Characterization of MgO-Based Magnetic Tunnel Junctions' Nonlinear Ferromagnetic Resonance Modes

We propose a new measurement technique to characterize ferromagnetic resonance (FMR) modes in magnetic tunnel junctions. Due to the selectivity and low-noise characteristics of the utilized radio-frequency (RF) receiver, which is often used in spectrum analyzers, we achieve large dynamic range FMR measurements. Instead of relying on weak coupling between the ac excitation and the magnetization fluctuations, we measure the noise floor difference between the two states: at zero and non-zero dc bias. The difference is due to the ferromagnetic layers' magnetization fluctuations adding to the noise floor of the ac excitation signal. This readout mechanism differentiates the proposed method from the previously reported measurement systems. Moreover, by separately adding a controllable RF source, we detect both the linear and nonlinear FMR modes.