FMR

Ferromagnetic resonance (FMR) is a resonance phenomenon in ferromagnetic materials in which the magnetization vector precesses about an effective magnetic field when excited by a microwave magnetic field. The resonance occurs at microwave frequencies and is detected as a peak in microwave absorption or a change in transmission as the external field or the frequency is varied.

In experiments, either a fixed microwave frequency is used while sweeping the static field, or the opposite is done. Modern FMR setups employ cavities or on-chip waveguides and vector network analyzers to measure absorption spectra, linewidths, and phase information. The resonance condition follows from the Landau-Lifshitz-Gilbert dynamics; for a uniformly magnetized thin film, the resonance frequency is approximately f = (γ/2π) sqrt(H_res (H_res + M_eff)), where H_res is the resonance field and M_eff is the effective magnetization, incorporating anisotropy and demagnetizing effects.

From the FMR linewidth and lineshape one obtains the Gilbert damping parameter α and information about magnetic anisotropy, inhomogeneous broadening, and exchange interactions. FMR is widely used to characterize magnetic thin films and multilayers, ferrites, and nanostructures, and it is central to research in spintronics and magnonics, including studies of spin pumping, magnetic relaxation, and microwave devices.

Note: FMR is an acronym with other meanings in different fields, but in physics it most commonly denotes ferromagnetic resonance.