Ferromagnetic resonance, or FMR, is coupling between an electromagnetic wave and the magnetization of a medium through which it passes.
G. Dorfman in 1923, when he suggested that the optical transitions due to Zeeman splitting could provide a way to study ferromagnetic structure.
A 1935 paper published by Lev Landau and Evgeny Lifshitz predicted the existence of ferromagnetic resonance of the Larmor precession, which was independently verified in experiments by J. H. E. Griffiths (UK) and E. K. Zavoiskij (USSR) in 1946.
Moreover, linewidths of absorption peaks can be greatly affected both by dipolar-narrowing and exchange-broadening (quantum) effects.
Furthermore, not all absorption peaks observed in FMR are caused by the precession of the magnetic moments of electrons in the ferromagnet.
The basic setup for an FMR experiment is a microwave resonant cavity with an electromagnet.
Furthermore, the resonant absorption of microwave energy causes local heating of the ferromagnet.
In samples with local magnetic parameters varying on the nanometer scale this effect is used for spatial dependent spectroscopy investigations.