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Paragraf and Oxford Instruments Test Graphene-based Sensors Under Cryogenic Temperatures

May 18, 2021 by Nicholas St. John

Paragraf, a spin-out company from the Department of Materials Science and Metallurgy of Cambridge University, has released a new Hall Effect sensor that, proven by an innovative test chamber created by Oxford Instruments, can operate in extremely low temperatures.

Hall Effect sensors are devices that can detect and measure external magnetic fields, and can have a myriad of applications in modern electronics, from non-invasive current measurement to proximity monitoring applications for products like automatic flushing toilets. They can be found in applications that are in more extreme environments, such as space travel and quantum computing. This is where some Hall Effect sensor technology tends to today fail – either giving inaccurate magnetic field measurements, producing too much heat to remain in the very low cryogenic environments they are within, or just simply breaking in such cold environments. Paragraf is looking to change that. 

A Magnet operating at cryogenic temperatures. Image used courtesy of Paragraf

A Magnet operating at cryogenic temperatures. Image used courtesy of Paragraf


According to Paragraf, the new GHS09CC sensor can measure fields very accurately while being exposed to 14T magnetic fields in temperatures belows 100mK. This kind of performance is unheard of, as Hall Effect sensor operation in the mK range alone was not possible before now. 

The device can also operate in high radiation environments as well. Normally, the low temperatures would cause the Quantum Hall Effect (QHE) would cripple any device, but the exceptionally pure and clean graphene that makes Paragraf’s sensor improves the range of the device before QHE kicks in, confirmed via Paragraf’s test results.

Prior to the Paragraf breakthrough, high energy physics labs, one of the main users of Hall Effect sensors in such harsh conditions, would be limited to devices rated at temperatures only as low as 1.5K, unless they could afford very expensive custom NMR probes. Still, the NMR probes would generate a lot of heat during operation, causing the environment that the probes are in to deviate from its minimum temperature. 

The GHS09CC sensor fixes both of these issues thanks to its ability to operate effectively down to 100mK while also dissipating 6 times less heat compared to NMR probes thanks to its graphene topology.

This device can do much more than just operate at cryogenic temperatures. The device is also rated for operation at up to 480K operation, while having parts per billion resolution over a 9T dynamic measuring range while exhibiting no planar Hall effect according to a press briefing

The planar Hall Effect occurs when the device responds to magnetic fields within the same plane as the sensor, rather than to ones orthogonally to it. Ideally, you want these sensors to measure one and only one direction in order to ensure you are measuring the field that you want, rather than measuring outside noise, and Paragraf ensures that by eliminating this effect. Furthermore, results of testing via a superconducting solenoid magnet and Oxford Instruement’s newly released Proteox dilution refrigerator, allowing 100mK temperatures, show the extreme efficiency of these sensors, only requiring picowatts of power, while producing measurement results that exceptionally linear, correctible to 0.01% with a third order polynomial fitting over the dynamic measuring range at room temperature. 

The Oxford Instruments ProtexMX dilution refrigerator. Image used courtesy of Oxford Instruments


Over its wide temperature range, it shows a very low maximum temperature coefficient of -0.8%/K at cryogenic temperatures. These parameters were tested multiple times and found to be very repeatable, showing a high reliability in the operation of these sensors. Lastly, the device is precise, giving an estimated 10 ppb resolution for measuring a 1T field at 1.8K, which is performance in that environment that is unmatched by all competitors.

The utilization of graphene has truly set Paragraf far apart from its competition. Setting the standard for sensors to come, they have created a very high bar to meet. While not only creating devices that can handle such harsh conditions, they excel in them, hitting marks in performance and efficiency that, in some cases, have yet to be reached in ideal conditions. This kind of graphene technology will push the technologies of today into tomorrow, and the Hall Effect sensor is just the beginning of that.