Voltage controllable magnets
Debye looked for a combined magnetic and electric moment in molecules (NO), but such a moment does not exist, as van Vleck showed, since these and other molecules are paramagnetic (or diamagnetic), but not ferromagnetic. However, B.D.H. Tellegen from the Philips research labs predicted in 1948 that it should be possible to make artificial particles that have such properties. The question became: “Can one ‘glue’ a magnet to an electric dipole and then switch the magnet with a voltage?” Remarkably, Tellegen's proposal to make orientable microscopic magnets coupled to electrets has never been realized before.
Figure: Schematic of microparticles with electric and magnetic dipole moments |
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Together with Nick Sheridon from Xerox, we recently succeeded in making a soft-matter magnet [1] that can be switched with a static electric field. The system is based on a form of electronic paper, known as Gyricon. A PDMS film contains millions of 50 to 100 micron polyethylene particles that carry both an electric dipole moment and a magnetic dipole moment (schematically depicted in the Figure). The particles, although embedded in the PDMS, sit in a small fluid cavity, where they are free to rotate. When no voltage is applied the particles are disordered and so there is no net magnetic field. However, when a voltage is applied all the particles orient due to their electric dipoles and automatically orient all the micro-magnetic moments with them. Reversing the voltage reverses the poles of this soft-magnet.
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| Figure: schematic of Janus particles (top left), schematic of PDMS film (right), (bottom) images of actual particles and film (left and right, respectively). |
We are currently exploring this interesting system, to see if it can be used to realize the fifth-network element, known as the “ideal gyrator”.
[1] A. Ghosh, N. K. Sheridon and P. Fischer, "," Small 4 (11), 1956-1958 (2008).
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