Generation of Spin Waves in Nonuniform Magnetic Fields. I. Conversion of Electromagnetic Power into Spin-Wave Power and Vice Versa

The conversion of electromagnetic power into spin-wave power and vice versa is investigated from a theoretical point of view. The analysis applies to a rod of ferromagnetic material whose two ends are each in a resonant cavity that is connected to a waveguide. The dc magnetic field is assumed to be nonuniform in such a way that the effective wavelength of the spin waves becomes large in those regions of the sample that protrude into the cavities. The theoretical analysis of the excitation process leads to a differential equation which is of the same form as the well-known wave equation except that the wavenumber is a function of position. The conversion efficiency depends on the solution of this wave equation through a simple integral over the wavefunction which has the physical significance of a coupling length. A numerical estimate indicates that substantially complete conversion should be possible.