. 23–25 The thermal treatments performed on the layered structure, such as the heating before the field cooling process, can induce redox reactions 26–28 and affect the device properties. 21,22 In exchange bias systems, where Fe is interfaced with an antiferromagnetic oxide, often the formation of Fe oxides is observed. For example, the oxidation of FeO films grown on Pt(111) induces the formation of a O–Fe–O trilayer, which is a key factor to activate the FeO/Pt(111) catalyst for low temperature CO oxidation. In heterogeneous catalysis, during the reaction the oxidation state of Fe oxide can be modified and influence the catalyst performances. The redox reactions occurring on Fe yO x layers change drastically their physical and chemical properties, affecting the performances of the devices in which they are integrated. investigated by means of ambient pressure scanning tunneling microscopy the effects induced by oxygen and carbon monoxide exposure on FeO(111) nano-islands grown on Au(111). reduced a bulk Fe 2O 3 sample to Fe 3O 4 by Ar ion sputtering followed by annealing. found that Fe 2O 3 films grown on Pt(111) can be reversibly reduced to Fe 3O 4 by annealing in vacuum, 18 while Tang et al. In this frame, several recent papers describe the redox reactions occurring on iron oxide samples: Freindl et al. The interconversion from one phase to another, depending on parameters like temperature and oxygen partial pressure, is particularly important, both from fundamental and applied points of view. After these investigations, it has been recognized that also new phases, with stoichiometries and physical properties deviating from those occurring in bulk samples, can be stabilized in epitaxial films with a thickness of few monolayers. ![]() 5–8 Thin and ultra-thin Fe oxide films supported on metallic substrates like Pt, 9,10 Ag, 11,12 Fe, 13,14, Ni, 15,16 have been investigated by using this approach. the preparation of well-defined model systems under highly controlled conditions, is the most appropriate to investigate the detailed atomic structure and chemical composition of oxide surfaces. The stoichiometric α-Fe 2O 3, below 955 K, is an antiferromagnetic insulator. Haematite (α-Fe 2O 3) adopts a corundum structure containing Fe 3+ ions in octahedral sites. Above the Verwey transition temperature ( T v ∼ 120 K), Fe 3O 4 is a half metal, while below T v it is insulating. Magnetite Fe 3O 4 is a ferrimagnet containing a mixture of Fe 2+ and Fe 3+ ions, which assumes an inverse spinel structure. Wüstite FeO is an antiferromagnetic insulator crystallizing in a rock salt lattice, where only Fe 2+ cations are present. 4 Owing to the different oxidation states assumed by Fe cations, Fe 2+ or Fe 3+, iron oxides can form various phases, with different stoichiometries and physical properties. Introduction Iron oxides have been investigated for decades in several scientific disciplines, spanning physical chemistry 1–3 to medicine. The Fe 3O 4 phase can be recovered by oxidation at 10 −6 mbar of molecular oxygen. The sample exposes the (001) surface of the rock salt structure, with a lattice parameter close to that of bulk wüstite. Upon annealing at 800 K in an ultra-high vacuum, AES reveals that magnetite transforms to FeO. STM topographic images of Fe 3O 4 are characterized by atomically flat terraces separated by highly oriented steps running along the (010) and (100) crystallographic directions of the substrate. The as-grown iron oxide samples display a square LEED pattern with a lattice constant compatible with a p(1 × 1) bulk terminated Fe 3O 4(001) surface. The reduction and oxidation of epitaxial Fe 3O 4 films grown by reactive deposition on a Fe-p(1 × 1)O surface have been investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunneling microcopy (STM). ![]()
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