Role of Tunneling Matrix Elements in Determining the Magnitude of the Tunneling Spin Polarization of3dTransition 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.