Measurement of the Spin-Transfer-Torque Vector in Magnetic Tunnel Junctions

Spin-polarized currents can transfer spin angular momentum to a ferromagnet, generating a torque that can efficiently reorient its magnetization. Achieving quantitative measurements of the spin-transfer-torque vector in magnetic tunnel junctions (MTJs) is important for understanding fundamental mechanisms affecting spin-dependent tunneling, and for developing magnetic memories and nanoscale microwave oscillators. Here we present direct measurements of both the magnitude and direction of the spin torque in Co60Fe20B20/MgO/Co60Fe20B20 MTJs. At low bias V, the differential torque vector d{tau}/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with a prediction for highly-spin-polarized tunneling. With increasing bias, the in-plane component d{tau}_{parallel}/dV remains large, in striking contrast to the decreasing magnetoresistance ratio. The differential torque vector also rotates out of the plane under bias; we measure a perpendicular component tau_{perp}(V) with bias dependence proportional to V^2 for low V, that becomes as large as 30% of the in-plane torque.