  The mode dependence of current-induced magnetic excitations in spin valves is
studied theoretically. The torque exerted on the magnetization by transverse
spin currents as well as the Gilbert damping constant are found to depend
strongly on the wave length of the excitation (spin wave). Analytic expressions
are presented for the critical currents that excite a selected spin wave. The
onset of macrospin (zero wavelength) vs finite wavelength instabilities depends
on the device parameters and the current direction, in agreement with recent
experimental findings.
