Effect of angular dependence of spin-transfer torque on zero-field microwave oscillation in symmetric spin-valves

We numerically investigate effect of the angular dependence of spin-transfer torque (STT) on current-induced precession mode in symmetric spin-valve structures where two ferromagnets are free to move. Two spin-valve structures with NiFe and Co as ferromagnets are studied, which show considerably different angular dependence of STT. We find that the zero-field microwave oscillation occurs in symmetric spin-valve structures. Precession frequency can reach around 10 GHz and decreases with increasing current. In contrast to a spin-valve structure with a single free layer, the blue shift is not observed due to highly nonlinear coupling of two magnetizations. We find that more symmetric angular dependence of STT allows the magnetic excitation at a lower current and provides a more monotonous increase of output power.