In room temperature it was observed RF absorption far away from the AFMR field. The value of this absorption is inversely proportional to the magnetic static field. This resonances are lineal with the power of the RF applied field. The mean amplitude of these resonances is independent of the width of the sample, what shows the surface nature of these resonance structure.
Te monotonic absorption of RF power can be associated with resonance excitation of phonons.
The fine structure can be explained if we suppose different magnetic properties for the sample near its boundaries. These excitations will have resonances in the conditions of standing waves with one node in the boundary of the sample and the surface were the magnons have zero wave vector will be acting as a reflection wall. The position of this reflection wall is shifted towards the boundary as field is increased.