Measurement of spin wave instability magnon distributions for subsidiary absorption in yttrium iron garnet films by Brillouin light scattering

The first evidence for a wide distribution in the critical mode wave vectors above the spin wave instability threshold for subsidiary absorption is reported. The Brillouin light scattering BLS measurements were done on 4.15 {μ}m thick yttrium iron garnet films with the static field and the 8.47 GHz linearly polarized microwave field in plane. The distribution in the critical mode wave number is broad, with widths of about 0.5×{}10⁴--2×{}10⁴ cm^-1 and shapes which change with field and power. These results cast doubts on the validity of simple two mode models for the analysis of self-oscillation, chaos, and related effects above threshold.