Elsevier

Surface Science

Volume 600, Issue 18, 15 September 2006, Pages 4166-4169
Surface Science

Spin polarization and structure of thin iron oxide layers prepared by oxidation of Fe(1 1 0)

https://doi.org/10.1016/j.susc.2006.01.140Get rights and content

Abstract

The structure and magnetism of thin iron oxide layers formed on a Fe(1 1 0) single crystal surface are investigated by Auger electron spectroscopy, ion beam triangulation, and spin polarized secondary electron emission. The formation of a FeO(1 1 1)-like film on top of the metal substrate is observed for oxidation at elevated temperatures. Additional oxygen exposure at room temperature is suggested to lead to a gradual conversion to Fe3O4.

Introduction

In order to understand the elementary mechanisms of metal oxidation, the interaction of oxygen with single crystalline iron surfaces has been studied [1], [2], [3], [4]. Depending on parameters such as the oxygen partial pressure, the substrate temperature, and the surface orientation, different types of iron oxide layers are obtained. Binary iron oxides differ in the oxidation state and the stoichiometry. FeO and Fe2O3 exhibit only one type of cations, Fe2+ and Fe3+, respectively. Both valences exist in Fe3O4. In the bulk, FeO crystallizes in the cubic sodium chloride structure, α-Fe2O3 in the hexagonal corundum structure, and Fe3O4 in the cubic inverse spinel structure. The structure and composition of thin iron oxide films prepared on iron single crystals is still a matter of debate [3]. In particular, it is difficult to establish and inspect homogeneity throughout the whole metal oxide film.

Binary iron oxides do not only reveal a versatile chemical and structural behavior, but they are also of considerable interest owing to their magnetic properties. Wustite (FeO) possesses antiferromagnetic order below a Néel temperature TN = 198 K [5]. Fe moments are aligned ferromagnetically within (1 1 1) planes while adjacent (1 1 1) planes have antiparallel magnetic orientation. Hematite (α-Fe2O3) is also an antiferromagnet (TN = 953 K) with ferromagnetically aligned Fe moments in hexagonal planes [6]. Different from FeO and α-Fe2O3, magnetite (Fe3O4) shows a net magnetic moment. Fe3O4 is ferrimagnetic with a Curie temperature TC = 858 K [7]. Due to a predicted half-metallic behavior and the high ordering temperature, Fe3O4 is a potential material for applications in spintronic devices.

In this paper, we report on investigations of the structural and magnetic properties of thin iron oxide layers via electron emission and ion scattering methods. Ion beam triangulation [8] is used for real-space information on the geometrical arrangement of the atoms in the surface layer. The chemical composition is analyzed by Auger electron spectroscopy (AES). Spin polarized secondary electron emission (SPSEE) [9] is used to study the magnetic properties.

Section snippets

Experiment

The experiments were performed in an ultrahigh vacuum (UHV) setup at a base pressure of about 5 × 10−11 mbar. The bcc(1 1 0) Fe single crystal disk was placed in the gap of a soft-magnetic FeCo yoke with a coil to remanently magnetize the sample. Exposure of the well-prepared atomically clean Fe surface to molecular oxygen was performed at a partial pressure of 5 × 10−7 mbar for a total duration of about 90 min with the sample kept at a temperature of 570 K. The oxygen exposure corresponds to about 2000 

Results and discussion

The polar angular distributions for 20 keV grazingly scattered He+ ions are shown in Fig. 1 for the clean and the oxidized Fe(1 1 0) surface. A well-defined peak is located around the direction of specular reflection Φ = Φin  Φout = 0. The angular distribution of scattered ions is strongly affected by the defect structure of the topmost surface layer [11]. For example, the subspecular foot is caused by the scattering of projectiles at downward surface steps, whereas the superspecular foot is caused by

Conclusion

For oxidation of a Fe(1 1 0) single crystal surface at an elevated temperature of 570 K and exposures of about 2000 L leads to the following experimental results which suggest the formation of a FeO(1 1 1)-like film on top of the metal substrate: An oxygen/iron Auger signal ratio of 2.6 which is close to the value expected for the monoxide, a hexagonal surface structure, and an average spin polarization of the secondary electrons close to zero for electron excitation. A reversal of sign of the spin

Acknowledgements

We thank K. Maass and F. Kirchner for their assistance in the preparation of the experiments. This work was supported by the Deutsche Forschungsgemeinschaft (Wi 1336).

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