High Frequency Microwave Emission of a Tri-layer Magnetic Tunnel Junction in the Absence of External Bias-Magnetic Field

We perform an experimental study of DC current induced microwave emission in a magnetic tunnel junction (MTJ) consisting of three active magnetic layers. For this tri-layer structure, in addition to a conventional bilayer orthogonal MTJ containing a perpendicular free layer and an in-plane fixed layer, a second perpendicular layer has been introduced. We found that the microwave emission frequency induced by spin-transfer torque (STT) can reach as high as 6 GHz in the absence of any applied magnetic field. Moreover, microwave emission is observed for both current polarizations where a redshift is seen with increase in magnitude of current. We discuss spin-dynamics of the observed bi-directional high-frequency emission and the physical origin of the red-shift. The distinct microwave emission properties exhibited in this tri-layer MTJ structure could potentially be useful for future applications in nanoscale spintronics devices such as microwave communication and neuromorphic computing.