FINE-STRUCTURE AND CRITICAL MODES AT THE 1ST-ORDER SUHL INSTABILITY IN YIG SPHERES

The FMR fine structure of the first-order Suhl instability in YIG sphe res, previously observed by Jantz and Schneider (1975), has been re-in vestigated both for the conditions of coincidence and subsidiary absor ption (3 and 9 GHz) from room temperature down to 1.6K. The observed f ine structure consists of several series of closely spaced resonances, indicating a critical wave vector k not-equal 0 even for half the pum ping frequency lower than the 45-degrees spin-wave branch, which is in contrast to the prediction of Suhl's theory. The spacing of these res onances was analyzed in terms of spherical modes. We were able to dete rmine the indices and the effective wave numbers (almost-equal-to 10(5 ) cm-1) of the critical modes. The very different dynamic behaviour ob served at room temperature and at 1.6K can be attributed to a competit ion of different damping mechanisms - mainly surface pit scattering an d three-magnon confluence processes - and their dependences on field, frequency, and temperature.