Radiation from a Localized Source in a Gyromagnetic Medium

Abstract

While wave propagation in gyromagnetic media has been investigated extensively in the technical literature, little attention has been given to the excitation of such waves by a prescribed source. This problem is treated in the present paper. To render the analysis tractable, the source function is taken as a time-harmonic line source of electric currents with a rapidly varying phase, embedded in an infinite homogeneous gyromagnetic medium whose axis of magnetization is perpendicular to the source direction. For further simplification, the operating frequency is taken to be near gyromagnetic resonance. The resulting model incorporates relevant anomalies of the radiation process, and also serves as a prototype for more practical thin slab configurations. The analysis is performed for permeability tensors in which spin exchange effects are ignored and included. In the former case, the refractive index surface may have open branches and gives rise to an infinity in the total radiated power (infinity catastrophe); this anomaly is removed when spin exchange is accounted for. Detailed study of the radiation fields and radiated power densities shows that except for certain initial directions, the outward power flow is almost entirely in the electromagnetic waves, and the total radiated power in the electromagnetic waves far exceeds that in the spin exchange waves, even when the source dimensions tend to zero. These results imply that a localized electromagnetic current source does not strongly excite the spin exchange waves.