Ferromagnetic resonance

Ferromagnetic resonance, or FMR, is a spectroscopic technique to probe the magnetization of ferromagnetic materials. It is a standard tool for probing spin waves and spin dynamics. It was unknowingly discovered by V. K. Arkad'yev when he observed the absorption of UHF radiation by ferromagnetic materials in 1911. A qualitative explanation of FMR along with an explanation of the results from Arkad'yev was offered up by Ya. G. Dorfman in 1923 when he suggested that the optical transitions due to Zeeman splitting could provide a way to study ferromagnetic structure. FMR is very similar to nuclear magnetic resonance except FMR probes the magnetic moment of electrons and NMR probes the magnetic moment of atomic nuclei.

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FMR arises from the precessional motion of a ferromagnetic material in an external magnetic field. The magnetic field puts a torque on the magnetization which causes the magnetic moment to precess. The precession frequency depends on the orientation of the material and the strength of the magnetic field.

The basic setup for an FMR experiment is a microwave resonant cavity with an electromagnet. The resonant cavity is fixed at a frequency in the super high frequency band. A detector is placed at the end of the cavity to detect the microwaves. The magnetic sample is placed between the poles of the electromagnet and the magnetic field is swept while the intensity of the microwaves are detected. When the precession frequency and the resonant cavity frequency are the same, absorption increases indicated by a decrease in intensity in the detector.

References

• S. V. Vonsovskii, Ferromagnetic Resonance (Pergamon: Oxford, 1966).
• S. Chikazumi, Physics of Ferromagnetism (Oxford: New York, 1996).