'Inverse spin Hall effect' harvests electricity from magnetism

April 22, 2016 // By R. Colin Johnson
'Inverse spin Hall effect' harvests electricity from magnetism
With microwaves on the rise worldwide, generated by cell phone towers, mobile devices, WiFi, Bluetooth, 5G and on and on, it's natural that scientists would investigate ways to harness these waves to generate energy. Scientists at the University of Utah (Salt Lake City, UT) have discovered a novel way of converting microwave energy into electricity in organic semiconductors.

In the lab, they have demonstrated a novel effect - called the inverse spin Hall effect - which can convert magnetic spin current into electrical current using microwaves as their source of magnetic spin. It sounds like taking the long way around, since cell-phone antennas already convert microwaves into electricity, however the point of their demonstration is not to preview an application, but to prove that the inverse spin Hall effect can indeed be harnessed and controlled as a tool for the 21st century. They predict applications in batteries, solar cells, mobile devices.

"The energy that we take out of the device is energy that is put into the device through microwave radiation - in that sense, the power conversion does exactly what an antenna does as well, namely convert electromagnetic radiation into an electrical current," University of Utah professor Christoph Boheme told EE Times in an exclusive interview.

"The difference is that the physical mechanism by which our device does this is fundamentally different. It is not induction that accomplishes the conversion, it is the inverse spin Hall effect. As a matter of fact, corroborating the fact that we do not see spurious effects such as electrical induction (such as a simple antenna effect) or other known phenomena was the goal of this study."

The inverse Hall effect was first demonstrated in 1984 by Soviet scientists and was studied more recently (2006) in semiconductors and (2013) in ferromagnetic metals. The concept is relatively simple: just as magnetic spins are induced in the atoms surrounding a wire conducting electricity--the direction of the spin being dependent on the direction of the current--likewise a current will flow in a wire if magnetic spin is induced in the atoms surrounding the wire.

However, the concept is simpler than the apparatus needed to demonstrate it - and that is where the microwaves come in. The earlier experiments with the inverse spin Hall effect used a constant bath of microwaves - like those inside a microwave oven. Unfortunately, that fried the rest of the apparatus making their experiments short-termed and ultimately of very limited success. Their failures may also doom the harnessing of stray microwaves in the environment, even though Boheme and his collaborator, fellow professor Valy Varden, think the idea has merit.

The device built on a small glass slide (top) exhibits a spin current to be converted to an electrical current using the inverse spin Hall effect. The key is a sandwich-like device (bottom) where an external magnetic field and pulses of microwaves create spin waves in the iron magnet which converted to an electrical current in the copper electrodes they hit the organic semiconductor (polymer). (Source: University of Utah, Kipp van Schooten and Dali Sun)

"That is an excellent idea and whether this will or will not become an application of the inverse spin Hall effect has yet to be shown," Boheme responded to my suggestion of harnessing stray microwaves to produce electricity.

He may have just been being polite, however, because his experiments used pulsed microwaves to eliminate the overheating problem. Also his suggested applications sounded much more feasible than mine.

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