Comparison of the spin-transfer torque mechanisms in a three-terminal spin-torque oscillator

Abstract

We have studied magnetization dynamics in three-terminal spin-torque oscillators (STOs) and present a direct method to compare the efficiencies of exciting oscillations in STOs through two mechanisms of spin transfer torque: the spin filtering torque (SFT) and the spin-orbit torque (SOT). The devices are composed of spin-valves patterned on a Pt wire that can be excited by a SFT and/or a SOT, depending on the pathway of the DC current. By varying the device and wire size, we tune and compare the efficiencies of both mechanisms in terms of current and current density. To a first approximation, for sufficiently narrow Pt wires (compared to the dimension of the spin-valve), significantly more current is required to excite the devices with SFT than with SOT, whereas in terms of current density, the SFT is up to three times more efficient than the SOT in the system studied. We investigate the limits of this comparison using control samples where the spin-valve is replaced by a magnetic tunnel junction (MTJ) or where the Pt wire is replaced by a Cu wire. A three terminal STO made with a MTJ is the most appropriate device to compare the efficiencies of both spin transfer mechanisms in order to avoid additional spin transfer torque induced by current shunted through the magnetic pillar.