Antimagnets: Controlling magnetic fields with superconductor-metamaterial hybrids

Arxiv – Antimagnets: Controlling magnetic fields with superconductor-metamaterial hybrids

Magnetism is very important in science and technology, from magnetic recording to energy generation to trapping cold atoms. Physicists have managed to master magnetism – to create and manipulate magnetic fields- almost at will. Surprisingly, there is at least one property which until now has been elusive: how to ‘switch off’ the magnetic interaction of a magnetic material with existing magnetic fields without modifying them. Here we introduce the antimagnet, a design to conceal the magnetic response of a given volume from its exterior, without altering the external magnetic fields, somehow analogous to the recent theoretical proposals for cloaking electromagnetic waves with metamaterials. However, different from these devices requiring extreme material properties, our device is feasible and needs only two kinds of available materials: superconductors and isotropic magnetic materials. Antimagnets may have applications in magnetic-based medical techniques such as MRI or in reducing the magnetic signature of vessels or planes.

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A metamaterial is a bizarre substance with properties that physicists can fine tune as they wish. Tuned in a certain way, a metamaterial can make light perform all kinds of gymnastics, steering it round objects to make them seem invisible. This phenomenon, known as cloaking, is set to revolutionise various areas of electromagnetic science. But metamaterials can do more. One idea is that as well as electromagnetic fields, metamaterials ought to be able to manipulate plain old magnetic fields too. After all, a static magnetic field is merely an electromagnetic wave with a frequency of zero.

Instead of a magnetic cloak -null interior fi eld and external fi eld unaff ected-, we want to design here an antimagnet, defi ned as a material forming a shell that encloses a given region in space while ful lling the following two conditions:
i) The magnetic field created by any magnetic element inside the inner region –
e. g. a permanent magnet – should not leak to outside the region enclosed by the shell.

ii) The system formed by the enclosed region plus the shell should be magnetically undetectable from outside (no interaction -e. g. no magnetic force- with any external magnetic sources).

In this work we will consider the case of a cylindrical cloak; results can be extended to other geometries.

Two important key ideas have been needed for achieving our goal: the design of a simpli ed cloak with homgeneous parameters, corresponding to a new space transformation, and the placement of a superconducting layer at the inner surface. Such an antimagnet would be passive and, provided that the superconductors are in the Meissner state and that the isotropic layers have a negligible coercivity (as if using superparamagnetic materials), also lossless. The strategy for antimagnet design presented in this work can be adapted to other geometries (e.g. spheres) or even to other forms of manipulating magnetic fields, such as magnetic fi eld concentrators. Antimagnet devices may bring important advantages in fields like reducing the magnetic signature of vessels or in allowing patients with pacemakers or cochlear implants to be allowed to use medical equipment based on magnetic fields, such as magnetic resonance imaging MRI or transcraneal magnetic stimulation. Moreover, by tuning one parameter like the working temperature of the device – below or above the critical temperature of the superconductor, for example – one could ‘switch o and on’ magnetism in a certain region or material at will, opening up room for some novel applications.

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