Interface Formation of Epitaxial MgO/Co2 MnSi(001) Structures: Elemental Segregation and Oxygen Migration
The interface formation in epitaxial MgO/Co2 MnSi(001) films was studied using in-situ X-ray photoelectron spectroscopy (XPS). MgO was deposited on single crystal Co2 MnSi(001) layers using e-beam evaporation: a technique which is expected to oxidize the Co2 MnSi layer somewhat due to the rise in oxygen partial pressure during MgO deposition while leaving the deposited MgO oxygen deficient. Not unexpectedly, we find that e-beam evaporation of MgO raises the oxygen background in the deposition chamber to a level that readily oxidizes Co2 MnSi, with oxygen bonding preferentially to Mn and Si over Co. Interestingly, this oxidation causes an elemental segregation, with Mn-Si effectively moving toward the surface, resulting is an MgO/CMnSi interface with a composition significantly differing from the original surface of the unoxidized Co2 MnSi film. As MgO is deposited on the oxidized Co2 MnSi, the Mn-oxides are reduced, while the Si oxide remains, and is only somewhat reduced after additional annealing in ultrahigh vacuum. Annealing after the MgO is grown on CMnSi causes oxygen to move away from the oxidized Co2 MnSi interface toward the surface and into the MgO. These findings are discussed in light of fabrication of MgO/Heusler based magnetic tunnel junctions, where the exponential decay of tunneling probability with contact separation exemplifies the importance of the ferromagnet/tunnel barrier interface.