We observe amplification of spin-wave packets propagating along a film of single-crystal yttrium iron garnet subject to a transverse temperature gradient. The spin waves are excited and detected with standard techniques used in magnetostatic microwave delay lines in the 1-2 GHz frequency range. The amplification is attributed to the action of a thermal spin-transfer torque acting on the magnetization that opposes the relaxation and which is created by spin currents generated through the spin-Seebeck effect. The experimental data are interpreted with a spin-wave model that gives an amplification gain in very good agreement with the data.

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Direct calculation of the electric field induced by a timevarying magnetic induction Am.Effect of a timevarying boundary on the sound field in an enclosure. I Substituting this equation into Eq. (C3) then yields an expression identical to Eq. (Cl) indicating that the solution (19) is the correct solution far from the plane z=O.The propagation of microwave magnetoelastic waves in a ferromagnet subjected to pulsed magnetic fields is investigated. In the first part of the theoretical analysis, which is restricted to spatially uniform bias fields, the interaction between the magnetic and elastic systems is neglected. Using a coupled-mode approach, it is found that plane spin waves propagate in magnetic field transients with negligible reflections, constant momentum and wavenumber, but variable frequency and power flow. These results are further elucidated by the study of a simple step transient case. In the presence of magnetoelastic interaction, the theory indicates that besides changes in frequency and in power flow, the field gradient causes group velocity modulation and changes in the time duration of wave packets. In addition there is an exchange of momentum between the different branches of the dispersion relation; this exchange is calculated in terms of critical field gradients, The analysis reveals that negligible coupling occurs for field gradients much smaller than the critical values, resulting in efficient magnon/phonon conversion. The critical gradients are given for both shear and longitudinal waves propagating along certain crystallographic directions. * Based on a thesis submitted to the Department of Electrical Engineering, M.LT.

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