We measure the injection locking of a three-terminal spin-torque oscillator (STO) excited by a spin-orbit torque. The device consists of a magnetic tunnel junction on top of a Pt wire. A DC and an AC current are applied through the Pt wire to respectively induce the oscillations and injection lock the STO. The injection locking is studied at fMW ≈ f0 or at fMW ≈ 2f0, where fMW is the microwave frequency and f0 is the free running frequency of the STO. We observe a frequency response which is qualitatively different from the injection locking generally reported experimentally and theoretically in STOs. Whereas typical phase-locking behavior is observed at fMW ≈ 2f0, we show that the injection locking at fMW ≈ f0 is only partial and exhibits a strongly asymmetric frequency response. Defining the frequency deviation range as the frequency range where the STO differs from its free running frequency, we show that the asymmetric interaction is characterized by: a pulling effect present on only one side of the frequency deviation range, the presence of a sideband inside the frequency deviation range, and an interaction of the STO with the microwave current that is wider than the frequency deviation range.
Applied Physics Letters
Spin-torque oscillator, injection locking, three-terminal device