Oxides

LEED Fe3O4 on STiO3

Growth of iron oxide on Ru(0001)

Molecular beam epitaxy consists on using a atomic beam of a given element generated from a doser (in our case, the doser is a metal bar heated by leem magnetiteelectron bombardement and with a water shroud to avoid heating the areas around the doser). Such method can be used to grow oxides if oxygen is supplied as a background gas during the process. One can use O2, O3 or NO2 to such end. In our case, we are using either O2 or NO2.

 

We have studied the initial growth stages of iron oxides on Ru (0001) by oxygen-assisted MBE. Iron oxides on metals are interesting for catalysis applications, and they have been studied quite a bit, so they are a kind of model system. On Ru, FeO grows initially, while at later stages Fe3O4 is obtained.

We have studied the initial stages of the FeO growth. And as usual in science, with surprising results. Depending on the oxygen pressure, we obtain either monolayer high FeO islands, or bilayer. We have proposed that if the density of oxygen preadsorbed on the Ru surface is too high, it will hinder the growth of FeO, so it will prefer to grow in bilayers. This work is being described in arxiv 1301.2519.

 

At the later stage where Fe3O4 grows, we have used low energy electron microscopy (LEEM) and nanospectroscopic techniques based on synchrotron radiation to study the growth of magnetite triangular islands. The results indicated that the islands (of about 1 nm thick) grow on a continuous atomic bilayer of FeO ("wetting layer"). These islands display magnetic contrast by PEEM and show dichroic signal when studied by XMCD, indicating that they are ferrimagnetically ordered at room temperature. The result is important because it demonstrates for the first time that it is feasible to prepare magnetically ordered nanometer thick films of magnetite, without evidence of superparamagnetic phenomena associated with its reduced thickness. The observed ferrimagnetic order is robust because it remains even up to 350 º C. These results are attributed to the structural perfection of the films formed. When these bicomponent films (magnetite islands grown on a "wetting layer" of FeO) are exposed to a pressure of 2·10-8 torr of NO2 at 150 °C, the islands and the "wetting layer" suffer a different evolution. While the magnetite islands are oxidized to maghemite (γ-Fe2O3), the FeO "wetting layer" is oxidized to hematite (α-Fe2O3). This result is explained taking into account the different crystal structures of the initial oxides that favor the formation of different Fe3+ oxides.

 

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