Stochastic DomainWall Depinning in Magnetic Nanowires 
Ferromagnetic wires of nanometer sizes are considered to be key components in future spintronic applications for novel classes of magnetic storage devices. One example is the concept of a racetrack memory, where instead of a spinning disk in which individual information bits fly by a read head, magnetic domain walls acting as information units are pushed by spin currents along a magnetic wire until they are read out by a stationary head. One of the fundamental issues for such schemes is the precise control of domainwall motion, which in turn is directly linked to the reproducibility of domainwall propagation, pinning, and depinning. To locally control the position and the motion of a domain wall, it is common to introduce artificial topological imperfections, such as notches or antinotches, into the wire. The potential created around such a notch is sufficient to trap and release the domain wall in a controlled way. Although a wealth of information has already been experimentally and theoretically obtained, the fundamental question of under what conditions, if any, the domainwall dynamics in the vicinity of artificial notches can be fully deterministic has not been addressed so far. The Berkeley–Hamburg group used the soft xray microscope at ALS Beamline 6.1.2 for an indepth investigation of the statistical behavior of the domain wall depinning field at a single notch in permalloy nanowires with different wire widths (w), notch depths (N_{d}), and film thicknesses (t). Magnetic images based on magnetic circular dichroism contrast with a spatial resolution of better than 25 nm were recorded as an applied magnetic field was gradually increased in steps. In a magnetically saturated wire (only one domain in the wire), increasing the field successively nucleates a second domain (and hence a domain wall) at one end of the wire, propagates the wall down the wire until it becomes pinned at a notch, depins the wall, and drives the wall to the other end of the wire. The stochastic nature of the domainwall depinning field for different notch depths and wire widths was systematically investigated by determination of the field distribution from depinning events in experiments repeated at least 40 times for each wire. For the first time, these results clearly showed that the domainwall depinning field exhibits stochastic behavior and the stochastic nature depends considerably on the wire width and the notch depth. A thorough analysis of the data allowed the researchers to conclude that it is the multiplicity of domainwall types (transverse, vortex, etc.) generated in the vicinity of a notch that is responsible for the observed dependence of the stochastic nature of the domainwall depinning field on the wire width and the notch depth.
While at first glance these findings seem to discourage a successful implementation of devices driven by domain walls, it also shows that a proper geometrical design of the wires could limit the domainwall types and hence minimize the stochastic behavior of the domainwalldepinning process, which should be easy to achieve with stateoftheart patterning and fabrication tools.
Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES) and the Deutsche Forschungsgemeinschaft. Operation of the ALS is supported by BES. Publication about this research: M.Y. Im, L. Bocklage, P. Fischer, and G. Meier, “Direct Observation of Stochastic DomainWall Depinning in Magnetic Nanowires,” Phys. Rev. Lett. 102, 147204 (2009).
