Dynamic properties of ferromagnets with single-ion anisotropy of easy-axis type

The dynamical properties of easy-axis ferromagnets are investigated by the method of expansion with respect to basis operators for magnetic systems with single-ion anisotropy. The method used to introduce the basis makes it possible to study systems with arbitrary value of the spin in a unified manner. It is shown that for sufficiently large values of the anisotropy (of the order of the exchange interaction) the energy spectrum of the system contains not only an acoustic branch but also optical branches – magnetic excitons associated with multiple excitation of an isoIated ion and stable with respect to decay into acoustic magnons. The excitons corresponding to ira, stitch from the ground state to the second excited level of an isolated ion are also stable at small values of the anisotropy if their wave vector is sufficiently large. The lifetimes of the excitations in processes in which they are scattered by one another are caleu!atedo The amplitudes of scattering with participation of acoustic magnons satisfy Adler's principle.