American researchers have developed a technique to control and measure the spin voltage of electrons, a move they believe will lead to future electronic components based on electron spin, from spin transistors, to spin gates and circuits.
The technique uses atomic-sized defects in diamonds to measure chemical potential, a bit like a nanoscale spin multimeter that allows measurements in chip-scale devices. These defects — in which one carbon atom in a diamond is replaced with a nitrogen atom and a neighboring atom is removed — can be used to detect minute magnetic fields.
“There is growing interest in insulating materials that can conduct spin,” said lead researcher Prof Amir Yacoby from Harvard University's School of Engineering and Applied Sciences. “Our work develops a new way to look at these spins in materials such as magnets.”
He explains that, in conducting materials, electrons can carry information by moving from point A to point B as an electric current. Spin, on the other hand, can propagate through insulating materials in waves, each electron standing still and communicating spin to its coupled neighbor. To drive these waves from point A to point B, the researchers needed to develop a technique to increase the spin chemical potential — spin voltage — at a local level.
The team used two spin-wave injection methods: in the first, they applied fast-oscillating, microwave magnetic fields to excite spin waves. In the second, they converted an electrical current 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 fellow researcher Toeno Van der Sar. “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 the team injected spin waves into the material, the next step was to figure out how to measure information about those waves. The researchers turned to nitrogen-vacancy (NV) defects in diamonds. These defects — in which one carbon atom in a diamond is replaced with a nitrogen atom and a neighboring atom is removed — can be used to detect minute magnetic fields.
The researchers fabricated tiny rods of diamond containing NV centers and placed them nanometers above the sample. As the spin waves move through the material, they generate a magnetic field, which is picked up by the NV center.
Based on NV-center measurements, researchers can now figure out the spin chemical potential, the number of spin waves, how they are moving through the material and other important insights.
“The nice thing about this technique is that it’s very local,” said Van der Sar. “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.”
[An illustration of atomic-size defects in diamonds being used to detect and measure magnetic fields generated by spin waves. Image: Peter and Ryan Allen/Harvard University]