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

Abstract The interface formation in epitaxial MgO / Co 2 MnSi ( 001 ) films was studied using in-situ X-ray photoelectron spectroscopy (XPS). MgO was deposited on single crystal Co 2 MnSi ( 001 ) layers using e-beam evaporation: a technique which is expected to oxidize the Co 2 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 the Co 2 MnSi 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 / Co 2 MnSi interface with a composition significantly differing from the original surface of the unoxidized Co 2 MnSi film. As MgO is deposited on the oxidized Co 2 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 Co 2 MnSi causes oxygen to move away from the oxidized Co 2 MnSi 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.