Amplified Ferrimagnetic Echoes

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Abstract
Multiple pulse echoes of the type originally discovered by Hahn in paramagnetic resonance have recently been observed in ferrimagnets. The behavior of the ferrimagnetic echo differs from the paramagnetic spin echo in two important respects. First, it is possible to obtain very large amplifications, that is, echoes exceeding the signal pulse by many orders of magnitude. Second, with increasing pulse separation τ the echo initially grows in an exponential manner, reaches a peak and then decays to zero. It appears that like spin echo this echo phenomenon depends upon storage of the magnetic excitation among a large number of resonators, which in the present case are provided by spin‐wave modes. The specific echo mechanism is, however, quite different, resulting primarily from nonlinear intermode coupling which causes some of the excited modes to grow exponentially at the expense of others through a mechanism similar to parametric pumping. This process operates only under conditions of an inhomogeneous internal magnetic field and amplification is therefore favored in irregularly shaped samples. The amplification and the τ dependence of the echo amplitude are accounted for by this model. The delay and correlation properties of this phenomenon allow application to pulse compression and signal processing. Since neither cooling nor a highly homogeneous magnetic field are required, devices based upon the effect can be small, lightweight, and operate at room temperature.

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