The frequency dependencies of the uniform transverse and longitudinal susceptibilities of Heisenberg ferromagnets with a dipole-dipole interaction are studied in different temperature regions. Within a broad range of parameters the major contribution to the longitudinal component of the susceptibility, as well as to the dispersion of the fluctuational corrections to the transverse one is found to be the process containing a couple of spin waves in the intermediate state. The ferromagnetic-resonance (FMR) linewidth, which at zero temperature exists due to the quantum fluctuations is calculated. Deviations of the FMR lineshape from a Lorentzian are shown to be rather significant at sufficiently high temperatures. The results obtained within the framework of the developed perturbation theory based on the approach of Vaks, Larkin, and Pikin are generalized for the critical dynamics region. The range of validity of the employed approach is discussed. It is shown that dipole-dipole forces cannot be considered as a perturbation not only in the close vicinity of the Curie point but also at any temperature at low enough frequencies. In particular, the longitudinal susceptibility reveals the infrared discontinuity in the zero-frequency limit.
Предметные области Scopus
- Физика конденсатов