THEORY OF FERROMAGNETIC-RESONANCE RELAXATION IN VERY SMALL SOLIDS

Two models for ferromagnetic resonance (FMR) relaxation in magnetic so lids which are sufficiently small for the discreteness of the magnon a nd phonon modes to be important are studied. The models exhibit a tran sition as a function of the number of atoms in the solid from a regime in which energy loss occurs, as evidenced by the fact that the energy of the solid increases as a function of time, to a regime in which th ere is no energy loss, as evidenced by the fact that the energy does n ot increase as a function of time on the average. It is shown that thi s phenomenon is due to a transition to chaotic behavior of the solid. These results open up the interesting possibility that sufficiently sm all magnetic solids could exhibit FMR with practically no energy loss.