Magnetic spectra

The various types of magnetic spectra are reviewed. In ferromagnetic metals, relaxations of various time-constants occur. Above 200 Mc/s, the magnetic dispersion is attributable to incomplete penetration of surface domains, due to skin effect, and internal phenomena are not observed. In low-conductivity paramagnetic materials, three types of relaxation have been found, namely spin-lattice, spin-spin, and a third, yet to be identified. The application of a static magnetic field normal to the h.f. field causes electron-spin resonance absorption at the Larmor precession frequency. This induced resonance, which has been observed in paramagnetic materials, in ferromagnetic metals and in ferrites, is broadened by relaxation effects. Similar induced nuclear magnetic resonance is found at lower frequencies. The magnetic spectra of the ferrites exhibit similar natural resonances in the absence of an applied static field. These resonances, which occur in both the microwave and r.f. regions, are due to domain spin rotations in the internal anisotropy field, and to domain wall displacements, and their identity is discussed. Magnetic dispersion due to relaxation of translational and other magnetization processes occurs at r.f. and lower frequencies. The relaxation of irreversible magnetization processes also occurs in the low-frequency region.