  We demonstrate that magnetic oscillations of a current-biased magnetic
nanocontact can be parametrically excited by a microwave field applied at twice
the resonant frequency of the oscillation. The threshold microwave amplitude
for the onset of the oscillation decreases with increasing bias current, and
vanishes at the transition to the auto-oscillation regime. The parametrically
excited oscillation mode is the same as the one in the auto-oscillation regime,
enabling studies of both the passive and the active dynamics of the oscillator.
Theoretical analysis shows that measurements of parametric excitation provide
quantitative information about the relaxation rate, the spin transfer
efficiency, and the nonlinearity of the nanomagnetic system.
