Antimagnetic cloak could be used in medicine, military and mischief

Photo credit: Sanchez, Navau, Prat-Camps and Chen, 2011. 

(From left to right) Magnetic field, as represented by the blue lines, of a single magnet, two magnets repulsing each other and then an antimagnet placed on top of one of the two magnets. In the last pane, the magnetic field lines are the same as those for a single magnet, demonstrating the antimagnet’s undetectability.

Stealth and health may have a new tool available to each of their agendas.

Researchers in Spain used computers to model what they say is a realistically producible antimagnetic cloak, a device that prevents objects from being magnetically detectable, including the antimagnet itself.

“The antimagnetic cloak is useful in all those applications in which you need two things,” Carles Navau, one of the researchers, said. “One, to protect magnetically something that you put inside the antimagnet and at the same time … not to disturb an external [magnetic] field.”

The proposed antimagnet keeps an object placed inside of it free from the influence of external magnetic fields while also leaving those fields unaffected.  In the article, published in the New Journal of Physics, the authors suggest that such a device could be used in the medical industry, such as for patients with pacemakers who need to undergo magnetic resonance imaging (MRI). Normally, MRI scans cannot be performed on such patients because of the possible interactions between the magnet in the machine and the metals in the pacemaker, which can damage the pacemaker and in some cases cause patient injury or death. However, containing the pacemaker within an antimagnet would protect against such interactions, as the magnet in the MRI would behave as if the pacemaker were not there at all.

The antimagnet could also be used to bypass security. “So if you put some metallic part inside an antimagnet, you could pass through detectors or magnetic scanners,” Navau said. Likewise, the antimagnet could be used to cloak metal-hulled ships and other vessels from detection.

The researchers’ design involves three components. “One of them is just a superconducting layer,” Navau said. “Magnetic fields that you put inside [that layer] cannot leak outside. But this superconducting layer distorts [an external] magnetic field. The other two types of layers are put there to compensate for the distortion.”

A superconductive material expels any magnetic fields inside of it. The other two layers alternate between a standard magnetic material and an array of superconducting plates. These two layers combine to produce a shell with the effective magnetic permeability necessary to prevent external field distortion. Permeability in effect refers to how easily a material can be peer pressured by external magnetic fields into becoming magnetic. It can have radial and angular components. One of the layers has different values for its radial and angular permeability (it is anisotropic), while for both outer materials, neither component depends on position (the materials are homogenous). Put together, these two alternating layers give rise to an overall homogenous, anisotropic material.

“All these layers have different magnetic properties and the point is that if you put those layers in a certain way, you get the full antimagnet properties,” Navau said. “If you remove one of them, you destroy the full properties.”

In the past, magnetic cloaks have been created but these only worked for a small range of radiation frequencies. Other researchers have proposed designs to create anti-magnets, but the materials necessary to build them required permeability values that could not be found in existent materials.

“We have gone one step beyond,” Navau said. “We have taken some of those ideas and designed a system … based on materials which, maybe they are challenging to build,  but [they] are there.”

There are some limits to the proposed antimagnet. “When the external field is quite large, superconductivity is destroyed [and] the antimagnet cannot work,” Navau said, adding that researchers are looking into using high temperature superconductors, which may be able to maintain their magnetic properties at larger fields.

Their model also assumes linear materials, those that respond directly to a change in magnetic field strength. “So if you double the magnetic field, the antimagnet reacts in the same way – it will react doubly – so it will cloak the same field,” Navau explained. “But at high enough fields, the materials are no longer linear, so they lose the properties that we need for the antimagnet.”

While the researchers showed that an antimagnet can feasibly be made from existing materials, creating one to be used in practical applications would require that other substances are added. These additional substances also exist, but the exact configuration for a usable device needs to be worked out.

“We are in contact with some experimental groups which are trying to make some reduced or some … simplified antimagnet just to prove the possibility of this idea,” Navau said. “Maybe in a few months a reduced antimagnet could be built and tested … but to fabricate a device, there is still some development to do. Navau said that to create a reduced antimagnet, one that exhibits antimagnet properties as opposed to one that can also be immediately used practically, the cost is “quite cheap.”

Reference:

Sanchez, Alvar; Carles Navau, Jordi Prat-Camps, Du-Xing Chen. 2011. Antimagnets: controlling magnetic fields with superconductor-metamaterial hybrids. New Journal of Physics. 13 093034

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