Spin Torque Oscillators and RF Currents-Modulation, Locking, and Ringing

We study the interaction between a nano-contact spin torque oscillator (STO) and injected radio-frequency and microwave currents. Modulation of the STO signal is observed over a wide frequency range from 100 MHz to 3.2 GHz. The modulation side-bands agree well with macrospin simulations. When the injected microwave frequency approaches that of the STO, we observe injection locking, frequency pulling/pushing, and intermodulation peaks. While the intermodulation peaks are reasonably well reproduced by macrospin simulations, they do not follow the Adler's model. We argue that this discrepancy is due to intrinsic ringing effects stemming from the internal dynamics of the STO.