  It is demonstrated that the decay time of spin-wave modes existing in a
magnetic insulator can be reduced or enhanced by injecting an in-plane dc
current, $I_\text{dc}$, in an adjacent normal metal with strong spin-orbit
interaction. The demonstration rests upon the measurement of the ferromagnetic
resonance linewidth as a function of $I_\text{dc}$ in a 5~$\mu$m diameter
YIG(20nm){\textbar}Pt(7nm) disk using a magnetic resonance force microscope
(MRFM). Complete compensation of the damping of the fundamental mode is
obtained for a current density of $\sim 3 \cdot 10^{11}\text{A.m}^{-2}$, in
agreement with theoretical predictions. At this critical threshold the MRFM
detects a small change of static magnetization, a behavior consistent with the
onset of an auto-oscillation regime.
