A STUMPY metal column less than one-fifth of a micrometre tall claimed the record this week as the world鈥檚 smallest microwave generator. It has also solved a long-standing mystery about how spinning electrons interact with magnetic fields 鈥� a key phenomenon in the emerging technology of 鈥渟pintronics鈥�.
One way to think of an electron is as a tiny ball spinning about an axis. Because it is charged, it generates a tiny magnetic field as it spins and this can be made to line up with an external field. It is this property of spin that a team led by Daniel Ralph from Cornell University, Ithaca, New York, has exploited to create the microwave generator.
鈥淚t is pleasantly simple in the way it works,鈥� says Ralph. A small current of electrons is made to pass through a nanoscale metal column made from a series of layers of metals with different magnetic properties. The first layer is cobalt, a naturally magnetic material with a strong field that forces the electrons鈥� spins to become aligned. The electrons then pass through a layer of copper into another much thinner slice of cobalt in which the magnetic field is unstable. The electrons interact with this field causing it to 鈥減recess鈥� like a spinning top. It is this precession that produces the microwaves (Nature, vol 425, p 380).
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While the signal from the miniature source is too weak to work over long distances, Ralph suggests it might be powerful enough to ferry messages around computer chips. But Jonathan Sun, a materials physicist at IBM Research in New York state, says the significance of the experiment for spintronics goes far deeper than any potential applications. Until now, no one knew for certain whether electron spins alone could flip a magnetic field. This experiment proves they can. 鈥淚t is a very clean, very beautiful demonstration of the mechanism,鈥� says Sun.
Ralph and his team are now investigating how the frequency of the microwaves can be tuned by changing the material the column is made of, or by varying the current. So far they have generated microwaves with frequencies from 1 to 40 gigahertz. If they can make the field in the cobalt layer precess more rapidly, the device should produce infrared radiation. If they slow it down, it should emit radio waves.
