Oxygen migration in epitaxial CoFe/MgO/Co₂MnSi magnetic tunnel junctions

The effects of post-growth annealing in ultrahigh vacuum (UHV) on the temperature-dependent transport properties of single-crystal, full-Heusler CoFe/MgO/Co₂MnSi magnetic tunnel junctions (MTJs) grown by molecular beam epitaxy have been correlated with in-situ X-ray photoelectron spectroscopy (XPS) studies of the MgO/Co₂MnSi interface. CoFe and MgO layers were grown on single-crystal Co₂MnSi at room temperature and annealed post growth. The structures were found to be epitaxial and single-crystal before and after annealing as assessed by in-situ reflection high-energy electron diffraction (RHEED). While annealing has little effect on RHEED patterns, post-growth annealing at temperatures as low as 200 °C has a dramatic effect on tunnel magnetoresistance and transport properties. XPS measurements conducted on MgO/Co₂MnSi structures reveal the presence of interfacial Mn and Si oxides which form as a result of the e-beam deposition process used for MgO. Mn oxides are observed to be reduced upon UHV annealing with a corresponding migration of oxygen from the MgO/Co₂MnSi interface into the MgO. In contrast to the case of Mn oxides, Si oxides were not significantly reduced following annealing at 300 °C. Transport measurements on fabricated MTJs show an increase in the tunneling magnetoresistance ratio and a significant alteration in the interfacial electronic structure with increasing annealing temperature. The changes observed in transport are interpreted to result from a reduction in interfacial oxides and a corresponding reduction in oxygen vacancy defect density in the MgO, consistent with XPS results.