Ferromagnetic Resonance near the Upper Limit of the Spin Wave Manifold

Measurements of line widths of polycrystalline garnets as a function of frequency and temperature are reported. The variation of these parameters allows the uniform precessional mode to move above the $\text{k\hspace{0.17em}=\hspace{0.17em}0}$ upper limit of the spin wave manifold so that this mode is no longer degenerate with the low $k$ value spin wave states. When this occurs the line width undergoes a radical change, since these low $k$ states can no longer take place in the relaxation process. When the uniform precession is thus “outside” the manifold, the behavior of the line width should be attributed to the theory of Clogston, Suhl, Walker, and Anderson. When the uniform precession is “inside”the manifold, the behavior is best described by the theory of Schlömann. It is believed that the measurements reported indicate convincingly the utility of the spin wave analysis of ferromagnetic relaxation.