(Phys.org)—Typically when two magnets are brought close together, they either attract or repel each other due to interactions between their magnetic fields. In a new study, researchers have designed a 3D magnetic invisibility cloak, inside of which they placed a magnetic object, and showed that the cloaked magnet is no longer affected by nearby magnetic fields. It appears as if the cloaked magnet has become demagnetized, but in reality the magnet is simply hidden.

The researchers, led by Yungui Ma at Zhejiang University in Hangzhou, China, have published a paper titled "Three-dimensional magnetic cloak working from d.c. to 250 kHz" in a recent issue of Nature Communications. Like other invisibility cloaks, the new cloak is made of metamaterials (man-made materials with repeating patterns) and works by manipulating electromagnetic waves in unusual ways.

To achieve the cloaking effect, the researchers used a new type of invisibility cloak called a bilayer cloak, first demonstrated in 2012 by Alvaro Sanchez and colleagues at the Autonomous University of Barcelona. The cloak has a spherical structure consisting of two shells: a superconducting inner shell (made of single-crystal YBCO) and a ferromagnetic outer shell (made of a nickel zinc composite).

The bilayer cloak consists of an inner superconducting shell and an outer ferromagnetic shell, whose opposite effects on an external magnetic field completely cancel each other out to shield a cloaked magnetic object (yellow) from the external magnetic field. Credit: Zhu, et al. ©2015 Nature Communications

Basis: Experimental realization of a magnetic cloak
A recent article in Science presents the successful experimental realization of a dual-layer cylindric cloak for magnetic fields [8]. Such a device creates a field-free region inside and no interferences outside a cylindrical volume, which is exactly the field configuration we want to achieve. This section summarizes the basis of this experiment and its results.

Fig. 2 (top) illustrates the basic idea: A ferromagnetic cylinder placed in a magnetic field (A) pulls in fields lines and reduces the flux inside the cylinder while distorting the field homogeneity. On the other hand, a superconducting cylinder (B) creates a field-free region inside the cylinder by pushing out the magnetic field lines and distorts the field homogeneity around the cylinder in the opposite way. For ideally homogeneous magnetic fields, the combination of a superconducting inner cylinder with a ferromagnetic outer cylinder of the right radius, thickness, and permeability (C) creates a field-free region inside and no interferences outside the cylinders.

Based on Maxwell’s equations, the permeability and radii of the ferromagnetic layer have to relate according to:

to achieve this perfect cloak. R1 and R2 are the inner and outer radius of the ferromagnetic layer and µ2 is the magnetic permeability of this layer. The superconducting layer does not need to have a specific thickness.
The cloak described in [8] is 12 mm long and has an inner diameter of 12.5 mm and an outer diameter of 17.5 mm. It consists of multiple layers of Fe18Cr9Ni alloy sheets for the outer (ferromagnetic) layer and a high temperature superconductor (ReBCO) on the inside. Both materials are commonly available, which allows for the construction of such a device at relatively low cost end effort.
Fig. 2 (bottom) shows the result of measuring the magnetic flux along a line 3 mm above the cloak in a homogeneous magnetic field of 40 mT. The presence of the cloak practically has no effect on the field homogeneity outside, which confirms the viability of this magnetic cloak design [8].

Physicists have already unveiled invisibility cloaks that can hide objects from light, sound, seismic and even water waves. Now researchers report a cloak that can hide objects from static magnetic fields. This 'antimagnet' could have medical applications, but might also subvert airport security.


The cloak's interior is lined with turns of tape made from a high-temperature superconductor. Superconductors repel magnetic fields, so any magnetic field enclosed within a superconductor would be undetectable from outside. But the superconductor itself would still perturb an external magnetic field, so the researchers coated its external side with an ordinary ferromagnet — the material that kitchen fridge magnets are made of. The superconductor tries to repel external field lines, whereas the ferromagnet tries to draw them in — together, the two layers cancel each other out.

To test the antimagnet, the Slovak group cooled the cloak with liquid nitrogen to activate the superconductor, and placed it in a static, uniform magnetic field with a strength of 40 millitesla. Using a measuring device called a Hall probe to map the magnetic field, the researchers found that the field lines did not enter the cloak, even through from the outside they appeared to pass straight through. They say that theirs is an 'exact' cloak — one for which the cloaking could, in principle, be made perfect using currently available materials.

Sanchez points out that the magnetic cloak is straightforward to make: it requires only off-the-shelf materials and costs in the region of €1,000 (US$1,300) — very little in research terms. He believes the cloak could have uses in medicine, protecting delicate pacemakers from the strong magnetic fields of magnetic resonance imaging machines. But he admits there could also be unsavoury applications — the technology could, for example, be used to hide metallic weapons from security portals. “I would prefer to consider it the other way — that our ideas can help to design safer security procedures,” Sanchez says.

Taking the wraps off cloaking, John Pendry
The real challenge of cloaking lies in deriving a theoretical prescription for the optical properties of the cloak and, even more challenging, realizing these properties in a material. Transformation optics provides the theoretical background and metamaterials provide the means of achieving the prescribed parameters.

Transformation optics
It was Michael Faraday who stressed the importance of “lines of force.” He could see magnetic lines of force aligning iron filings placed near his magnets, and for him they represented physical reality. Lines of force are continuous and their density represents the strength of a field. Likewise, electric field lines are also continuous, at least in the absence of electrical charges. In fact, any conserved vector quantity can be represented in this way, and one can add the Poynting vector, representing the flow of electromagnetic energy, to this set. The Poynting vector is merely the mathematical representation of a “ray” of light.

Maxwell’s equations are a mathematical realization of Faraday’s work. They describe the phenomena of classical optics and it has long been known that their form is invariant under coordinate transformations.

Transformation optics was born of the realization that lines of force are effectively glued to the coordinate system. As the system is distorted it carries with it all the associated fields. Hence to guide the trajectory of a ray of light, only a distortion in the underlying coordinate system is needed, automatically taking with it the light ray. Knowledge of the transformation in turn provides the values of μ and ε required to steer the light in this way.

Invisibility
Figure 3 (top left) shows a ray of light travelling in free space. We wish to hide the contents of a sphere radius R1 by directing the rays around this region, but requiring that any distortion of trajectories is confined within a larger sphere radius R2 (Fig. 3, top right). In this way an external observer would be aware neither of the presence of the cloak nor its contents. The illusion of empty space has been created.