DC magnetic cloak.

DOI: 10.1002/adma.201104012 Isolation from external magnetic fields is a fundamental requirement that appears in various scientific experiments today. In particular, for applications where static or slowly varying magnetic fields need to be screened, enclosures made of superconductors or high magnetic permeability metal alloys are typically utilized. These are called “shields” as they isolate the inner space from the external fields. They achieve this interior shielding by either expelling the fields from or drawing the fields into the shield materials, which inevitably results in the distortion of the external field profile by the presence of the shields themselves. In this communication, we report the first experimental realization of a “cloak” for dc magnetic fields. We construct a hollow cylindrical cloak with a material consisting of an artificially patterned network of superconducting and soft ferromagnetic elements. We show that, when an external dc magnetic field is applied, the interior of the cloak is completely shielded while the field outside remains unperturbed. A dc magnetic cloak that shields the inner region without affecting the exterior space requires a highly anisotropic medium characterized with a response that is diamagnetic in one direction and paramagnetic in the perpendicular directions.[1,2] For the former diamagnetic property, arrays of superconducting strips have been investigated,[1–10] and we incorporate such structures into our cloaking material. See Figure 1(a). We thermally evaporate 200 nm-thick Pb film onto 77m-thick polyimide sheets and pattern the film into an array network using photolithography. When the magnetic field is applied perpendicularly to the superconducting structures, the Meissner effect funnels the field into the regions between the adjacent plates. When the field encounters the next layer of superconducting planes, it becomes expelled, distorted, and then guided in a direction perpendicular to the original incoming direction. The field that penetrates the second layer through the gap between superconducting structures again sees the third layer to repeat the same process, until all applied field has been guided in-plane. In this stacked superconducting meta-material, we cascade the layers so that the positions of superconducting structures do not align perfectly from layer to layer to encourage field distortion within the material. The strength of diamagnetism (for fields perpendicular to the plates) depends on the dimension of material building blocks (i.e., superconducting structures) and the lattice spacing for their network.[1–5] This provides us with a knob to tune the field path by using the “graded” geometry with variations in lattice parameters. The spatial variation that we prescribe is that the gap between the superconducting plates narrows as the field penetrates deeper into the material. This allows the magnetic field to enter the cloak freely with little field perturbation while forcing it to be more distorted and guided within the material as it penetrates further into it. The outermost layer has 2000 m 2000 m superconducting plates with a 600 m gap. The innermost layer has the plates of the same size with 4 m spacing. The gap is reduced in five discrete steps. See Table 1 for the physical parameters of different superconducting layers. The superconducting plate thickness (200 nm) is chosen to be smaller than the lattice spacing but larger than the London penetration depth. The second requirement to realizing a dc magnetic cloak is to engineer a material with a tendency to shunt the magnetic field that is in plane. To achieve this specific profile of permeability, we deposit a soft ferromagnetic material between the neighboring superconducting layers and turn the space into a low reluctance path for guided magnetic field lines. Permalloy Supradeep Narayana and Yuki Sato* DC Magnetic Cloak