Spin Waves in Narrow Bands

A variational method is used to improve upon the random‐phase‐approximation result for the energy of a spin wave in the fully aligned ferromagnetic state of a narrow $s$ band. A calculation for long‐wavelength spin waves in a narrow $s$ band has been reported, and the effect of including correlation is to restrict the range of parameters in which the ferromagnetic state is stable. In this paper we generalize this result to the case of finite wavevector, and present numerical results for the case of a simple cubic lattice with nearest‐neighbor interaction.