Low-field current-hysteretic oscillations in spin-transfer nanocontacts

We measured spin-transfer-driven, large-amplitude, current-hysteretic, low-frequency $(l500\phantom{\rule{0.3em}{0ex}}\mathrm{MHz})$, narrowband oscillations in nanocontacts made to spin valve structures. The oscillations occur in zero field, persist up to $5\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ for in-plane applied fields, and to beyond $400\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ for out-of-plane fields. Unlike for previous measurements, here the oscillation frequency is well below that for uniform-mode ferromagnetic resonance, is a weak function of the applied field and current, and is highly anharmonic. The oscillations are hysteretic with dc current, appearing at high currents but persisting to lower currents upon decrease of the current. We suggest that these observations are consistent with the dynamics of a nonuniform magnetic state, one nucleated by both the spin-transfer torque and dc current-generated Oersted fields, with dynamics driven by spin transfer. The electrical oscillations are large amplitude and narrowband, with the largest amplitudes on the order of $1\phantom{\rule{0.3em}{0ex}}\mathrm{mV}$ and the narrowest linewidths below $1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$.