Redshift and Blueshift Regimes in Spin-Transfer-Torque Nano-Oscillator Based on Synthetic Antiferromagnetic Layer

The spin-torque-driven in-plane precession mode of synthetic antiferromagnets (SAFs) is characterized by a frequency current dependence that changes its slope from negative redshift to positive blueshift upon increasing the applied magnetic field. Here, we show that the transition from redshift to blueshift is due to additional torques arising from the dynamic interlayer exchange coupling of the SAF. These torques depend on the precession amplitude that changes with current and field. We present an analysis of the corresponding energy surface that is able to predict such behavior for any spin-torque-driven excitation of a coupled two-layer system.