Measurements of the subsidiary absorption spin‐wave instability threshold hcrit in high‐quality single crystals of yttrium iron garnet reveal anomalous structure over a field interval extending several hundred Oe below the butterfly curve minimum at Hc. Such measurements have now been extended to cover the entire butterfly curve for sphere and plate samples. At the low‐field end of the structure region, hcrit increases rapidly as the static field H is decreased to a ’’kink’’ field H′c. As H is reduced further, the increase in hcrit is much more gradual. The nature of the onset changes drastically at H′c, from a gradual increase to an abrupt jump in reflected power at threshold. Theoretical analysis of the data shows: (1) The instability at Hc is for spin waves with a polar angle ϑk of 22° rather than the 35° value often cited. (2) The field interval for the instability of zero wave number (k=0) modes is not confined to H≳Hc, but extends over a wide range down to H′c. (3) The kink at H′c corresponds to an abrupt transition to k≠0 spin waves at lower fields. These features are all due to the effect of the k dependence of ΔHk, the spin‐wave linewidth, on hcrit. However, a steplike increase of ΔHk for k≠0 modes, rather than the linear dependence inferred from parallel pumping, is needed for a quantitative fit. This observation, while in partial agreement with the existing three‐magnon confluence theory, shows the need for a refined calculation. The structure between Hc and H′c varies with sample shape and appears to be related to low‐k magnetostatic mode excitations in the spin‐wave spectrum.
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