In situ soft x-ray absorption spectroscopic study of Fe/MgO interfaces

TL;DR

This study uses in situ soft x-ray absorption spectroscopy to analyze the formation and characteristics of iron oxide interfaces with MgO substrates, revealing thermodynamic and kinetic factors influencing interfacial phases.

Contribution

It provides a detailed in situ analysis of Fe/MgO interfaces, highlighting how substrate surface state affects oxide formation and interfacial chemistry.

Findings

Approximately two monolayers of Fe3O4 form at the interface.

FeO forms on MgO (001) surface due to oxygen availability.

Magnetic properties of FeO are significantly altered by proximity to ferromagnetic iron.

Abstract

Interfacial reaction between iron and MgO has been studied in situ during deposition of iron on MgO surface, using soft x-ray absorption spectroscopy. High sensitivity of the technique which is capable of detecting even a sub monolayer of a given phase, combined with in situ measurements as a function of iron layer thickness, allowed one to make a quantitative estimate of interfacial phases. Two different substrates namely, MgO (001) single crystal, and a polycrystalline MgO film on Si substrate have been used in order to elucidate the role of the state of MgO surface in controlling the interface structure. It is found that at the interface of iron and MgO film, about two monolayers of Fe3O4 is formed. Fe3O4 being the oxide of iron with the highest heat of formation, the reaction appears to be controlled thermodynamically. On the other hand, on the interface with MgO (001) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behaviour of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.