Large tunnel magnetoresistance in magnetic tunnel junctions using Co2MnX (X = Al,Si) Heusler alloys

摘要

We fabricated B2-ordered Co2MnAl and L2(1)-ordered Co2MnSi Heusler alloy films by optimizing various fabrication conditions (substrate, composition of sputtering target, substrate and post-annealing temperature, etc) and applied these films to bottom electrodes of magnetic tunnel junctions (MTJs). We used Al-oxide insulating tunnel barriers for our MTJs and varied oxidation times of Al films to control qualities of the Al-oxide insulating layer and Heusler-alloy/Al-oxide interface. Observed tunnel magnetoresistance (TMR) ratios were extremely sensitive to the structure and surface morphology of the prepared Heusler alloy films. Epitaxially grown Heusler alloy films showed very flat surfaces and enhanced TMR ratios. The good structural quality, behaviour of the TMR ratios towards oxidation time for the preparation of the Al-oxide barriers and the measurement temperature dependence of the TMR ratios were quite different between the MTJs with Co2MnAl and Co2MnSi electrodes. The obtained TMR ratio of 83% at 2 K in the MTJ with epitaxially grown B2-ordered Co2MnAl was large among the MTJs with an amorphous Al-oxide tunnel barrier. This result suggests that B2-ordered Co2MnAl is a highly spin-polarized material, as predicted by our theoretical calculation. Moreover, we observed a very large TMR ratio of 159% at 2 K in the MTJ with a high-quality epitaxially grown L2(1)-ordered Co2MnSi electrode. This TMR ratio is the highest value to date in MTJs using an amorphous M-oxide tunnel barrier. Spin-polarization of the Co2MnSi bottom electrode obtained from Julliere's formula was about 0.89. This value is also the largest achieved to date for a Heusler material and is much larger than those of conventional ferromagnetic materials such as Co-Fe. This large spin-polarization is attributed to a half-metallic band structure, as predicted by theoretical calculations.