Direct observation of the coherent precession of magnetic domain walls propagating along permalloy nanowires

The dynamics of the motion of domain walls (DWs) in magnetic materials has been extensively explored theoretically(1-3). Depending on the driving force, conventionally magnetic field and, more recently, spin-polarized current(4-13), the propagation of DWs changes from a simple translation to more complex precessional modes(14). Experimentally, indirect evidence of this transition is found from a sudden drop in the wall's velocity(15-18), but direct observation of the precessional modes is lacking. Here we show experimentally, using a combination of quasi-static and real-time measurement techniques, that DWs propagate along permalloy nanowires with a periodic variation in the chirality of the walls. The frequency of this oscillation is consistent with a precession of the propagating DW, increasing linearly with field according to the Larmor precession frequency. Current in the nanowire, large enough to significantly influence the DW velocity(18,19), has little effect on the precession frequency but can be used to adjust the phase of the wall's precession. The highly coherent and reproducible motion of the DW revealed by our studies demonstrates that the DW is a well-defined macroscopic object whose phase is inextricably interlinked to the distance travelled by the DW.