Bias-voltage dependence of perpendicular spin transfer torque in asymmetric MgO-based magnetic tunnel junctions

It has been demonstrated [1] that the magnetic tunnel junction (MTJ) has a sizable perpendicular spin-transfer torque (p-STT), which could substantially affect current-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of p-STT is quadratic [2], it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence [3]. In this talk, we present experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs [4]. The linear contribution is significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the p-STT, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena. [1] I. Theodonis et al. PRL 97, 237208 (2006); C. Heiliger {\&} M. D. Stiles, PRL 100, 186805 (2008). [2] J. C. Sankey et al. Nature Phys. 4, 67 (2008); H. Kubota et al., ibid 4, 37 (2008). [3] J. Xiao, G. E. W. Bauer {\&} A. Brataas, PRB 77, 224419 (2008). [4] S.-C. Oh et al. Nature Phys., published online (2009).