Role of tunneling matrix elements in determining the magnitude of the tunneling spin polarization of 3d transition metal ferromagnetic alloys

We have measured the tunneling spin polarization and associated tunneling magnetoresistance for Co-Pt and Co-V alloys using Al2O3 and AlN barriers. These properties are insensitive to the Pt content of Co-Pt alloys for up to 40 at.% Pt, whereas the spin polarization of Co-V alloys decreases significantly with the addition of small amounts of V. We attribute these different behaviors to the relative strengths of bonds formed between the alloy constituents and oxygen in the Al2O3 tunnel barrier, which thereby influence the corresponding tunneling rates. The results for AlN barriers are consistent with this argument. © 2005 The American Physical Society.