  An effect is considered of alternating (high-frequency) current on the
spin-valve type magnetic junction configuration. The stability with respect to
small fluctuations is investigated in the macrospin approximation. When the
current frequency is close to the eigenfrequency (precession frequency) of the
free layer, parametric resonance occurs. Both collinear configurations,
antiparallel and parallel ones, can become unstable under resonance conditions.
The antiparallel configuration can become unstable under non-resonant
conditions, also. The threshold current density amplitude is of the order of
the dc current density switching the magnetic junction.
