Domain wall dynamics in ferromagnetic rings
Kornel Richter (Institut für Physik)

Ferromagnetic nanorings are promising candidate for development of nonvolatile data storage devices based on domain wall (DW) propagation [1-2]. Two possible chiralities of vortex states are supposed to code the logical values [3], while the change of its logic state is performed by DW motion. Consequently, a key-prerequsite for their high functional stability is a stable domain wall with a well-controllable velocity. In turn, the DW propagation in symmetric ring is characterized by a non-constant DW velocity even though it is driven by rotating field of constant amplitude [4]. Such a behaviour was attributed to instrinsic and extrinsic effects such as spin structure transformations and pinning. Here, we examine ferromagnetic rings, in which domain wall velocity was controlled on a local scale utilizing variations of DW potential landscape that has been introduced to the sample via non homogeneous ring-width.

Ferromagnetic rings were prepared by electron beam lithography (EBL) system and lift off pattern transfer. A series of asymmetric rings with outer diameter of 5.5µm having different spacing in the narrowest part (from 100nm to 500 nm) provided inhomogeneous potential for domain wall. The DW dynamics was investigated by use of time-resolved scanning transmission X-ray microscopy (STXM) imaging technique providing high spatial and time resolution.

Observation of domain wall motion in rotating magnetic fields reveals that the phase shift between the direction of magnetic field and domain wall position is strongly related to the asymmetric shape of the sample, namely, the varying ring width. While domain wall entering the narrowest part exhibits negative phase shift (it seems to be pinned in this region), the wall entering the widest part is characterized by positive phase shift. Such relative change of the phase shift result in variations of DW velocity profile, thus allowing us to control the local DW velocity.
Additionally, the asymmetric shape of the sample was found to be strong enough to control the intra-spin structure changes of domain wall moving above Walker breakdown. When domain wall crosses the narrowest part of the ring, the large phase shift between the field and a domain wall position results in the local drop of domain wall velocity, in which Walker breakdown occurs. Surprisingly, the same behavior was observed for DW with any starting position in the magnetic ring, thus allowing to spatially synchronize the DW velocity. Our qualitative analyses of results are supported by micromagnetic simulations. The obtained results could serve for better understanding of dynamical processes involved in DW motion in curved nanowires with non-constant width and potentially help the improvement of future spintronic devices based on this material.
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[2] M. Negoita, et al., Appl. Phys. Lett. 100, 072405 (2012).
[3] D. K. Singh, et. al. Phys. Rev. B 79, 184409 (2009).
[4] A. Bisig, et al., Nat. Comm. 4, 2328 (2013).