Quantum sensor illuminates novel transistor design
AI faces an energy crisis, stemming from processors continually shuffling data between separate computing and memory nodes (the 'von Neumann bottleneck').
One solution is spintronics, using an electron’s 'spin' (or its intrinsic magnetic orientation) to improve efficiency. Such devices would be based around 'spin' transistors that combine magnetic bits with a semiconducting switch, allowing them to compute and store data simultaneously.
“The major challenge is understanding how magnetism and electrical current interact in nanoscale devices,” said Boston College professor of Physics Brian Zhou, whose group has now demonstrated an architecture using the magnetic compound semiconductor, CrSBr, that allows signals to be switched either by magnetism or voltage.
In a new study published in Physical Review Letters, the team describes using a specially developed single-spin quantum microscope to observe magnetic states inside the atomically thin devices as they actively process electrical information.
“Traditionally, these devices require joining two different materials together: a magnet and a semiconductor,” said Zdeněk Sofer, a materials synthesis expert at the University of Chemistry and Technology, Prague. “By engineering a single van der Waals crystal, CrSBr, that inherently possesses both semiconducting and magnetic properties, we eliminate losses at interfaces entirely.”
Zhou’s research group fabricated the transistor using two-layer-thick CrSBr, placing laterally separated electrodes on opposite layers. Their unique construction forces current to travel both across and between the two magnetic layers. The device can be switched 'on' and 'off' either by changing the voltage on a nearby gate electrode or by changing the relative magnetic orientations of the two CrSBr layers, similar to a traditional CMOS transistor, but with an added magnetic twist.
To characterise the spin transistor, the researchers combined electrical measurements with a high-resolution quantum sensing probe, scanning nitrogen-vacancy (NV) centre magnetometry. This specialised imaging technique maps the local magnetic field by tracking variations in the magnetic resonance of a single atomic defect. The quantum microscope vividly revealed the correlated magnetic and electrical behavior of the device: how spatial changes in the magnetisation modify the device conductance, and how gate voltage flips the magnetic layers between parallel and antiparallel states.
A key factor that enhanced the performance of the developed spin transistor was the researchers’ ability to access 'space-charge-limited' conduction. Within this regime, the internal build-up and mutual repulsion of charges in the material alter the current-voltage relationship, causing it to display a power law scaling instead of the conventional linear, ohmic behavior.
“The rapid power law scaling allows us to dramatically tune the conductivity,” said graduate student Thomas K. M. Graham, the lead author of the study. “Our device achieves an electrical on/off ratio of a million percent and a magnetic on/off ratio of 3000 percent, the latter significantly higher than previous efforts.”
By merging switching logic with a nonvolatile memory bit, the developed architecture paves the way for ultra-efficient, “instant-on” processors that do not need to fetch data from memory, alongside reconfigurable computing circuits that can be reprogrammed after manufacturing.
To realise this potential, Zhou says researchers must continue to advance nanoscale imaging techniques and the electrical control of magnetic states.
Pictured above: The interior of a spin transistor during switching is captured by a scanning quantum sensor developed by Boston College researchers.
