Published by the Students of Johns Hopkins since 1896
January 28, 2022

Invisibility cloak hides magnets

By Mo-Yu Zhou | October 5, 2011

While invisibility cloaks are still mostly associated with fantasy (and Harry Potter), physicists and engineers have already developed rudimentary invisibility cloaks that shield objects from light, sound, and water waves. Now, they have moved cloaking technology another step forward with the development of a cloak that can shield an object from a static magnetic field, keeping the field undisturbed – an anti-magnet cloak.

Protecting an object from a constant magnetic field using a superconductor is relatively easy. When a magnetic field is applied, a conductor will cause currents to flow and counteract the applied field. An ordinary conductor has resistance and, thus, such currents will quickly be suppressed. However, a superconductor, which is a material that carries electrical current without resistance when it is cooled close to absolute zero, allows such currents to continue flowing.

Thus, it creates a magnetic field that cancels out the applied field, resulting in a total field within the container of zero. If a superconductor is used to encase an object, it should be able to shut out a magnetic field.

However, this is insufficient to create an antimagnetic cloak. Outside of the container, the superconductor will produce a field altering the applied field, thus making its presence known. If a field can be thought of in terms of lines of force, a superconducting shield creates a hole in the distribution of the lines by pushing them outward. In order to make an antimagnetic cloak, it would be necessary to counteract such a distortion.

In 2007, John Pendry of the Imperial College London, who co-invented the idea of cloaking, proposed that a cloak for static magnetic fields could be made using a material that would attract magnetic fields in one direction while repelling them in the opposite direction. Unfortunately, such a material does not exist.

However, recent research by Alvaro Sanchez and his colleagues at the Autonomous University of Barcelona, published recently in the New Journal of Physics, suggests that there is a way to approximate such a self-contradicting material. If their hypothetical material can be realized, it could have medical applications. The researchers propose wrapping a cylindrical shell of superconductor in layers of different materials, each of which will perform one job at a time.

Certain layers, easily magnetized, can pull outside magnetic field lines around the cylinder. These would be made out of tiny magnetic particles such as submicroscopic iron filings, mixed into a nonmagnetic material like plastic. These layers can alternate with other layers of superconducting plates that push on the field so that it will not come straight towards the center of the cylinder.

The cloak can accommodate fields of any shape or strength, limited only by the capabilities of the superconductor (too strong an external field could compromise the resistance-free state of the superconductor). With ten layers, the cloak could work almost perfectly, although even four layers would be effective. In addition, while a closed cylinder shape works best, an open cylinder or plate would also suffice.

This cloak causes no distortion in an applied external magnetic field, unlike a simple superconductor container. It could alternatively be used to conceal a magnetic object, preventing its magnetic field from extending outward, which could potentially compromise magnetic security tags used by retail stores.

In a more exciting development, the antimagnetic cloak developed by Sanchez and his fellow researchers could have medical applications. Currently, electronic implants create distortions up to 15 centimeters across in magnetic resonance imaging (MRI) images. If those implants were covered in a magnetic cloak, the image would be improved, and the patient and implant would also be protected.

A team at the Slovak Academy of Sciences in Bratislava led by Fedor Gömöry is looking to create a version of this hypothetical cloak, having already gathered the necessary equipment. Gömöry believes that an experimental confirmation of this antimagnetic cloak could be reached within a few months.

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