An international research team, led by the University of Tsukuba, has used electron spin resonance (ESR) to monitor the number and location of unpaired spins going through a molybdenum disulfide transistor. ESR uses the same physical principle as the MRI machines that create medical images. The spins are subject to a very strong magnetic field, which creates an energy difference between electrons with spins aligned and anti-aligned with the field. The absorbance of photons that match this energy gap can be measured to determine the presence of unpaired electron spins.
The experiment required the sample to be cooled to just four degrees above absolute zero, and the transistor to be in operation while the spins are being measured. "The ESR signals were measured simultaneously with the drain and gate currents," corresponding author Professor Kazuhiro Marumoto says. "Theoretical calculations further identified the origins of the spins," coauthor Professor Małgorzata Wierzbowska says. Molybdenum disulfide was used because its atoms naturally form a nearly flat two-dimensional structure. The molybdenum atoms form a plane with a layer of sulfide ions above and below.
The team found that charging the system with the additional electrons in a process called n-type doping was important for creating the spins. "In contrast with previous work on other 2D materials, the n-type doping allowed us to achieve better control of the electronic spins," Professors Marumoto and Wierzbowska explain. The scientists believe that molybdenum disulfide will prove to be an important testbed for spintronic devices as the technology advances towards future consumer products.