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 degrees 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 degrees 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. Published by AIP Publishing.