Co on Fe3O4(001): Towards precise control of surface properties.

Raquel Gargallo-Caballero,Adrián Quesada,Lucía Aballe,Cecilia Granados-Miralles,Roland Bliem,Juan De La Figuera,Michael Foerster,Laura Martín-García,Peter Blaha,José F Marco,Gareth S Parkinson

doi:10.1063/1.4942662

Abstract

A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations. Co is deposited at room temperature on the reconstructed magnetite (001) surface filling first the subsurface octahedral vacancies and then occupying adatom sites on the surface. Progressive annealing treatments at temperatures up to 733 K diffuse the Co atoms into deeper crystal positions, mainly into octahedral ones with a marked inversion level. The oxidation state, coordination, and magnetic moments of the cobalt atoms are followed from their adsorption to their final incorporation into the bulk, mostly as octahedral Co(2+). This precise control of the near-surface Co atoms location opens up the way to accurately tune the surface physical and magnetic properties of mixed spinel oxides.

Highlights

The catalytic properties of specific oxides stem partially from the particular arrangement of Fe and Co cations at the surface under the given operation conditions
A novel approach to incorporate cobalt atoms into a magnetite single crystal is demonstrated by a combination of x-ray spectro-microscopy, low-energy electron diffraction, and density-functional theory calculations
In order to follow the evolution of the properties as the cobalt atoms are incorporated into the magnetite structure, we combine low-energy electron diffraction (LEED), density functional theory (DFT) calculations, x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) in photoemission electron microscopy (PEEM)

Summary

Introduction

The catalytic properties of specific oxides stem partially from the particular arrangement of Fe and Co cations at the surface under the given operation conditions. In order to follow the evolution of the properties as the cobalt atoms are incorporated into the magnetite structure, we combine low-energy electron diffraction (LEED), density functional theory (DFT) calculations, x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) in photoemission electron microscopy (PEEM). This approach enables controlling the particular location and oxidation state of the Co cations at the surface by adjusting the Co dose and the annealing procedure. The evaluation of the magnetic moments of the specific cations is brought forward

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Discussion

Conclusion