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Physicists Devise Perfect Magnetic Shield

22 September 2011 3:14 pm
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(illustration) J. Prat-Camps; A. Sanchez, C. Navau, D.-X. Chen/Autonomous U. of Barcelona

Hideaway. The magnetic cloak calls a truce on warring magnetic fields. On the left, the magnetic field of a lone cylinder-shaped magnet. In the middle, a second magnet, pointing the opposite way, disrupts its field. On the right, the second magnet's field is hidden in the cloak, which also allows the first magnet's field to extend as if the second weren't there at all.

The sneaky science of "cloaking" just keeps getting richer. Physicists and engineers had already demonstrated rudimentary invisibility cloaks that can hide objects from light, sound, and water waves. Now, they've devised an "antimagnet" cloak that can shield an object from a constant magnetic field without disturbing that field. If realized, such a cloak could have medical applications, researchers say.

"This will take cloaking technology another step forward," says John Pendry, a theorist at Imperial College London and co-inventor of the original cloaking idea, who was not involved in the present work.

In fact, shutting out a static magnetic field to protect an object isn't that hard. All a researcher needs to do is to encase the object in a container made of a "superconductor," a material that will carry electrical current without any resistance when it is cooled sufficiently close to absolute zero. If the container encounters a magnetic field, currents within the conductor will flow to generate a field that counteracts the applied field. In an ordinary conductor, the resistance of the metal quickly snuffs out those currents. In a superconductor, however, those currents just keep flowing, creating a magnetic field that exactly cancels the applied field and zeroing out the total field within the container.

But that doesn't make a superconducting can a magnetic cloak. That's because outside the can, the field produced by the superconductor will alter the applied field and reveal its presence. In a nutshell, the field can be thought of as a distribution of lines of force that vaguely resembles a weather map of winds. The superconducting shield pushes the magnetic field lines outward, creating a hole in the field. So the trick to making a cloak for static magnetic fields is to counteract that distortion. In 2007, Pendry and Ben Wood, also of Imperial College London, proposed that such a cloak could be made of a material that repels magnetic fields in one direction and attracts them in the opposite direction. Unfortunately, this self-contradicting material doesn't exist.

But Alvaro Sanchez of the Autonomous University of Barcelona in Spain and colleagues propose a way to approximate the impossible stuff by wrapping the cylindrical shell of superconductor in layers of materials that do one job at a time. Some layers are easily magnetized and will essentially pull the external magnetic field lines around the cylinder; those layers alternate with shells of superconducting plates that push on the field, preventing it from coming straight in toward the center. The attracting layer would be made of tiny magnetic particles, like submicroscopic iron filings, mixed into a nonmagnetic material such as plastic.

The cloak could handle fields of any shape and any strength within what the superconductor can stand. If the external field gets too strong, the magnetically induced current becomes so powerful that it knocks the superconductor out of its resistance-free state and ruins its field-repelling qualities. Computer simulations showed that the cloak could work with as little as four layers, but with 10, it would guide a magnetic field nearly as well as a perfect cloak, as Sanchez and colleagues report today in the New Journal of Physics. "It doesn't need to be a closed cylinder; it can be an open cylinder or open plate, although in this case the magnetic cloaking properties are reduced," Sanchez says.

The hypothetical device would work as a magnetic cloak by creating a space that is protected from an external magnetic field while at the same time causing no telltale distortion of the field. Alternatively, it could also be used to conceal a magnetic object and prevent its magnetic field from extending out into space—a pie-in-the-sky dream for shoplifters trying to steal clothes pinned with magnetic security tags.

More seriously, the magnetic cloak could have medical applications. For example, sensitive electronic implants create voids or distortions in MRI images 10 to 15 centimeters across, says Ariel Roguin, a cardiologist at Rambam Medical Center in Haifa, Israel. So a strategically placed magnetic cloak would not only protect the patient and implant but also could preserve the image, Pendry says. Such a cloak could soon be more than just an idea, too. Fedor Gömöry of the Slovak Academy of Sciences in Bratislava says his group already has the equipment and is preparing to make a version of the antimagnet cloak: "I think that such an experimental confirmation could be reached within a few months."

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