Phase-resolved ferromagnetic resonance using a heterodyne detection method

This paper describes a phase-resolved ferromagnetic resonance (FMR) measurement using a heterodyne method. Spin precession is driven by microwave fields and detected by 1550 nm laser light that is modulated at a frequency slightly shifted with respected to the FMR driving frequency. The evolving phase difference between the spin precession and the modulated light produces a slowly oscillating Kerr rotation signal with a phase equal to the precession phase plus a phase due to the path length difference between the excitation microwave signal and the optical signal. We estimate the accuracy of the precession phase measurement to be 0.1 rad. This heterodyne FMR detection method eliminates the need for field modulation and allows a stronger detection signal at higher intermediate frequency where the $1/f$ noise floor is reduced.