Due to limitations in size and power dissipation, many prevalent measurement methods are incompatible with low temperature applications. Hall effect sensors have emerged as the superior option because of their compact size and low-power requirement. Until now, conventional Hall sensors have themselves been limited by material capabilities and the quantum Hall effect (QHE). Paragraf®’s Cryogenic Graphene Hall Sensors (GHS-C), however, enhance the ability of companies that make and/or use superconducting magnets to conduct continuous measurements in low temperature and high-field environments. This means ending the reliance on factory calibration or current-to-field measurements to evaluate these systems. With its patented graphene deposition process, Paragraf averts limitations by producing the GHS-C, which achieves operation in temperatures down to mK and field measurements of over 30 T. 

Graphene and Robustness

Conventional Hall effect sensors can experience thermal expansion and contraction of their components when moving between high and low temperatures, leading to degradation of materials and failure of the sensing function. In contrast, owing to its two-dimensional structure, graphene is an exceptionally flexible and robust material, capable of withstanding extreme temperatures and temperature shocks. 

Paragraf’s GHS-C makes use of that flexibility, while enhancing the overall material strength of the sensor. Using a patented technique, they incorporate the graphene layer into the sensor in a structurally sound and contamination-free manner. This produces a sensor with a superior, highly durable construction. 

The QHE

The QHE involves a loss of linearity for voltage outputs above a particular field strength in low temperatures. Once sensors encounter fields above that threshold, the corresponding readings increase in plateaus and sharp inclines, rather than in a continuous, proportional trend line. This makes the device unusable as a cryogenic magnetic field sensor. Unique to Paragraf’s graphene deposition is a technique that enables them to modify the sensitivity of the sensors. Through experimenting with the construction process of the GHS-C, Paragraf has found it can delay the onset of QHE so that it is producing sensors that maintain their linearity up to ~30 T, as demonstrated at The High Field Magnet Laboratory in Nijmegen, The Netherlands. 

Ease of Use

The Paragraf GHS-C’s ability to operate at low temperatures means that it does not require the use of additional inserts to protect it. This allows for enhanced flexibility in locating a device within the sample loader. The very low-power dissipation (a few nanowatts) and the development of the line of smaller sensors means that the device can be placed closer to the field of interest while having the accuracy of its measurements improved.

GHS-C Benefits

Paragraf’s GHS-C offers a number of advantages over conventional Hall sensors in cryogenic environments. The small form factor eases installation, and the low current requirement prevents interference with cryogenic operations. The 2D nature of graphene provides very high-resolution and eliminates the planar Hall effect to which conventional sensors are prone. The sensors are able to operate at temperatures down to millikelvin temperatures – with no room temperature inserts – while maintaining their linearity of measurement even in fields of 30T and higher. 

Applications

Paragraf GHS-C sensors can be employed across a range of cryogenic applications. They are capable of monitoring the operations of quantum computers that require extremely low temperatures. Similarly, they can be used to map superconducting magnets and circuits, to accurately calibrate magnets, to monitor magnet drift, and to identify flux pinning. The sensors can be built into superconducting magnet coils (in active or persistent mode) or onto sample stages designed for cryogenic use. Further, the same sensor can be employed on both the inside and outside of a shield to characterize the shielding materials and the superconducting magnet. Rosie Baines, a Paragraf scientist, recently gave a presentation to the Graphene Council explaining more about the value of Paragraf sensors in extreme environments. (To view her presentation, go to youtu.be/fSqs-SWL0Kg.) 

Paragraf harnesses graphene to create the next generation of sensors. It is the first company to mass produce graphene-based Hall effect sensors that measure magnetic fields using less power and achieving greater sensitivity than conventional Hall sensors. Paragraf’s sensors have unique advantages for a spectrum of applications that include industrial automation and manufacturing, automotive and transportation, consumer electronics, research, computing and healthcare applications. www.paragraf.com

Image: Paragraf GHS mounted on an Oxford Instruments‘ Proteox development fridge. Credit: Paragraf