This paper presents a micromagnetic study about the nonlinear properties exhibited by spin torque oscillators, implemented as nanoscale exchange biased spin-valves of elliptical cross-sectional area. The analysis is based on numerical simulations which also include the back-torque effect in the pinned layer. The external bias field is applied along the in-plane hard axis. Our results demonstrate that there exists a critical field at which the current dependence of the oscillation frequency changes passing from a “red shift” (typical of in-plane magnetization) to a “blue shift” (typical of out-of-plane magnetization). Such results are in qualitative agreement either with recent experiments and with an analytical non-linear theory which identifies the transition field as the field at which the non-linear frequency shift vanishes. In such condition, the generation linewidth exhibits a minimum, the phase noise is independent of the power noise, and the oscillation frequency is independent of both current and power.
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