Effect of Impurities on the Microwave Properties of Yttrium Iron Garnet

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Abstract
Simplified physical models are given for the giant anisotropy theory, the longitudinal relaxation theory, and the rapidly relaxing impurity (i.e., transverse relaxation) theory for the ferromagnetic resonance linewidth ΔH, and field for resonance Hres. A classification of the types of temperature T and frequency ω dependence of ΔH and Hres which may result from the longitudinal relaxation mechanism is given. Recent experiments on giant anisotropy peaks in ΔH and Hres and on maxima in ΔH and Hres vs T are interpreted with reference to this classification. By observing both ΔH and Hres, it is possible to distinguish between the transverse and longitudinal mechanisms even when ΔHis proportional to ω above and below the maximum of ΔH vs T. A new interpretation is given for observed linewidths at the angles for the giant anisotropy effect, and the possibility of having a longitudinal relaxation mechanism without maxima—such as T−1 sech2(ℏωn/2kBT) —in ΔH and −Hres from the population factor is discussed.