First-principles study of Co concentration and interfacial resonance states in Fe$_{1-x}$Co$_x$ magnetic tunnel junctions

TL;DR

This study uses first-principles calculations to identify the optimal Co concentration in FeCo/MgO magnetic tunnel junctions for maximum TMR, highlighting the role of interfacial resonance states and disorder effects.

Contribution

It provides a detailed analysis of how Co doping and interface disorder influence interfacial resonance states and TMR, offering insights for optimizing magnetic tunnel junctions.

Findings

10-20% Co concentration maximizes TMR

Minority d-type IRS is filled at optimal Co levels

Disorder at the interface quenches IRS and reduces conductance

Abstract

The optimal Co concentration in Fe$_{1-x}$Co$_{x}$/MgO magnetic tunnel junctions (MTJs) that maximizes tunneling magnetoresistance (TMR) is still under investigation. We perform a first-principles transport study on MTJs using disordered electrodes modeled using the virtual crystal approximation (VCA) and ordered alloys with various MgO barrier thicknesses. We find that 10-20$%$ Co concentration maximizes TMR using VCA to represent disorder in the electrodes. This TMR peak arises due to a minority d-type interfacial resonance state (IRS) that becomes filled with small Co doping, leading to a decrease in antiparallel conductance. Calculations with ordered Fe$_{1-x}$Co$_{x}$ electrodes confirm the filling of this minority d-type IRS for small Co concentrations. In addition, we construct a 10x10 supercell without VCA to explicitly represent disorder at the Fe$_{1-x}$Co$_x$/MgO interface, which demonstrates a quenching of the minority-d IRS and significant reduction in available states at the Fermi level that agrees with VCA calculations. These results explain recent experimental findings and provide implications for the impact of IRS on conductance and TMR in Fe$_{1-x}$Co$_x$/MgO tunnel junctions.