Radio Frequency Field-Induced Effects in Ferromagnetic Materials

Abstract

The possibility of influencing the Mossbauer effect with an external radio frequency (rf) field was recognized in the early years of the “Mossbauer era.” It was found that the high-frequency piezoelectric vibration of a Mossbauer single-line source, induced by a quartz transducer, results in a modulation of the Mossbauer γ-radiation that may be observed as sidebands in the spectra.1 In this experiment, the source as a whole was vibrated mechanically. However, when the rf magnetic field is applied to a ferromagnetic magnetostrictive material, each atom is forced to vibrate with the external field frequency and the spectrum then consists of the original carrier hyperfine split pattern and the infinite set of satellite lines formed at the positions determined by the rf field frequency. These satellite lines, called the rf sidebands, were first observed for metallic iron.2–5 Soon a new effect was discovered, the so-called rf collapse effect.5,6 It occurs when the magnetic rf field forces the fast reversal of the sample magnetization. This results in the oscillation of the magnetic hyperfine field at the iron nuclei in response to the applied rf field. When the frequency of the rf field is larger than the Larmor precession frequency, the magnetic hyperfine field averages to zero, and in the Mossbauer spectrum a collapse of the entire Zeeman pattern to a single line or a quadrupole doublet is observed. This effect was studied for various soft ferromagnetic alloys and ferrites.5–11