The magnetic hose

A guide that routes static magnetic fields as easily as fiber optics carry light could one day see applications as diverse as stepping up voltage in a transformer or manipulating a tiny quantum system. Now researchers led by Alvar Sanchez of the Autonomous University of Barcelona have taken the first step toward developing that routing technology, with a device they call the magnetic hose. The hose works because it is made from a material that has an anisotropic magnetic permeability and thus responds differently to magnetic fields entering it from different directions. The figure illustrates the theory for the extreme case of an infinite slab with infinite permeability in the vertical (z) direction and zero permeability in the orthogonal directions. Within the material, the field (white lines) is totally vertical. As a result, the dipole field (whose z component is illustrated by the colors) is faithfully transmitted across the slab. Sanchez and company fabricated their finite-sized, finite-permeability hose by surrounding a cylindrical ferromagnet with a coaxial superconducting shell. The ferromagnetic core gives the hose a large permeability in the axial direction, analogous to the z direction in the figure, whereas the field-expelling superconductor enforces nearly zero permeability in the radial directions. To assess their device’s performance, the researchers placed a current loop slightly below the hose and measured the dipole field slightly above it. For each of two different hose lengths (6 cm and 14 cm), they found the measured field was at least twice as great as for the ferromagnetic core alone. In theory, additional alternating shells of ferromagnet and superconductor could improve the hose’s ability to transmit magnetic fields. (C. Navau et al., Phys. Rev. Lett., in press.)