Bias dependent oscillations in spin polarized tunneling*

We investigated the bias dependence of spin polarized tunnelling in (pinned)CoFeB/ MgO/(free)CoFeB and (pinned)CoFeB/MgO/(free)NiFe tunnel junctions as a function of temperature and applied field angle. The differential magnetoresistance (MR) exhibits oscillations about zero MR when the free layer of the asymmetric devices is above +0.7 V; no oscillations were observed for negative bias. Oscillations were not observed for any bias in the symmetric devices. The zero-crossing voltages were independent of temperature and relative magnetization angle between the two ferromagnetic layers. A model using spin-split free electron energy bands in the ferromagnets and a trapezoidal tunnel barrier demonstrates qualitative agreement with the experimental data.