Part of the book: Ferromagnetic Resonance
We describe studies on ferrites that use the Low Field Absorption (LFA) technique. These are made in soft magnetic polycrystalline ferrites magnetite, garnet iron, and nickel-zinc ferrite, where we show LFA spectra, highlighting their main characteristics and illustrating how magnetic phase transitions and magnetic anisotropy are detected employing these measurements. In this chapter, we also present a review of ferromagnetic resonance (FMR) studies in ferrites. Ferromagnetic materials exhibit various behaviors when subjected to ac magnetic fields, such as domain wall relaxation (DWR) and FMR. For instance, most ferrites generally find DWR at relatively low frequencies, below 10 MHz, while FMR must satisfy the Larmor equation. Microwave power absorption can be associated with magnetization processes as a function of dc fields, such as LFA and FMR. Microwave absorption centered at zero magnetic fields has been observed in various materials: superconductors, ferrites, and semiconductors. For ferrites, the LFA signal with the same phase of the ferromagnetic resonance follows the thermal behavior of the magnetization. For magnetically ordered materials, the LFA signal is evidence of the existence of a ferromagnetic phase, this signal is not present in the paramagnetic state, and it appears as the temperature is decreased below Curie temperature.
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