How to hide from a magnetic field

Researchers in Europe have built a magnetic cloak that, in theory, is reasonably practical to manufacture. An object concealed by the new cloak, the researchers claim, is magnetically undetectable, while the cloak itself is made from materials available in many physics labs the world over. This means that it is, in principle, the first cloak that should be reasonably practical to manufacture.

Cloaks and shields

In 2011 Alvaro Sanchez and colleagues at Universitat Autònoma de Barcelona, Spain, developed a theory for a type of magnetic cloak they called an "antimagnet" that would have two crucial properties. One is that any magnetic field created within the cloak would not leak outside the cloaked region and the other is that the cloak and the cloaked region would be undetectable by an external magnetic field; that is, the field would not be distorted by the cloak. Now, Sanchez along with Fedor Gömöry and colleagues from the Slovak Academy of Sciences, has designed and demonstrated a modified version of the cloak proposed last year.

The new cloak is a simple bi-layer cloak made up of two common materials – an inner superconducting layer made up of a high-temperature superconducting tape and an outer ferromagnetic layer composed of a few turns of a thick FeNiCr commercial alloy sheet. "The cloak we proposed last year was more of an ideal cloak," explains Sanchez. "But it was complicated with 10 layers and included superconducting plates. This new cloak, while not perfect, is a much simpler design for achieving similar results using a static uniform magnetic field." He adds that it is fair to say that this is the first cloak that is an exact cloak that can be feasibly implemented in practice.

The superconducting layer on its own repels the magnetic field, while a ferromagnetic layer on its own attracts the magnetic field lines; so both independent layers distort the field. The cloak is the accurate combination of the two layers, determined by a specific radius, which adjusts for the permittivity (μ) such that there is no external field distortion at all. This radius is calculated using Maxwell equations. "It is quite amazing that almost 160 years after Maxwell equations were first developed, we are still finding new solutions based solely on them!" says Sanchez.

Artist's impression of how the magnetic cloak works. The ferromagnet attracts magnetic field lines (left), the superconductor repels magnetic field lines (centre), and the superconductor-ferromagnetic bilayer cloaks a magnetic field (right). The result is that an object inside the cloak is magnetically undetectable.

Perfection problems

Sanchez tells physicsworld.com that the entire team is highly inspired by the initial work on building invisibility cloaks using transformation optics carried out by John Pendry and colleagues at Imperial College London since 2006. "There are generally two ways of achieving a cloak – either using transformation optics or using plasmonics. The problem with the first is that, while it is theoretically the perfect cloak, it is nearly impossible to physically create. With plasmonics, while the materials are available, you get a slight shadowing or scattering effect, not a complete cloak at all. This is the first time that you get both using commercially viable materials," Sanchez explains.

Sanchez points out that an advantage in developing a cloak for a static magnetic field is that, for such a field, the magnetic and electric effects decouple and the researchers only have to consider the magnetic permeability. The team tested its cloak using a static field of 40 mT – which is greater than the Earth's magnetic field. Currently, the cloak has been built on a small but reasonable scale – 12.5 × 12 mm. Sanchez explains that another advantage is that, for a static magnetic field, the cloak can work on any length scale – from microns to metres – as there is no intrinsic cut-off, unlike other cloaks that work at fixed wavelengths.

Because the cloak is capable of running under relatively strong magnetic fields and relatively warm liquid-nitrogen temperatures, and as it is made from commercially available materials, it could be readily put to practical use, the researchers say. The team is also looking at other methods to manipulate and control magnetic fields into different "shapes", for purposes other than cloaking, in the coming months