Phase locking of a spin-torque nano-oscillator to a strong microwave magnetic field

Magnetization precession excited by spin transfer effect in a current-driven spin-torque nano-oscillator (STNO) can be phase-locked to an external microwave signal having frequency $f_{e}$ close to the frequency $f_{0}$ of the STNO precession. In previous studies, only the phase-locking of STNO to a microwave \textit{current} was observed. In this work we studied experimentally STNO phase-locking to a microwave \textit{magnetic field} having amplitude $h_{e}$ up to 20 Oe rms. We observed both main ($f_{e}$/$f_{0}$ = 1) and secondary ($f_{e}$/$f_{0}$ = 2) locking regimes. For sufficiently large driving field $h_{e}$ both regimes of the STO phase-locking become hysteretic, with the boundaries of the locking interval dependent on the direction of the sweep of the external frequency $f_{e}$. The bandwidth of the main ($f_{e}$/$f_{0}$ = 1) locking regime was larger than the bandwidth of the secondary ($f_{e}$/$f_{0}$ = 2) regime when the dc bias magnetic field was perpendicular to the microwave field $h_{e}$. In contrast, for parallel orientations of dc and microwave magnetic fields, the secondary synchronization regime was more pronounced.