Theory of ferromagnetic resonance relaxation in very small solids

Abstract

Two models for ferromagnetic resonance (FMR) relaxation in magnetic solids which are sufficiently small for the discreteness of the magnon and phonon modes to be important are studied. The models exhibit a transition 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 there is no energy loss, as evidenced by the fact that the energy does not increase as a function of time on the average. It is shown that this phenomenon is due to a transition to chaotic behavior of the solid. These results open up the interesting possibility that sufficiently small magnetic solids could exhibit FMR with practically no energy loss.