Key Role of Non Equilibrium Spin Density in Determining Spin Torque

In summary, starting from the NEGF formalism, we have showed that the current induced torque is proportional to the spin density at the ferromagnetic surface. This automatically gives the well known Slonczewski and field terms in the current torque thus showing the relation between their relative amplitudes without the phenomenology commonly used in literature. We obtain a reasonable quantitative agreement with experimental data that validates our model. From the same simulation, we show that when the spin injection efficiency is 100%, the torque transfer efficiency saturates at 50%. This result is significant considering that the existing devices are not far from this limit and hence novel device designs [S. Salahuddin and S. Datta, 2006] going beyond the conventional three layer structure may be necessary to reduce the switching current to technologically viable levels (~105 A/cm2).