Instability and Echo-Pulse Amplification in a Ferrimagnetic Spin System in an Inhomogeneous Field

Nonlinear interactions are analyzed for the spin system of a ferromagnetic medium in an inhomogeneous magnetic field. Attention is focused on a certain class of excitations referred to as quasistationary spin waves which are in some respects analogous to plane spin waves in a uniform field. These waves have the property that at some time during their evolution they correspond to a uniform precession of the magnetization. Such waves are formed by sharp spatially uniform incident pulses, and conversely, are responsible for the radiation of sharp so-called "echo" pulses. Large quasistationary waves are unstable in a manner reminiscent of the Suhl instability of the uniform precession in a homogeneous field. This instability is accompanied by the exponential growth of small perturbing quasistationary waves and can be observed experimentally in the form of amplified echo pulses. The theoretical model accounts for the principal experimental features of the amplified echo, namely, the initial exponential increase with pulse separation τ and the rapid decrease at large τ, as well as the requirement for an inhomogeneous field.