A new way to obtain electricity from magnetism

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image: the upper part of this illustration shows the device, built on a small glass slide, which was used in experiments showing that what is called spin current could be converted into electric current using several semi -different organic polymer conductors and a phenomenon known as the reverse Hall effect of rotation. The lower illustration shows the key sandwich-shaped part of the appliance. An external magnetic field and microwave pulses create spin waves in the iron magnet. When these waves hit the polymer or organic semiconductor, they create a spin current, which is converted into an electric current at the copper electrodes.
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Credit: Kipp van Schooten and Dali Sun, University of Utah

SALT LAKE CITY, April 18, 2016 – By showing that a phenomenon called the “reverse spin Hall effect” works in several organic semiconductors – including carbon 60 buckyballs – physicists at the University of Utah have transformed the magnetic “spin current” into electric current. The effectiveness of this new method of energy conversion is not yet known, but it could be used in future electronic devices, including batteries, solar cells and computers.

“This paper is the first to demonstrate the Hall effect of reverse spin in a range of organic semiconductors with unprecedented sensitivity,” although a 2013 study by other researchers demonstrated it with less sensitivity in any of these materials, said Christoph Boehme, lead author of the study published April 18 in the journal Natural materials.

“The reverse spin Hall effect is a remarkable phenomenon that transforms the spin current into an electric current. The effect is so strange that no one really knows what it will ultimately be used for, but there are a lot of technical applications that can be considered, including some very strange new ones. power conversion schemes ”, explains Boehme, professor of physics.

His senior author colleague, distinguished professor Z. Valy Vardeny, says that by using microwave pulses, the reverse spin Hall effect, and organic semiconductors to convert spin current into electricity, this new force electromotive generates an electric current in a different way from existing sources.

Coal, gas, hydroelectric, wind and nuclear power plants all use dynamos to convert mechanical force into changes in the magnetic field and then into electricity. Chemical reactions power modern batteries, and solar cells convert light into electric current. Another method is converting spin current to electric current.

Scientists are already developing such devices, such as a thermoelectric generator, using traditional inorganic semiconductors. Vardeny says organic semiconductors show promise because they are cheap, easy to process, and environmentally friendly. He notes that organic solar cells and organic LED (light emitting diode) television screens were developed even though silicon solar cells and inorganic LEDs were widely used.

Vardeny and Boehme pointed out that the efficiency with which organic semiconductors convert spin current to electric current remains unknown, so it is too early to predict to what extent it could one day be used for new conversion techniques. power in batteries, solar cells, computers, telephones and other consumer electronics.

“I want to invoke some caution,” said Boehme. “It’s a power conversion effect that is new and for the most part not studied. ”

Boehme notes that the experiments in the new study converted more spin current to electric current than in the 2013 study, but Vardeny cautioned that the effect “should always be increased several times to produce voltages equivalent to household batteries. “.

The new study was funded by the National Science Foundation and the University of Utah-NSF Materials Research Science and Engineering Center. The co-authors of the study with Vardeny and Boehme were these physicists from the University of Utah: research assistant professors Dali Sun and Hans Malissa, postdoctoral fellows Kipp van Schooten and Chuang Zhang, and graduate students Marzieh Kavand and Matthew Groesbeck.

From spin current to electric current

Just as atomic nuclei and the electrons that revolve them carry electrical charges, they also have another inherent property – spin, which causes them to behave like tiny magnetic bars that can point north or south.

Electronic devices store and transmit information using the flow of electricity in the form of electrons, which are negatively charged subatomic particles. Zeros and ones in computer binary code are represented by the absence or presence of electrons in silicon or other inorganic semiconductors.

Spin electronics – spintronics – hold promise for faster and cheaper computers, better electronics and LEDs for displays, and smaller sensors to detect everything from radiation to magnetic fields.

The reverse spin Hall effect was first demonstrated in metals in 2008 and then in inorganic semiconductors, Vardeny explains. In 2013, researchers elsewhere showed that it occurs in an organic semiconductor named PEDOT: PSS when exposed to relatively weak continuous microwaves to avoid frying the semiconductor.

But Boehme and Vardeny say that the electric current generated in this study by the reverse spin Hall effect was small – nanotensions – and was obscured by microwave heating of the sample and other unwanted effects.

“We thought, let’s build different devices so that these parasitic effects are eliminated or very weak compared to the effect we wanted to observe”, explains Boehme.

In the new study, the researchers used shorter, more powerful microwave pulses to use the reverse spin Hall effect and convert a spin current to electric current in seven organic semiconductors, mostly at room temperature.

One organic semiconductor was PEDOT: PSS – the same material in the 2013 study. The others were three platinum-rich organic polymers, two so-called pi-conjugated polymers, and the spherical carbon-60 molecule named buckminsterfullerene because it looks like a pair of geodesic domes popularized by the late architect Buckminster Fuller.

Carbon-60 has surprisingly been found to be the most efficient semiconductor at converting spin waves into electric current, Vardeny says.

How the experiments were carried out

Physicists in Utah take several steps to convert spin current to electric current. They start with a small glass slide, about 2.1 inches long and one-sixth of an inch wide. Two electrical contacts are attached to one end of the glass slide. Thin, flat copper wires run the length of the slide, connecting the contacts at one end to a “sandwich” at the other end that includes the glass at the bottom, the tested organic polymer semiconductor in the middle, and a nickel ferromagnetic. – iron at the top.

This device is then inserted lengthwise into a metal tube about 1 inch in diameter and 3.5 inches long. A non-conductive material surrounds the device inside this tube, which is then inserted into a table-sized magnet that generates a magnetic field.

“We apply a magnetic field and leave it more or less constant,” says Boehme. “Then we connect the two contacts to a voltmeter and start measuring the voltage coming out of the device as a function of time.”

With just the magnetic field, no electric current was detected. But then Utah physicists bombarded the organic semiconductor device with microwave pulses – as powerful as those from a home microwave oven but in pulses ranging from just 100 to 5,000 nanoseconds. (the latter is equivalent to one 200,000th of a second).

“All of a sudden, we saw tension during this pulse,” Boehme said.

Vardeny says the microwave pulses generate spin waves in the magnet of the device, then the waves are converted to spin current in the organic semiconductor, and then into an electric current that is detected as a voltage.

Compared to the 2013 study, the use of microwave pulses in the Utah experiments meant “our power is much higher but the heating is much less and the reverse spin Hall effect is around 100 times stronger “, explains Boehme.

This is because pulsed microwaves provide a way to enhance the reverse spin Hall effect so that it can be used to convert power, Vardeny adds.

The new study also showed that the conversion of spin current to electric current works in organic semiconductors via “spin-orbit coupling” – the same process found in inorganic conductors and semiconductors. conductive – although the phenomenon in inorganic and organic materials works in a fundamentally different way. , says Boehme.

This coupling is much weaker in organic semiconductors than in inorganic semiconductors, but “the great success that we have achieved has been to find an experimental method sensitive enough to reliably measure these very weak effects in organic semiconductors, ”says Boehme.

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