Theoretical aspects of magnon–magnon interactions in Heisenberg ferromagnets

Abstract

Spin-wave interactions in ferromagnetic insulators have a well-established theoretical foundation, in contrast to the situation for antiferromagnets. These interactions may be classified into two groups according to whether the dominant aspects are of one- or two-magnon character. The weak interactions between spin waves excited at low temperatures are responsible for the success of "free" (or noninteracting) spin-wave calculations in explaining low-temperature thermodynamic properties. In contrast, the two-magnon aspects appear in connection with two-magnon Raman scattering, where pairs of magnons are created in close proximity and consequently interact strongly. Parallels with analogous systems, especially antiferromagnetic spin waves and phonons, are noted in reviewing the ferromagnetic case. Emphasis is placed on the structure of the theory, rather than on the wealth of experimental activity.