Dynamic domain wall chirality rectification by rotating magnetic fields
Applied Physics Letters
We report on the observation of magnetic vortex domain wall chirality reversal in ferromagnetic rings that is controlled by the sense of rotation of a magnetic field. We use time-resolved X-ray microscopy to dynamically image the chirality-switching process and perform micromagnetic simulations to deduce the switching details from time-resolved snapshots. We find experimentally that the switching occurs within less than 4 ns and is observed in all samples with ring widths ranging from 0.5 lmt
... ging from 0.5 lmt o2lm, ring diameters between 2 lma n d5lm, and a thickness of 30 nm, where a vortex domain wall is present in the magnetic onion state of the ring. From the magnetic contrast in the time-resolved images, we can identify effects of thermal activation, which plays a role for the switching process. Moreover, we find that the process is highly reproducible so that the domain wall chirality can be set with high fidelity. V C 2015 AIP Publishing LLC.[http://dx. The controlled manipulation of magnetic domain walls in nanowires has potential for data storage 1 and domain wall logic. 2,3 For example, in the proposed racetrack memory and logic devices, one data bit is represented by the presence or absence of a magnetic domain wall. 1 To implement domain wall logic operations, the position of the domain walls is controlled by spin-polarized currents injected through straight nanowires 1,3,4 or by rotating magnetic fields and domain walls traveling through a complex network of curved nanowires. 2 In magnetic nanowires, there are typically two basic types of domain walls: 5,6 the transverse domain wall, where the magnetization continuously rotates in-plane, and the vortex domain wall, where the magnetization curls in-plane around the central vortex core. The magnetization within the vortex core points perpendicularly to the plane to reduce exchange energy, 7 and is either parallel (p ¼þ1) or anti-parallel (p ¼À1) to the z-direction, defining the vortex core polarity p. The sense of rotation of the in-plane magnetization component, either counter-clockwise (CCW) or clockwise (CW), defines the vortex chirality c ¼þ1 or c ¼À1, respectively. Therefore, using vortex domain walls, two data bits can be represented by the vortex core polarity p ¼ 61 and the vortex chirality c ¼ 61. 8, 9 The chirality can be used to define the path (or trajectory) of domain walls traveling in magnetic branched networks 10 and to control the domain wall position, the use of artificial pinning sites, such as notches or stray fields from neighbouring elements, has been suggested. 6 However, the dynamic pinning at such sites strongly depends on the detailed domain wall structure, in particular, the chirality of the vortex domain walls.     To reliably operate devices based on the motion of vortex domain walls, the domain wall chirality must thus be controllable. It can be set by using the ends of magnetic nanowires as chirality rectifiers. 15 However, for this to work, the vortex domain walls must be trapped at an impasse. Alternatively, the chirality of nucleated vortex domain walls can be controlled by the direction of a saturating field in an optimized injection pad, 16 or by the combination of a well tuned spatially confined field generated by an injection line and a notch in the magnetic nanowire. 10 But these methods are cumbersome for real device applications, where the chirality needs to be set during the motion of the wall, and thus, a different approach is needed. In this letter, we report experimental observation that the vortex domain wall chirality is fundamentally controlled by the sense of rotation of a rotating magnetic field in circular nanowires. Using time-resolved scanning transmission X-ray microscopy (STXM), we dynamically image vortex domain walls driven by rotating magnetic fields and find that the domain wall chirality switches after a reversal of the sense of rotation of the field. By comparing the timeresolved images with micromagnetic simulations, we find that the dynamic chirality-switching process is triggered by the nucleation of a vortex-antivortex core pair and is thermally activated. The observed switching is a result of the interplay between a dynamic distortion of the vortex domain wall structure and the radial magnetic field. The permalloy (Ni 80 Fe 20 ) rings, in which we study the domain wall motion and structure transformations, are 500-750 nm wide, 30 nm thick, and were fabricated on top of a 100 nm thick X-ray transparent Si 3 N 4 membrane by a) andre.