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Introducing the ‘antimagnet’

04 Aug 2011 Tushna Commissariat
Iron filings encircling a small bar magnet

Researchers in Spain have proposed a new type of invisibility cloak that could hide objects from magnetic fields. The cloak – which has not been built yet – is designed to have a dual effect. It ensures that a magnetic field generated inside the cloak does not leak outside, and it ensures that the cloak and its contents cannot be detected by an external magnetic field. If it can be realized as a practical device, the technology could prove useful in industrial applications that require specific magnetic environments.

The first invisibility cloak was unveiled in 2006 and worked for electromagnetic radiation in the microwave range. It and subsequent cloaks have been based on metamaterials – materials that have been artificially engineered to have specific electromagnetic properties. In principle it should be possible to create a cloak that works for static magnetic fields – which are simply electromagnetic waves at zero frequency.

In 2008 John Pendry and colleagues at Imperial College London proposed such a magnetic cloak based on a metamaterial that has a magnetic permeability that is smaller than one in a given direction and larger than one in a direction perpendicular to it. While superconductors have a permeability of zero and ferromagnetic materials have a permeability greater than one, developing a material that has both properties simultaneously proved problematic. Pendry’s 2008 design involved a stack of superconducting plates that promised to screen weak magnetic fields but was not a complete cloak.

Magnetic shroud

Now, Alvaro Sanchez and colleagues at Universitat Autònoma de Barcelona have expanded on Pendry’s concept and come up with a design that they say can be easily built using practical metamaterials. Instead of calling the device a cloak, the researchers use the term “antimagnet”, which they define as having two key properties. The first is that any magnetic field created within the cloak (by a permanent magnet, for example) cannot leak outside the cloaked region. The second property is that the cloak and the cloaked region should not be detectable using an external magnetic field.

A design that meets these conditions involves repeating layers of different metamaterials – some of which have an isotropic response to a magnetic field and others that have an anisotropic response. The pattern begins with an inner superconducting layer with a magnetic permittivity of zero. The next layer is an isotropic ferromagnetic material with constant permeability. This layer could be made from ferromagnetic nanoparticles embedded in a non-magnetic medium. The next layer would be anisotropic while having a constant value of radial permeability. Sanchez and colleagues believe this could be built using the array of superconducting plates that Pendry proposed in 2008.

The team used a computer model to simulate the antimagnet as it enclosed a small single magnet. The researchers found that the cloak almost completely shielded the outside world from the internal magnet and vice versa.

While the current cloak design is cylindrical, the researchers say that it could be extended to other geometries. They believe antimagnets would be useful in many practical applications, including allowing patients with pacemakers or cochlear implants to access medical equipment based on magnetic fields, such as those used for magnetic resonance imaging. They also says that by tuning the working temperature of the device to above or below the critical temperature of the superconductor, one could switch the magnetism in a certain region or material off and on at will, potentially opening up more applications.

The research is available online at the arXiv preprint server.

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