Origin of the fine structure of subsidiary absorption in tangentially magnetized YIG films

Parametric excitation of spin waves (SWs) under conditions of perpendicular pumping was investigated in epitaxial films of yttrium iron garnet (YIG). The related microwave power absorption was measured at the frequency 9.1 GHz in the magnetic field range 1200-1600 Oe. A series of absorption peaks closely spaced on the magnetic field scale were observed near the threshold of first-order Suhl instability of spin waves. The peaks spaced by 0.2-0.7 Oe originate from the shallow ripple of the butterfly curve. This is the first observation of the fine structure at subsidiary absorption in YIG films. The peak spacing was measured as a function of dc magnetic field H, film thickness d, and in-plane sample dimensions L for square samples. In the field range 1300-1400 Oe the spacing did not depend on H and was inversely proportional to L in the range of 0.6-3 mm. No thickness dependence was found in the range d=3-45 μm. Explanation is given in terms of standing spin waves set in the film plane and related to the exchange part of SW dispersion curves in ferrite film.