The geometric structure of MgO deposited on Fe(001) in ultrahigh vacuum by
electron evaporation was determined in detail by using surface x-ray diffra
ction. In contrast to the common belief that MgO grows in direct contact on
the Fe(OO I) substrate, we find an FeO interface layer between the substra
te and the growing MgO structure which has not been considered thus far. Th
is result opens new perspectives for the understanding of the Fe/MgO/Fe(001
) interface and the tunneling magnetoresistance effect in general.