Current-driven destabilization of both collinear configurations in asymmetric spin valves

Spin-transfer torque in spin valves usually destabilizes one of the collinear configurations (either parallel or antiparallel) and stabilizes the second one. Apart from this, balance of the spin-transfer and damping torques can lead to steady precessional modes. In this Letter we show that in some asymmetric nanopillars, spin current can destabilize both parallel and antiparallel configurations. As a result, stationary precessional modes can occur at zero magnetic field. The corresponding phase diagram as well as frequencies of the precessional modes have been calculated in the framework of macrospin model. The relevant spin-transfer torque has been calculated in terms of the macroscopic model based on spin diffusion equations.