Imaging of the grain-to-grain epitaxy in NiFe/FeMn thin-film couples

Abstract

It is known that a metastable crystalline thin-film structure can often be stabilized by choosing a suitable substrate. For single-crystal thin films, this pseudomorphic growth has been attributed to the stabilization of the metastable phase by epitaxial growth. However, in the polycrystalline structure, the mechanism is less clear. In this study, we have observed through plane-view and cross-section transmission electron microscopy (TEM) that the metastable fcc FeMn phase was stabilized by the underlaying NiFe layer. The continuous moiré fringes across the FeMn/NiFe interface as revealed by the cross-section TEM image indicate the existence of the epitaxial relationship between FeMn and NiFe. The epitaxial strain involved has also been found to be confined locally within each grain and was accommodated by the coherent strain across the interface. Consistent with the epitaxial strain observation, it is also found that this metastable fcc FeMn phase exists at the interface of NiFe and FeMn, and away from this interface, the stable A12 cubic FeMn phase is formed. Microdiffraction study showed that the epitaxially grown FeMn lattice was rotated through a small angle relative to the NiFe lattice. This rotation suggests that, in addition to the conventional lattice-dilation strain-epitaxy mechanism observed in single crystals with small misfit, there may be a rotation-epitaxy mechanism in the polycrystalline thin-film epitaxial structure.