(a) Schematic pendulum models in the uniform gravity and driven by a tangential force. (b) The phase diagrams of the static and oscillation free layer magnetization states as functions of current and field. The theory predications of the critical currents I_c and I_b for OP precession are indicated by red and olive curves, respectively. The corresponding simulation results are indicated by the blue and black squares. P and AP denote the parallel state and antiparallel state with respective to the IP analyzer, respectively. (c) Simulated OP magnetization precession of the free layer on the unit sphere phase plane for two combinations of currents I and applied fields h_z.