Macrospin description of the perpendicular polarizer-planar free-layer spin-torque oscillator

For the ``perpendicular polarizer-planar free layer'' spin torque oscillator a complete current-field state diagram in the macrospin description is presented. This diagram is quite different from one of the more commonly known ``planar polarizer-planar free layer'' spin torque oscillators. In particular, various regions of bistability between two static states or a static and a dynamic state are evidenced in the current-field plane. The boundaries of the regions, where the static states become unstable, were deduced using a generalized ferromagnetic resonance formalism that requires only the knowledge of the second derivatives of the system energy for an arbitrary spin-polarization direction and field orientation. The dependence of the frequency as a function of current and bias field are described for the spin current induced steady-state oscillations as well as for the small amplitude damped oscillations around the static states. Finally, the results will be compared to recent experiments and the effect of temperature will be discussed.