  Spin wave resonances with enormously large wave numbers corresponding to wave
vectors 10^5-10^6 cm^{-1} are observed in thin plates of FeBO3. The study of
spin wave resonances allows one to obtain information about the spin wave
spectrum. The temperature dependence of a non-uniform exchange constant is
determined for FeBO3. Considerable softening of the magnon spectrum resulting
from the interaction of magnons, is observed at temperatures above 1/3 of the
Neel temperature. The excitation level of spin wave resonances is found to
depend significantly on the inhomogeneous elastic distortions artificially
created in the sample. A theoretical model to describe the observed effects is
proposed.
