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.

We have developed a novel ferromagnetic resonance (FMR) measurement technique using the magneto-optical Kerr effect. The measurement technique uses microwave-frequency, intensitymodulated light to stroboscopically measure the Kerr angle due to the magnetization precession. We demonstrate that this stroboscopic magneto-optical Kerr effect provides a frequency domain and phase sensitive FMR measurement. The measurement is sensitive enough to detect the precessing magnetization with the precession cone angle below 1. V