In an FMR experiment, a sample is subjected to a constant external magnetic field and a perpendicular oscillating magnetic field, typically in the microwave frequency range. When the frequency of the oscillating field matches the natural precession frequency of the magnetic moments in the material, resonance occurs, leading to maximum energy absorption. This resonance condition is described by the Kittel equation, which relates the resonance frequency to the external magnetic field and the material’s magnetic properties.
