How can magnetic fields be blocked?

Researchers develop invisibility cloaks for magnetic fields

For the first time, researchers have developed the concept for a magnetic “magic hat”. As they show in computer simulations, such a cover could completely shield magnetic metals from surrounding magnetic fields. Such a camouflaged object would then be “invisible” to detectors and other magnets, the scientists report in the specialist magazine “New Journal of Physics”. The magnetic invisibility cloak does not require any exotic materials, but can be implemented with already available building blocks and technologies.

It has long been known that so-called superconductors block magnetic fields. Superconductors are materials that no longer have any electrical resistance when cooled below a threshold value. If such a material forms a sphere, for example, it does not allow magnetic fields from the inside out and those from the outside not into it. Instead, the superconductor directs the magnetic field lines around itself.

The researchers say that the distortion of the field lines caused by such a superconductor is easy to determine. A way was therefore sought to block magnetic fields without such distortion. The scientists report that a cylindrical structure with an inner superconductor layer and an outer shell has proven to be the solution.

The shell comprises nine layers made of two alternating materials. The first material consists of magnetic nanoparticles that are embedded in a non-magnetic base substance. “The second material can be realized by a specific arrangement of superconducting plates,” the researchers write. The combination of these covering layers makes the cloak and its contents virtually invisible to magnetic sensors. No interactions with existing field lines can be measured in a magnetic field.

Shielding of pacemakers and anti-mine protection

Such a device could be used in the future, for example, to isolate pacemakers from the strong magnetic fields in medical nuclear spin examinations, say the researchers. The “invisibility cap” is also useful to keep magnetic fields invisible inside. This could be used, for example, to protect ships from water mines, which normally react to the magnetizable metal of the ship's hulls.

"We believe and hope that some laboratories could soon start building the first antimagnet," says lead author Alvaro Sanchez from the University of Barcelona. Of the two required components, superconductors are already available everywhere. Developing the magnetic layers with the desired features may still require a little work, but in principle the ingredients are also available.

Invisibility cloak could also bring security problems

However, the new technology does not only enable new, positive applications, warn the researchers. "It is conceivable that the principle of this device could also be used to hide the magnetic signature of forbidden objects," says Sanchez. Criminals could use it, for example, to outsmart security systems at airports or shops.

“For this reason, this research should also be followed up by security agencies so that they can develop more secure detection systems and protocols based on it,” says Sanchez. In general, however, he is confident that applications of such a magnetic invisibility cloak will bring more advantages than disadvantages for society.

(New Journal of Physics, 09/26/2011 - NPO)

September 26, 2011