Quantitative measurement of voltage dependence of spin-transfer torque in MgO-based magnetic tunnel junctions

When an electric current passes from one ferromagnetic layer via a non-magnetic layer into another ferromagnetic layer, the spin polarization and subsequent rotation of this current can induce a transfer of angular momentum that exerts a torque on the second ferromagnetic layer(1-4). This provides a potentially useful method to reverse(3,5-7) and oscillate(8) the magnetic momenta in nanoscale magnetic structures. Owing to the large current densities required to observe spin-torque-induced magnetization switching and microwave emission (similar to 10(7) A cm(-2)), accurately measuring the strength, or even the direction, of the associated spin torque has proved difficult. Yet, such measurements are crucial to refining our understanding of the mechanisms responsible and the theories that describe them(9,10). To address this, we present quantitative experimental measurements of the spin torque in MgO-based magnetic tunnel junctions(11-14) for a wide range of bias currents covering the switching currents. The results verify the occurrence of two different spin-torque regimes with different bias dependences that agree well with theoretical predictions(10).