Dynamic interaction between domain walls and nanowire vertices

The dynamic behavior of magnetic domain walls incident upon a $\mathsf{Y}$-shaped junction between three nanowires is explored. Details in the micromagnetic structure of the domain wall and the local spin texture at a vertex result in a complex interaction which ultimately determines the propagation direction of the domain wall following the vertex. This interaction has been explored through both magneto-optical Kerr effect experiments and micromagnetic simulations on single-vertex structures. Differences in the micromagnetic structure of incident domain walls in the dynamic regime have a significant influence on the interaction. We observe an oscillatory path selection with increasing domain wall propagation distance preceding the vertex. Our analysis shows that this behavior originates from cyclic reversals of the transverse domain wall chirality due to Walker breakdown processes.

Figure 1

Scanning electron microscope image of the fabricated structures comprising a hook structure (a) separated from a 120° vertex (b) by a nanowire of length $L$. MOKE measurement positions are indicated by dotted ellipses. The insets show higher magnification images of the (a) hook, (b) vertex, and (c) tapered ends of the structures. (d) Magnetic force microscopy shows the injection of a domain wall from the hook structure.

Figure 2

Magnetization as a function of field showing hysteresis loops for the 20-μm-long horizontal bar of a 20-nm-thick structure with a down hook. Two loops are shown when the field is rotated in the clockwise and anticlockwise directions.

Figure 3

Hysteresis loops from the upper branch of a hook down structure with clockwise rotating fields. Three loops correspond to different lengths, $L$, of the horizontal bar: (a) 1.4 μm, (b) 1.1 μm, and (c) 0.7 μm.

Figure 4

Micromagnetic simulations showing the interaction between a DW with (a) up and (b) down chirality with a nanowire vertex. The equilibrium state is shown for several fields: (i) 50 Oe, (ii) 180 Oe, (iii) 190 Oe, (iv) 370 Oe, and (v) 380 Oe.

Figure 5

Switching field of the upper branch of a (a) 20- and (b) 15-nm-thick hook down structure in a clockwise rotating field as a function of hook-vertex separation. The dashed lines show the DW injection field, $H_{\mathrm{INJ}}$, and switching fields at $H_{C1}$ and $H_{C2}$ corresponding to reversal from the transmission of the incident DW or nucleation of a secondary DW, respectively. Shaded regions highlight the periodic nature of the data.

Figure 6

Micromagnetic simulations showing the final state of a dynamic DW-vertex interaction as a function of applied field and initial DW-vertex separation. Regions in white show where the DW becomes pinned at the vertex while red and blue show where the DW propagates along the lower and upper branch, respectively.