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Boston University's Tiny Torsion Oscillator
A research group at Boston University recently published their successful measurement of the torque produced by a nano-scale motor less than a micron in size in Nature Nanotechnology. Although the motor's tiny size is remarkable, even more interesting is the principle on which it operates; direct conversion of a change in an electron's angular momentum into torque.
All electrons possess a fundamental quantum property called "spin" that corresponds to their intrinsic angular momentum. An electron can only have two possible values for its spin: +1/2 or -1/2. The new motor creates torque by passing a current of electrons that all possess the same spin through a junction between a ferromagnetic and nonmagnetic material. When the electrons move from the ferromagnetic material to the nonmagnetic material, they change the sign of their spin. Because this requires the electrons to change their angular momentum, conservation of angular momentum causes torque to be exerted on the junction.
The research group, which was lead by Professor Pritiraj Mohanty, created the motor by using electron beam lithography and surface nanomachining to connect a cobalt wire with a gold wire in the center of a tiny torsion oscillator that measured a mere 12 x 6 micrometers. By sending a current of one microampere through the cobalt-gold junction, they were able to measure the 2.3E-22 Nm of torque produced by the device.
The tiny torsion oscillator on which the cobalt/gold junction was mounted is a significant breakthrough in nano-motor research, as it is roughly an order of magnitude more sensitive than current optical teweezer techniques. The authors speculate that their torsion sensor could be useful for experiments involving measurements of DNA untwisting, torsion-producing molecules, and magnetoelectronics.
The paper containing the research is currently available from Nature online, but requires a subscription.