  Recent experimental and theoretical studies on the magnetization dynamics
driven by an electric current have uncovered a number of unprecedented rich
dynamic phenomena. We predict an intrinsic chaotic dynamics that has not been
previously anticipated. We explicitly show that the transition to chaotic
dynamics occurs through a series of period doubling bifurcations. In chaotic
regime, two dramatically different power spectra, one with a well-defined peak
and the other with a broadly distributed noise, are identified and explained.
