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, and unconventional computing. Here, we present experimental and theoretical studies of the quantized spin wave eigenmodes in perpendicular MTJs focusing on the 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 the excitation of two spin-wave eigenmodes in response to an applied microwave current. These modes show opposite frequency shifts as a function of an out-of-plane magnetic field. Our micromagnetic simulations accurately reproduce the dependence of the mode frequencies on the 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 a weak interlayer exchange coupling within the SAF enhances the mode amplitudes. We also calculate the response of the detector as a function of an in-plane magnetic field bias and find that its sensitivity significantly grows with increasing field strength. We experimentally confirm this prediction via ST-FMR measurements as a function of the in-plane magnetic field. Our results provide a deeper understanding of the 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.