As humans we have greatly benefited from our five natural senses, which have supported us in overcoming major barriers pertaining to our survival in the past from lighting our first fires to navigating vast expanses such as the oceans and deserts. However, as our needs and wants expanded we have lacked further senses such as a magnetic sense to satiate our exploratory nature.
Figure 1: Our five senses, but, no magnetic sense?[1]
Human ingenuity came to the rescue through the invention of magnetic detection tools, from early technologies such as the simple compass, to electronic magnetic sensors now widely used in everyday life. Of these electronic magnetic sensors the Hall sensor has seen an increased demand and growth, with a global market expected to grow to nearly 1.5 billion USD by 2023 according to Marketsandmarkets.[2] The Hall effect sensor finds itself in numerous applications such as in smartphones, the automotive industry, automation, non-destructive testing and medical technology.
But how does it work?
The Hall effect sensor is made up of a semiconductor material, often patterned in the form of a Greek cross - as pictured in Figure 2. This has a current passed through it between opposite leads. When positioned in a magnetic field, the charge carriers are deflected due to the magnetic force, with electrons and holes going to opposite sides of the material. The collection of these charges causes a potential difference which is directly proportional to the present magnetic field. By measuring this potential difference, one can gain a quantitative measure of the magnetic field.
Figure 2: A schematic of a common Hall probe, with IH denoting the applied current, VH denoting the measured Hall voltage/potential difference, B the applied magnetic field and w and l the width and length of the probe respectively.
At the University of Bath we have taken the application of the Hall sensor a step further by integrating the Hall sensor with a Scanning Tunnelling Tip (STM) to build a Scanning Hall Probe Microscope (SHPM), illustrated in Figure 3. SHPM can be used to create magnetic field maps of a material of interest by using a gold tip (acting as the STM tip) to approach to within a few nm of a material of interest and to maintain this separation as the Hall probe measures the varying magnetic field while it rasters over the surface. The nanometer separation, which allows quantum tunneling, is necessary so as to gain the maximum magnetic signal from the sample.
Figure 3: A schematic of the operation of an SHPM. The Piezoelectric tube (PZT) allows for the precise rastering of the Hall probe above the material under study.
SHPM currently relies upon GaAs as the Hall probe material, whose figure-of-merit deteriorate significantly above cryogenic temperatures. This has excluded SHPM from room temperature applications such as routine non-destructive testing of additive manufactured artefacts, and characterisation of high-capacity magnetic data media. Beyond industry, the technique can also be used to optimise high temperature superconducting cables, which can deliver electricity at significantly higher efficiencies than tapes used in the electricity grid. Here the amount and distribution of impurities inside the cables still needs to be optimised to maximise the amount of current they can carry. By mapping these tapes, valuable information can be fed back to manufacturers enabling them to improve their production process.
Figure 4: Magnetic field penetrations into a superconductor imaged by an SHPM at the University of Bath.[3]
A Hall sensor’s performance is most often judged by it’s signal-to-noise ratio. This is a result from a combination of its magnetic sensitivity, often called the Hall coefficient, and the low frequency electronic noise. Therefore by reducing the electronic noise and increasing the Hall coefficient, one can improve the performance of a Hall sensor. A second essential figure-of-merit for magnetic imaging is the spatial resolution, which is often determined by the width of the Hall cross leads - labelled w in Figure 2. Currently this parameter has been limited to 100s of nm in GaAs based probes due to the rapid deterioration in performance as its size shrinks. To address the limitations of GaAs we have recently looked to make the most of graphene’s ultrafast electrical properties and atomically thin size to push the limits of Hall sensor performance. Combined with graphene’s chemical and physical versatility, we can achieve sub 100 nm spatial resolution by using precise nanopatterning techniques while maintaining high magnetic sensitivities and low electronic noise to achieve excellent signal-to-noise ratios.[4]
Figure 5: A scanning electron micrograph of one of our Hall probes fabricated from graphene.[4]
A further important characteristic of a Hall probe is its stability in ambient conditions. With this, we have also shown improved electronic performance and stability by encapsulating graphene with a protective chemical known as HSQ.[5]
Building on the above advances, our target now is to fabricate scanners with graphene to further improve their performance, allowing us to meet the latest needs of industry and academic researcher’s new demands. Readers can follow future and past research at the university and its collaborators in this technology area via the university’s research portal as well as from some of the references below authored by the group.
References:
[1] Original creator: Allan-Hermann Pool. Creative Commons 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en)
[2] https://www.marketsandmarkets.com/Market-Reports/magnetic-field-sensors-market-521.html
[3] A. Grigorenko, S. J. Bending, T. Tamegai, S. Ooi, M. Henini. Nature, 414, 728-731 (2001)
[4] D. Collomb, P. Li, S. J. Bending. Sci. Rep., 9(1), 1-10 (2019)
[5] P. Li, D. Collomb, S. J. Bending. Mater. Lett. 257, 126765 (2019)
