Chaotic magnetization dynamics excited by spin transfer torque in elliptical nanodots

Recent theoretical work has shown that ac-driven magnetization dynamics near saddle equilibria in magnetic elliptical nanodots exhibits chaotic behavior. The tools of nonlinear dynamical systems allow the definition of the concept of erosion of the stability region of magnetic equilibria in absence of the ac drive. From an energy perspective, the effect of the erosion is similar to a reduction of the magnetic potential barrier separating two stable equilibrium states of the nanodot, as exploited in energy assisted magnetic reversal schemes. In this framework, we study experimentally this phenomenon by measuring thermal telegraph noise of the free magnetic layer in nanoscale elliptical magnetic tunnel junctions (MTJs) in the presence of AC drive provided by the spin transfer torque arising from radio-frequency (RF) current applied to the MTJ. We follow the switching rate of the free layer, which serves as an indicator for the occurrence of chaotic dynamics as a function of both the frequency and power of the applied microwave excitation. The experimental results are compared to analytical solutions and micromagnetic simulations.