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.
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.
The research is published in Science 335 1466.
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