Potential New Energy Source: ‘Inverse Spin Hall Effect’

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2016

The dynamic is achieved through using the spin of electrons.

 

KEY CONCEPTS
• A completely electric-current free spin current can be converted into an electric current through a phenomenon known as the ‘inverse spin Hall effect.’
• Organic semiconductors are used to generate the spin current at a magnitude not seen before.
• Further research is needed to determine how effective the inverse spin Hall effect will be as a power conversion effect.

IN AN EFFORT TO DEVELOP more efficient ways to generate electricity, researchers are looking at how best to take advantage of existing energy sources. A case in point is the development of more effective thermoelectric materials that can convert heat into electricity.

In a previous TLT article, the development of a new thermoelectric material based on magnesium, tin and germanium is described (1). This material displays a peak power factor above 40 and a ZT value that exceeds one, which are two important criteria needed to convert heat to electricity.

Electricity occurs commonly through the flow of electrons in a wire generating a current. But another consideration in examining electrons is that they exhibit angular momentum as they move through atoms, and they can even have a spin with a quantum number of 1/2. The spin causes electrons to act as small bar magnets that have a north and south pole. It is also known that electrons can display spin in different directions, for instance, either in an upward, positive direction, or a downward, negative direction.

Christoph Boehme is a professor in the physics and astronomy department of the University of Utah in Salt Lake City, Utah. He says, “Application of a flow of magnetically polarized electrons, a so-called spin current, perpendicular to a semiconductor surface and a magnetic field, will cause those electrons with a spin pointing up to move into one direction, for instance to the right, and those electrons with a spin pointing down to move into the opposite direction, for instance to the left.”

This effect caused by a spin current can even take place when the spin current itself does not involve a flow of electric charge, so it is free of an electric current. However, because a spin current can move electric charge perpendicular to its direction, it actually can produce an electric current perpendicular to the spin current direction. Boehme says, “This mechanism allows a completely electric-current free spin current to turn into an electric current. It is an entirely different way to convert the energy in a magnetic material into electric energy.”

Boehme indicates that the “inverse spin Hall effect” was only discovered in metals in 2008 and then in organic semiconductors in 2013. But in the more recent study, the electrical current produced was quite small.

Boehme and his colleague, Zeev Valy Vardeny, distinguished professor of physics at the University of Utah, then decided to corroborate the earlier work and determine if there is a technique that can be developed to increase the magnitude of the electrical current produced.

SHORT MICROWAVE PULSES
Boehme and Vardeny detected voltage and electric current generated by the inverse spin Hall effect by using several organic semiconductors that contained platinum atoms to a degree never before seen. Organic semiconductors are used in this study as compared to traditional inorganic semiconductors due to easier processing and more reasonable costs.

Boehme says, “The organic semiconductors are mainly polymer materials prepared with carbon-carbon double and triple bond backbones. Platinum atoms are integrated in every second, third or fifth repeat unit because they exhibit stronger spin-orbit coupling since they are heavier elements. A unique material used in this study is a spherical molecule containing 60 carbon atoms known as buckminsterfullerene.”

The researchers used the semiconductor device shown in Figure 1 to generate the spin current. In the experimental set-up, thin flat copper wires running the length of a glass slide connected copper electrodes at one end of the slide with the organic semiconductor at the other end of the slide. The semiconductor is part of a sandwich also shown in Figure 1 where this material is surrounded by copper wiring on the sides and a ferromagnet on top.


Figure 1. The experimental set-up used to study the inverse spin Hall effect is shown in the top part of the figure. The bottom illustration shows the sandwich type device used and where it is placed in the experimental set-up. (Figure courtesy of Kipp van Schooten and Dali Sun, University of Utah.)

This device is then inserted into a metal tube surrounded by a nonconducting material that is inserted into a magnet that produces the magnet field. Upon application of the magnetic field, the researchers did not detect any voltage. Then short microwave pulses ranging from 100-5,000 nanoseconds in duration are applied directly to the organic semiconductor. The result is the sudden production of voltage and electric current during the pulsing.

Boehme says, “We have determined that the voltage produced using our experimental procedure is 100 times stronger than what was detected in the initial study.”

A noteworthy inverse spin Hall effect was observed with the fullerene derivative which was evaluated without the use of any heavy elements. Boehme says, “This C60 material exhibited stronger spin-orbit coupling because its spherical geometry enabled the electrons to move in circular trajectories enhancing the inverse spin Hall effect.”

Future work is underway by the researchers to identify organic materials that are even more effective than buckminsterfullerene. Boehme says, “We are evaluating other carbon-based spherical materials and are also looking to use an organic magnet so that we can produce a completely carbon-based organic transducer.”

Boehme wishes to caution that the inverse spin Hall effect is a relatively new power conversion effect. He says, “We do not know how effective this technique can become as we are still very much in the learning stage.”

Additional information can be found in a recent article (2) or by contacting Boehme at boehme@physics.utah.edu.

REFERENCES
1. Canter. N. (2015), “High-power-factor  thermoelectric material,” Tribology and Lubrication Technology, 71 (7), pp 12 – 13.
2. Sun, D., Schooten, K., Kavand, M., Malissa, H., Zhang, C., Groesbeck, M., Boehme, C. and Vardeny, Z. (2016), “Inverse spin Hall effect from pulsed spin current in organic semiconductors with tunable spin-orbit coupling,” Nature Materials, DOI: 10.1038/NMAT4618.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.