Interface formation of epitaxial MgO/Co2MnSi(001) structures: Elemental segregation and oxygen migration

The interface formation in epitaxial MgO/Co2MnSi(001) films was studied using in-situ X-ray photoelectron spectroscopy (XPS). MgO was deposited on single crystal Co2MnSi(001) layers using e-beam evaporation: a technique which is expected to oxidize the Co2MnSi 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 the Co2MnSi surface, 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 in an MgO/Co2MnSi interface with a composition significantly differing from the original surface of the unoxidized Co2MnSi film. As MgO is deposited on the oxidized Co2MnSi, 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 Co2MnSi causes oxygen to move away from the oxidized Co2MnSi interface toward the surface and into the MgO. This observation is consistent with an increase in the tunneling magnetoresistance ratio with post-growth annealing measured in fabricated magnetic tunnel junctions (MTJs). The findings are discussed in light of fabrication of MgO/Heusler based MTJs, where the exponential decay of tunneling probability with contact separation exemplifies the importance of the ferromagnet/tunnel barrier interface.