BUTTERFLY CURVES AND CRITICAL MODES FOR 2ND-ORDER SPIN-WAVE INSTABILITY PROCESSES IN YTTRIUM-IRON-GARNET FILMS

Combined Brillouin light-scattering (BLS) and microwave pumping techni ques were used to measure the second-order spin-wave instability thres hold microwave field amplitudes and characterize the critical modes fo r ferromagnetic resonance (FMR) saturation in 4.15 mu m-thick yttrium iron garnet films at 8.47 GHz with the static magnetic field in-plane, To The best of our knowledge, this work represents the first BLS meas urements for second-order FMR saturation processes in ferrite material s. Several new results were obtained: (1) A new type of butterfly curv e for second-order processes. (2) Strong scattering from modes at low wave numbers in the 10(4) rad/cm range. (3) An observed in-plane propa gation direction far these low wave number modes at 90 degrees to the in-plane static field direction. These results differ significantly fr om the behavior expected from the bulk sample instability theories of Suhl and Schlomann, which predict critical modes at wave numbers and p ropagation directions in the 10(5)-10(6) rad/cm and 0 degrees ranges, respectively, and a sharp cusplike response centered at FMR. These res ults indicate that significant modifications of the bulk theory, which take into account the real modes supported by the film, an needed to realistically model second-order instability processes in films. (C) 1 996 American Institute of Physics.