In a Spin – New Technique for Controlling Electron Spin Voltage

Researchers from Harvard University have established a technique to control and measure electron spin voltage, a development which will have significant implications for information technologies of the future. However, Scientists first need to better understand how to control spin and learn how to build the spin equivalent of electronic components such as spin transistors, spin gates and circuits.

Researchers used atomic size defects in diamonds to detect and measure magnetic fields generated by spin waves (Image courtesy of Peter and Ryan Allen/Harvard University)

Rather than employing electric current – where information is carried from A to B through conducting materials using electrons – technologies of the future are likely to use an electron’s spin to carry information. Spin can propagate through an insulating material like a wave, with each electron remaining stationary and communicating spin to its coupled neighbor.

There is growing interest in insulating materials that can conduct spin. Our work develops a new way to look at these spins in materials such as magnets.

Amir Yacoby, Professor of Physics in the Department of Physics and of Applied Physics, Harvard

Yacoby and a team from the School of Engineering and Applied Sciences developed a technique to control and measure the spin voltage - or spin chemical potential - using atomic-sized defects in diamonds to measure chemical potential. The technique is essentially a nanoscale spin multimeter that allows for measurements in chip-scale devices.

The Researchers knew that spin can travel through material in waves, but had to determine a way to drive the waves from A to B by increasing the spin chemical potential at a local level.

If you have a high chemical potential at location A and a low chemical potential at location B, spin waves start diffusing from A to B. This is a very important concept in spintronics, because if you are able to control spin-wave transport, then you can use these spin waves instead of electrical current as carriers of information.

Chunhui Du, a Postdoctoral Fellow, the Department of Physics

Two spin-wave injection methods were employed; in the first, Researchers applied fast-oscillating, microwave magnetic fields to excite spin waves. In the second, electrical current was converted into spin waves using a platinum metal strip located at one end of the magnet.

“What’s remarkable is that this material is an insulator; it doesn’t conduct any current and still you can send information in the form of spin waves through it,” said Toeno Van der Sar, a Postdoctoral Fellow at the Department of Physics.  “Spin waves are so promising because they can travel for a long time without decaying, and there is barely any heat produced because you don’t have moving electrons.”

Once spin-waves were injected into the material, the next task was to determine how to measure the waves’ information. The Researchers turned to nitrogen-vacancy (NV) defects in diamonds. These defects – in which one of the diamond’s carbon atoms is replaced with a nitrogen atom and a neighboring atom removed - can be used to detect minute magnetic fields.

The Researchers made miniscule rods of diamond containing NV centres and positioned them just nanometres above one of their samples. As the spin wave travels through the material, a magnetic field is generated and detected by the NV centre. Based on these NV center measurements, Researchers can determine the spin chemical potential, the number of spin waves, how they are traveling through the material and other important insights.

The nice thing about this technique is that it’s very local. You can do these measurements just a few nanometers above the sample, which means that you can spatially study the chemical potential in a chip-scale spin-wave device, for let’s say a spin-wave computer. This is not possible with some of the other state-of-the-art techniques.

Toeno Van der Sar, a Postdoctoral Fellow at the Department of Physics

The system could also offer an insight into more colorful physics, such as the spin-wave Hall effect - production of a voltage difference or Hall voltage, across an electrical conductor, transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It may also show that spin-wave transport is hydrodynamic.

“The principle we use to control and measure the spin chemical potential is quite general. It opens ways to study more exotic spin phenomena in novel materials and aids the development of new spintronic devices,” said Du.

The work has been published in Science.

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