Spin wave eigenmodes in nanoscale magnetic tunnel junctions with perpendicular magnetic anisotropy

Magnetic tunnel junctions (MTJs) are key enablers of spintronic technologies used in a variety of applications including information storage, microwave generation and detection, as well as unconventional computing. Here, we present experimental and theoretical studies of quantized spin wave eigenmodes in perpendicular MTJs focusing on a coupled magnetization dynamics in the free (FL) and reference (RL) layers of the MTJ, where the RL is a synthetic antiferromagnet (SAF). Spin-torque ferromagnetic resonance (ST-FMR) measurements reveal excitation of two spin wave eigenmodes in response to applied microwave current. These modes show opposite frequency shifts as a function of out-of-plane magnetic field. Our micromagnetic simulations accurately reproduce the dependence of the mode frequencies on magnetic field and reveal the spatial profiles of the excitations in the FL and RL. The FL and RL modes generate rectified voltage signals of opposite polarity, which makes this device a promising candidate for a tunable dual-frequency microwave signal detector. The simulations show that weak interlayer exchange coupling within the SAF enhances the mode amplitudes. We also calculate the response of the detector as a function of in-plane magnetic field bias and find that its sensitivity significantly increases with increasing field. We experimentally confirm this prediction via ST-FMR measurements as a function of in-plane magnetic field. Our results provide deeper understanding of quantized spin wave eigenmodes in nanoscale MTJs with perpendicular magnetic anisotropy and demonstrate the potential of these devices for frequency-selective dual-channel microwave signal detectors.