Tunnel magnetoresistance in ultrathin MnGa-based perpendicular magnetic tunnel junctions utilizing bcc-Co based interlayers

摘要

Tetragonal Mn-based perpendicular magnetic tunnel junctions (p-MTJs) attracts attention for magnetic random access memory and advanced THz devices1. Past studies of Mn-based p-MTJs have been reported small TMR ratio at room temperature below 32%without magnetic interlayer2, and maximum of 60%even if FeCo interlayers were used3. Thus, it is crucial to find new interlayer materials to further enhance the TMR ratio for realizing Mn-based p-MTJs for practical applications. In this study, we focused on bcc-CoMn binary alloy possessing highly spin-polarized Δ1 band4 as new interlayer material for ultrathin MnGa-based p-MTJs5, and investigated its TMR properties6. The stacking structure of the p-MTJs are MgO(001) substrate/Cr(40)/Co_(55)Ga_(45)(30)/ Mn_(61)Ga_(39)(3)/ CoxMn_(100-x)(0.8) /Mg(0.4)/MgO(2)/ Fe_(60)Co_(20)B_(20) (1.2)/Ta(3)/Ru(5) (thickness is in nanometers). After microfabrication using conventional photolithography and Ar ion milling process, the MTJs were annealed at 250°Cin a vacuum furnace. TMR measurements were performed by a PPMS. Figure 1 shows the TMR curves of the MTJs with Co_(80)Mn_(20) interlayer measured at 300 and 10K. The Co_(80)Mn_(20) interlayer shows the TMR ratio up of approximately 85%(209%) at 300 and 10 K. respectively. Meanwhile, the shape of the TMR curve and the significant increase of coercivity indicate that Co_(80)Mn_(20) interlayer strongly and antiferromagnetically couples with perpendicularly-magnetized MnGa layer. The TMR ratio of 209%is larger than the value expected from Julliere’s relation with spin-polarization of fcc-Co and CoFe(B). Thus, this high TMR ratio would originate from coherent tunneling between highly spin-polarized ⊿ 1 bands in bcc-Co (Co-Mn) and CoFe(B) electrodes, even though the Co-Mn interlayer thickness is very small, 0.8 nm.