Structural transformations and interfacial iron reduction in heterostructures with epitaxial layers of 3d metals and ferrimagnetic oxides

Abstract

The present paper sheds light on the interfacial structural transformation taking place upon epitaxial growth of ferromagnetic 3d metal compounds – Co, Ni and CoFeB – on the surface of magnetically ordered iron oxides – Fe3O4, Fe2O3 and Y3Fe5O12. The reducing conditions due to the excess of neutral metal atoms at the surface cause reduction of the Fe3+ ions of the underlying iron oxide layer to the Fe2+ state and subsequent recrystallization of the subsurface region to an MeO-like rock salt crystal structure. The corresponding change of the lattice symmetry can be readily recognized by in situ high-energy electron diffraction reciprocal-space mapping and ex situ X-ray diffraction techniques. From an analysis of diffraction and X-ray absorption data a tentative model of the structural transformation is proposed, based on penetration of Me atoms into the near-surface region of the iron oxide lattice. Once a few nanometre thick oxidized layer is formed, further growth of pure metallic film with native crystal structure takes place. An important observation made is that the oxidation/reduction effects do not occur when 3d metals are deposited onto more stable MgO and Gd3Ga5O12 surfaces. This circumstance makes it appropriate to propose these materials for use as buffer layers to prevent formation of antiferromagnetic monoxide transition regions in multilayered magnetic structures. The presented results are supposed valuable for development of hybrid ferromagnetic heterostructures attractive for various spintronic applications.