Guiding superconducting vortices with magnetic domain walls

We demonstrate a unique prospect for inducing anisotropic vortex pinning and manipulating the directional motion of vortices by using the stripe domain patterns of a uniaxial magnetic film in the superconducting/ferromagnetic hybrid. Our observations can be described by a model, which considers interactions between magnetic charges of vortices and surface magnetic charges of domains resulting in the enhanced pinning of vortices on domain walls.

Figure 1

(Color online) Magnetization loops of Py film at $290\mathrm{K}$ for (a) in-plane and (b) persspendicular fields. The inset expands the low field region revealing the domain-collapse fields $\sim \pm 400\mathrm{Oe}$.

Figure 2

(Color online) $20\times 20\mu {\mathrm{m}}^2$ MFM images of stripe domains in Py film after applying and turning off an $H=1\mathrm{kOe}$ parallel magnetic field along the directions indicated by the arrows.

Figure 3

Magneto-optical images of flux entry (with increasing perpendicular field) and exit (two pictures on the right obtained at decreasing field from $H_z^{\max }=250\mathrm{Oe}$) at $T=4.5\mathrm{K}$ following the preparation of the stripe domain structures by turning on and off an in-plane field of $H=1\mathrm{kOe}$ at an angle of $\sim 45°$ with respect to the sample edge at $T>T_c$. The brightness of the magneto-optical contrast corresponds to the vortex density. Values of the perpendicular field normal to the sample surface are shown on the pictures. The double-line arrow shows the preferential flux entry direction coinciding with the direction of the stripe domains in the Py. The single-line arrow marks the sample edge. The diagonal dotted line is a scratch in the indicator film. The scale bar is $200\mu \mathrm{m}$.

Figure 4

Same as Fig. 4 after application and switching off of $H=1\mathrm{kOe}$ along the sample edge at $T>T_c$. Two pictures on the right marked with black arrows are taken at decreasing field.

Figure 5

Same as Figs. 3, 4 after application and switching off of the in-plane field $H=1\mathrm{kOe}$ perpendicular to the sample edge at $T>T_c$. The two pictures on the right show the flux exit at decreasing field.

Figure 6

(Color online) (a) Schematics of the domains and stray fields in high-$Q$ FM films with perpendicular anisotropy. (b) At $T<T_c$, the magnetic stray fields from the FM domains are expelled from the top SC layer and doubled inside the FM resulting in the reduction of the domain width (see text). The SC vortex interacts with surface magnetic charges in the domains as a magnetic monopole. The interaction potential and the force (it diverges for zero width domain walls) calculated following Ref. 35 are shown in (d) and (e). In low-$Q$ Py, (c) the magnetization distribution within domains is inhomogeneous, although the stray field pattern is qualitatively similar to (a). We approximate the vortex-magnetization interactions with (f) a sinusoidal function yielding (g) the appropriate force landscape. Arrows in (f) and (g) show the force direction. Vortices prefer to stay in the middle of domains with $M∕∕{\Phi }_0$.