Magnetic domain wall dynamics and asymmetric rings
Andrea Krone (Institut für Physik)

Ferromagnetic nanorings are promising candidates for a development of future spintronic devices based on domain wall propagation. Information transport through domain walls (DWs) is achieved via rotating magnetic driving fields. Saving information is possible via the two-fold degeneracy of the vortex state without DWs having either clockwise or counter clockwise magnetization configuration along the ring perimeter. A key-prerequisite for high functional stability and fast devices is a stable domain wall with a well-controllable high velocity. Recently it was found that the DW propagation in symmetric rings is characterized by non-constant DW velocity even when driven by a constant rotating field [1]. Such a behavior was attributed to intrinsic and extrinsic effects such as spin structure transformations and pinning with one or the other dominating depending on the velocity regime. Since the geometry sets the energy landscape it can be used to tailor the wall propagation. Here, we examine asymmetric ferromagnetic rings with gradually changing potential landscape via non homogeneous ring width to control spin structure changes.
The rings have an outer diameter of 5,5 µm but different spacing in the narrowest part (from 100 nm to 500 nm). DW dynamics were investigated by time-resolved scanning transmission X-ray microscopy (STXM).
Observation of domain wall motion in a rotating magnetic field reveals that the phase shift between the direction of the magnetic field and the DW position is strongly related to the varying ring width of the sample. The interplay between domain wall energy and Zeeman energy results in variations of the domain wall velocity but with angular dependence. Therefore the DW velocity is controlled locally and globally. It seems that the asymmetric shape has high enough influence on the phase shift that the radial movement of the vortex core (VC) is suppressed. A circular trajectory of VC is achieved only interrupted by the controlled angular position of the WB.
In addition, the asymmetric shape of the sample give rise to presence of DW zero field motion (automotion). This effect can be understood in terms of minimization of the total energy. When both domain walls approach each other, they minimize the energy originating from non-constant ring width (short-range local forces) as well as the mutual interaction of both walls (long-range forces).
Nucleation processes from vortex state to onion state can appear with ripple creation which is a highly reproducible phenomenon in the asymmetric rings. Using rotating fields with engineered field rotation direction with respect to the magnetization rotation direction of the vortex state allows to tailor the resulting nucleation process.

[1] A. Bisig et al. Nat. Comm. 4, 2328(2013)