Domain-Wall Movement Control in $\mathrm{Co}/\mathrm{Au}$ Multilayers by ${\mathrm{He}}^{+}$-Ion-Bombardment-Induced Lateral Coercivity Gradients

Defined perpendicular anisotropy gradients in the Co sublayers of a $[\mathrm{Co}(0.6\mathrm{nm})/\mathrm{Au}(2\mathrm{nm}){]}_3$ sputter-deposited multilayer have been introduced by light ion bombardment through a wedged Au stopper layer. Within such a layer system, domain walls between up- and down-magnetized areas are controllably movable by an external perpendicular homogeneous magnetic field. This method and layer system is very promising for a controlled magnetic particle transport within the stray fields of the moving domain walls.

Figure 1

Schematic view of the sample. The $[\mathrm{Co}(0.6\mathrm{nm})/\mathrm{Au}(2\mathrm{nm}){]}_3$ multilayer (alternating black and white bands) was capped by a wedged Au stopper layer (0–100 nm). Different stripes were bombarded by ${\mathrm{He}}^{+}$ ions (1, 5, and $10\times {10}^{14}\mathrm{\text{ions}}{\mathrm{cm}}^{-3}$). The intervening stripes were left unexposed for reference.

Figure 2

Mean coercive fields $H_C$ from polar Kerr effect measurements as functions of the Au stopper thickness for areas exposed to different ${\mathrm{He}}^{+}$ ion fluences. The $x$-coordinate range along the Au wedge corresponds to the Au stopper thickness range from 0 to approximately 34 nm. For two highest ion fluences, points for small $x$ are not shown as they correspond to easy-plane anisotropy of the Co layers (see the leftmost loop of Fig. 3, upper panel). The solid line shows the relative MOKE signal in remanence for the stripe exposed to $10\times {10}^{14}\mathrm{\text{ions}}{\mathrm{cm}}^{-3}$; the dashed line replaces the part of the dependence which is noisy due to low signal through the thick Au stopper.

Figure 3

Magnetic domain images of the fragment of the $[\mathrm{Co}(0.6\mathrm{nm})/\mathrm{Au}(2\mathrm{nm}){]}_3/\mathrm{Au}(0–100\mathrm{nm}\mathrm{\text{wedge}})$ multilayer exposed to IB by $10\times {10}^{14}\mathrm{\text{ions}}{\mathrm{cm}}^{-2}$ taken for different $H_{\perp }$ pulse values. Each image stripe corresponds to the same 0.26 mm wide and 6.5 mm long fragment of the sample. The thickness of the Au wedge increases from left to right from 0 to 34 nm. The upper panel shows exemplary MOKE hystereses for different Au wedge thicknesses.

Figure 4

DW positions as functions of the applied field in the $[\mathrm{Co}(0.6\mathrm{nm})/\mathrm{Au}(2\mathrm{nm}){]}_3$ multilayer obtained from the analysis of the images like those shown in Fig. 3.