  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.
