‘Inverse spin Hall effect’ harvests electricity from magnetism

‘Inverse spin Hall effect’ harvests electricity from magnetism
Technology News |
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.
By Rich Pell


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.

“We know from other spintronics applications, such as hard-disk read heads, that spintronics may fill technological gaps for magnetic-field to electrical-current conversation where simple induction fails – meaning where induction becomes too insensitive and too inefficient (in hard discs this was the case when the read heads became too small),” Boheme told EE Times. “It is conceivable to make inverse spin Hall effect devices out of organic semiconductor layers as monolithic, nanometer sized thin-film devices on flexible substrates (essentially foils) at very low cost, so the range of applications can not be foreseen at this point. If efficiencies permit (which we don’t know at this point!), then it is also conceivable that this could be used to take microwave radiation out of our environment and use the energy therein for other applications.”

The long and short of the inverse spin Hall effect is that it works, that it is a new use of spintronics that in some ways complements the already growing toolbox of spintronic effects and devices that can harness them. Next, their efficiency needs to be accurately measured and some appropriate trial applications need to be tried, in order to gauge just how useful the inverse spin Hall effect will be for organic semiconductors in the future.

“Our study’s goal was to show how to measure the inverse spin Hall effect in a ‘straight forward’ manner [that is] show a strong and directly observable inverse spin Hall effect with no or very little simple microwave induction effects and other signals,” Boheme told EE Times. “We have achieved this by building devices and conducting experiments which make the inverse spin Hall effect about a 100 times stronger than what was previously seen and, at the same time, we accomplished the oppression of the spurious effects. So now we have devices on which we can easily observe this effect. For the near future, we (and probably other research groups as well) will use this progress to really study this effect in detail. Part of these studies will, of course, be aimed at the question of how well can this effect be used for potential technical applications.”

A view of the University of Utah physics laboratory where researchers showed that a phenomenon named the inverse spin Hall effect works in several organic semiconductors when pulsed microwaves are applied to the materials. The effect converts so-called spin current to electric current and may find use in future generations of batteries, solar cells and electronic devices. (Source: University of Utah, Christoph Boehme)

So the proof is still in the pudding, and these researchers have merely come up with a baseline recipe. It will up to them and others – in future experiments – to gauge the usefulness of the inverse spin Hall effect in future applications. Personally, I hope this ends up solving the “microwave overload” from communications towers that is slowing cooking everybody in their own juices, but if I had to bet, my money would be on small on-chip applications such as new spintronic devices for the ultra-low-power organic semiconductors of the future.

The researchers proved that the inverse spin Hall effect works in three organic semiconductors: PEDOT:PSS and in three platinum-rich organic polymers, two of which were pi-conjugated polymers and the other was spherical carbon-60 molecules (buckey balls) the latter of which proved to be the most efficient. For all the details see Inverse Spin Hall Effect from pulsed Spin Current in Organic Semiconductors with Tunable Spin-Orbit Coupling

Funding was provided by the National Science Foundation (NSF) and the University of Utah’s NSF Materials Research Science and Engineering Center. Other contributors were research assistant professors Dali Sun and Hans Malissa, postdoctoral researchers Kipp van Schooten and Chuang Zhang, and doctoral candidates Marzieh Kavand and Matthew Groesbeck.

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