Improvement in Spin-Torque Ferromagnetic Resonance Using Frequency Modulation

Spin-torque ferromagnetic resonance (ST-FMR) is the most common method for measuring spin-orbit torques in heterostructures with in-plane magnetic anisotropy. Almost always, amplitude modulation (AM) of the microwave current and homodyne detection are employed to improve the signal-to-noise ratio. Quantitative results can then be calculated either from the amplitudes of the resulting resonant Lorentzian lineshapes (LS) or from the DC-current-induced linewidth modulation (LW); however, the two methods frequently disagree. We show, using a Pt(6 nm)/Py sample series, that the AM causes changes to the resonant lineshape that are not captured by the conventional theoretical models of ST-FMR. If we substitute frequency modulation for the amplitude modulation, the line shapes match the theoretical model much more closely and the LS and LW measurements are brought into agreement.