The influence of radiation reaction damping on the ESR lineshape of non-Lorentzian lines and magnetostatic modes

In the quasi-two-dimensional ferromagnet (C₂H₅NH₃)₂CuCl₄ magnetostatic modes have been observed in the temperature rangeT=4.2...20 K by a shorted waveguide technique. The spectra differ significantly from recently reported ESR measurements in a cavity. The discrepancy can be explained by radiation damping effects which do not only broaden a line but can also essentially affect the lineshape. A simple experimental method is proposed to test the non-Lorentzian character of ESR lineshapes.