Soft magnetic lithography and giant magnetoresistance in superconducting/ferromagnetic hybrids

We demonstrate an approach to create a tunable pinning potential in a superconducting/ferromagnetic (SC/FM) hybrid, allowing the switching of their electronic properties through the application of a small magnetic field. Using direct magneto-optical imaging, macroscopic transport, and magnetic measurements, we show that the alignment of stripe domains in the ferromagnet provides a remarkable directionality for the superconducting vortex motion. An analysis of the anisotropic flux motion demonstrates a substantial critical current anisotropy in the superconductor. The possibility of aligning stable lattices of stripe domains in select directions using in-plane magnetic fields allows the realization of soft magnetic lithography for efficient manipulation of supercurrent flow in SC/FM bilayers. Furthermore, in our samples we observed a pronounced magnetoresistance effect yielding 4 orders of magnitude resistivity change in a few millitesla in-plane field.

Figure 1

(Color online) The resistive transition of lead film.

Figure 2

(Color online) (a) Critical fields obtained from the disappearance of the diamagnetic signal on (b) $M\left(T\right)$ curves and from (c) the collapse of magnetic hysteresis loops $M\left(H\right)$ recorded at different temperatures.

Figure 3

(Color online) Magnetization loops of the Py film at $T=8\text{K}$ in the (a) perpendicular and (b) in-plane fields. (c) MFM pictures of the stripe domains oriented by in-plane fields in different directions.

Figure 4

(Color online) (a) Magneto-optical images of the flux penetration into the Pb/Py sample with horizontally aligned FM stripe domains at 4.2 K. Insets show fields in Oe. Pictures marked with down arrows are taken at decreasing field. (b) A scheme of meandering currents yielding spatially alternating field pattern.

Figure 5

MO pictures of the flux penetration when stripe domains are aligned in the vertical direction $\left(T=4.6\text{K}\right)$.

Figure 6

(Color online) (a) $I\text{−}V$ characteristics of the samples for stripes aligned parallel (filled circles, red) and perpendicular (open circles, blue) to the current direction. (b) The ratio of critical currents along and across the stripes close to $T_c$.

Figure 7

(Color online) $I\text{−}V$ curves at $T=7.11\text{K}$ in different $H_{\text{in-pl}}$. Arrow on the right shows the change in voltage ($10000$ change of resistance) due to application of $\sim 10\text{Oe}$ along the stripes oriented perpendicular to the current direction.

Figure 8

(Color online) Scheme of the magnetic charges on the surface of a FM film with large perpendicular anisotropy in contact with a SC layer. In the case of the total Meissner screening, magnetic charges are doubled on the SC/FM interface compared to the free FM surface. The superconducting vortex carry magnetic charges near the SC surfaces which can be described as magnetic monopoles with a charge of two flux quanta.