Transient Growth of the Spin-Wave Population in a Strong RF Magnetic Field Applied Parallel to the DC Field

Abstract

The transient phenomena that occur immediately after a strong microwave pulse has been applied to the sample are analyzed. The calculation is performed on a quantum‐mechanical basis and assumes that the spin waves are in thermal equilibrium before the pulse is applied. Saturation effects are neglected. For this reason the theory is applicable only as long as the spin‐wave amplitudes are not too large. General expressions for the transient time dependence of the dc and rf magnetic moments and for the power absorbed are derived. Simple results are obtained for the asymptotic behavior, realized after the microwave pulse has been applied for a long time. It is shown that the apparent critical field, at which spin‐wave instability first becomes detectable, should depend approximately linearly on the inverse of the pulse length.