Reduction of the damping induced by nonlinear effects

G. de Loubens, V.V. Naletov and O. Klein

By measuring simultaneously M_z (the component parallel to the effective field direction) and absorption power, when ferromagnets are excited by microwave fields at high power levels, we found a diminution of the damping with increasing power. These changes are interpreted as reflecting the properties of longitudinal spinwaves excited above Suhl’s instability.

The high power dynamics of magnetic structures is receiving much attention owing to the potential application to spin electronic devices. In this regime, the nonlinear contributions contained in the torque term of the gyroscopic equation become important as soon as the precession angle exceeds a couple of degrees. Although these effects were first discovered in the ferromagnetic resonance response of insulators, they apply to every magnets. While the consequences of these nonlinearities on the microwave susceptibility have been thoroughly investigated, their effects on M_z have never been established. We exploit the exquisite sensitivity of Magnetic Resonance Force Microscopy to follow the changes in M_z at the resonance saturation. Our data obtained at room temperature on an yttrium iron garnet (YIG) sample of micron-size. It is found that M_z drops dramatically at the saturation of the main resonance. These findings indicate that the spin-lattice relaxation rate of the system decreases with increasing power. This diminution of the damping with increasing power is not specific to ferrites and it challenges some of the assumption used in the Landau-Lifshitz-Gilbert equation.