Researchers at Harvard University have taken an important step in the realization of “spintronic” devices by developing a technique for both measuring and controlling the spin voltage of electrons, also known as their spin chemical potential.
This spin voltage characterizes the tendency of electron spins to diffuse through a material. Whereas in a conducting material electrons can physically move from point A to point B (an electric current), with spin voltage the spin of an electron can be transferred to a neighboring electron. This communication of spins between electrons creates a wave that makes it possible to diffuse the spin of electrons through even an insulating material without the electrons actually moving.
The Harvard method exploits defects in diamonds. These nitrogen-vacancy (NV) defects provide a way to increase the spin voltage locally in an insulating material so that it becomes much easier to detect. The benefits of being able to read this information in an insulating material are key for spintronic devices.
“The spin lifetime in insulators is typically longer than that in conductors, and there is no heat associated with moving electrons, thus spin in an insulator can travel a much longer distance without decaying, which can be important for long-range quantum information transfer,” explained Chunhui Du, a postdoctoral fellow at Harvard and co-first author of the paper, in an e-mail interview with IEEE Spectrum.
The field of “spintronics” in which the spin of electrons encodes information has tantalized us with its capabilities since the introduction of read heads in today’s hard disk drives.
One of the main challenges for making spintronics extend beyond read heads is that the spin of electrons in a material is a fragile and short-lived affair. In read heads for hard drives that short life span doesn’t pose a problem because the information that the spin imparts only needs to travel a few nanometers. However, if you could extend the spin over a greater distance, you would have a greater opportunity to use it for transmitting information.
The Harvard research is so important because it simultaneously extends the spin of electrons over a greater distance through measuring the spin voltage in an insulating material and the technique the researchers have developed takes those measurements within nanometers of the material.
In research described in the journal Science, the technique starts with NV defects in diamonds. These defects are where a nitrogen atom replaces a carbon atom and a neighboring atom in the lattice of a diamond. This makes it possible for the diamond to detect minute magnetic fields. The researchers fabricated these NV diamonds into nanorods that contain the NV centers. They then placed these nanorods a few nanometers above the sample.
Equipped with the sensor, the researchers then triggered a spin wave in the insulating material by shooting fast-oscillating, microwave magnetic fields into the material. In another approach, they converted an electrical current into spin waves using a platinum metal strip located at one end of a magnet. The sensor detects the spin wave and provides information about spin wave distribution in the material.
Of course, there are other techniques for measuring spin voltage. However, Du and his colleagues see that the technique they have introduced is fundamentally different.
“Our technique uses a single spin sensor to measure local magnetic field fluctuations, said Du. “This approach is non-invasive and provides spatial resolution in nanometer scale determined by the distance between the sensor spin and the system under study.”
“The nice thing about this technique is that it's very local," said Toeno Van der Sar, a postdoctoral fellow at Harvard and co-first author of the paper, in a press release. "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."
In future research, Du and his colleagues intend to use this method to continue to explore spin transport in insulating materials.