Controlling the properties of vortex domain walls via magnetic seeding fields

The seeding of vortex domain walls in soft magnetic V-shaped nanowires by magnetic fields has been investigated via scanning electron microscopy with polarization analysis and micromagnetic simulations. It is found that the orientation of the magnetic seeding field determines the sense of rotation and the position of single vortex domain walls in the state of remanence. The topology of the magnetic microstructure in combination with symmetry considerations gives the key for the explanation of this behavior.

Figure 1

Scanning electron microscope image of V-shaped wires of 400 nm width. The symmetry axis (bisection) is indicated by the white dotted line. The black arrow gives the direction of the external field used for seeding the domain walls.

Figure 2

(Color online) SEMPA micrograph showing two wires carved out of a continuous film via FIB milling. The orientation of the magnetization is color coded according to the color wheel. In the bright region where the ferromagnetic material has been removed no magnetic signal could be found within the uncertainty of the experiment. The adjacent film shows a domain pattern caused by the sharp edges of the structuring.

Figure 3

(Color online) [(a)–(d)] SEMPA images of vortex domain walls in V-shaped wires after field application in the indicated directions (gray arrows). The labels (i)–(IV) refer to the four arrangements shown in Fig. 1. The orientation of the magnetization is indicated by the black arrows and color coded according to the color wheel. The images have been rotated so that the bisections of the wires are aligned with the black dotted line. In the images only the signal of the wire, i.e., within the black solid lines, is displayed. [(e)–(h)] Micromagnetic simulation of the corresponding geometries from (a)–(d) after pretreatment in an external field of $B_{\text{ext}}=60\text{mT}$ in the directions of the gray arrows.

Figure 4

(Color online) Possible configurations of a head-to-head vortex wall. In (a)/(c) the counterclockwise/clockwise configuration is displayed, respectively. The direction of magnetization is indicated by black arrows (obtained via OOMMF). In (b) and (d) the $z$ component of the curl of the vector fields of (a) and (c) is shown, red denotes positive values, blue negative values, and white is equal to zero. In (e) a sketch of the magnetization orientation around the edge defects located at the kinks is shown.

Figure 5

(Color online) Details of the simulation of the relaxation from the fully saturated state (a) with a high damping constant of $\alpha =0.5$. The field is aligned $5°$ toward the right-hand side of the symmetry axis. The relaxation steps shown are: (b) 50 steps, (c) 80 steps, (d) 300 steps, and (e) 400 steps. The simulation converges after 11647 steps, yielding the configuration of Fig. 3h. The white dots are marker points which are discussed in the text.