Morphology induced magnetoresistance enhancement of tunneling junctions with the Heusler electrode Co2Cr0.6Fe0.4Al

A large magnetoresistance is expected when half-metallic ferromagnets are used in magnetic tunneling junctions with polycrystalline AlOx barrier. We demonstrate by in situ tunneling microscopy how the atomically smooth morphology of Co2Cr0.6Fe0.4Al Heusler thin films results in unfavorable wetting properties of the Al layer, which forms the tunneling barrier after oxidation. The insertion of a very thin (2.5 A) Mg layer at the Co2Cr0.6Fe0.4Al/Al interface changes the growth mode of the Al drastically. As a result the tunneling magnetoresistance (TMR) is clearly enhanced [Co2Cr0.6Fe0.4Al/AlOx/Co70Fe30 junction: TMR(4 K)=101%]. The corresponding Julliere spin polarization amounts to 67%, larger than previously reported values for this compound.