Effect of Rare-Earth Impurities on the Ferrimagnetic Resonance and Nuclear Relaxation in Yttrium Iron Garnet

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Abstract
The ferrimagnetic resonance anomalies of Yb‐doped yttrium iron garnet have been satisfactorily interpreted recently with the aid of the so‐called ``slow relaxation theory.'' In an attempt to relate this theory to that of ``fast relaxation,'' we begin by relating the anomalous properties of the impure garnet to the susceptibilities of the rare‐earth ions which it contains. This enables us to obtain as limiting cases, the results of Kittel, De Gennes, Portis, on one hand, and of Teale and Tweedale, on the other. Then making the slow relaxation hypothesis, we rigorously calculate the susceptibilities of the rare‐earth ions and deduce that, in order to be successful, the slow relaxation theory requires that there exist a coupling between the transverse magnetization of the irons, and components of the rare‐earth magnetic moments with nonzero diagonal matrix elements. Finally we compare the thermal variations of the ferrimagnetic resonance linewidth and of the relaxation time of the 57Fe nuclei.