  We have directly measured coherent high-frequency magnetization dynamics in
ferromagnet films induced by a spin-polarized DC current. The precession
frequency can be tuned over a range of several gigahertz, by varying the
applied current. The frequencies of excitation also vary with applied field,
resulting in a microwave oscillator that can be tuned from below 5 GHz to above
40 GHz. This novel method of inducing high-frequency dynamics yields
oscillations having quality factors from 200 to 800. We compare our results
with those from single-domain simulations of current-induced dynamics.
