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Spins turn light off

10 Apr 2007

Researchers in Canada and the US have unveiled a new material that can switch a light beam on and off by varying the spin-polarization of electrons in the material. The material is made up of tiny magnetic particles of cobalt that are partially coated with gold. According to the researchers, it could one day be used in devices for processing information that exploit both light and electron spin (Phys. Rev. Lett. 98 133901).

Many researchers are trying to develop "spintronic" devices that use the spin of the electron as well as its electrical charge to store and process information. Others, meanwhile, are trying to exploit the interaction between light and the collective oscillations of electrons on the surfaces of metals (called plasmons) to create “plasmonic” devices for processing and transmitting data. Now, Abdul Elezzabi and Kenneth Chau at the University of Alberta, along with Mark Johnson at the Naval Research Laboratory in Washington, DC, have created a material that combines spintronics and plasmonics to switch a beam of terahertz light.

When their material is exposed to an external magnetic field, the electrons in the micrometre-sized cobalt particles become spin polarized. If light in the terahertz frequency range is shone on the material, the electromagnetic field associated with the light drives some spin-polarized electrons from the cobalt into the partial gold coating (see figure). This leads to an electrical resistance between the gold and cobalt called anisotropic magnetoresistance (AMR).

With the magnetic field turned off, there is no AMR and much of the light is transmitted through the material via plasmons. However, as the field is turned on, the increased resistance brought on by AMR impedes the flow of plasmons and the light transmission drops by over 70% in some samples.

Elezzabi told Physics Web that the team has observed this switching effect in other magnetic materials and is currently trying to find materials that enhance the effect. He said that in principle the effect is not restricted to terahertz light – which is sandwiched between the ultraviolet and microwave bands – but it would be difficult to achieve at higher (microwave) frequencies because the light’s electric field oscillates so rapidly that spin-polarized electrons would not accumulate in the gold coating.

Elezabbi also said that the group are in the process of filing a patent related to several devices based on the effect.

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