Graphene Nanosensors for Scanning Hall Microscopy and Susceptometry

Many of the most important advances in science and technology have only been made possible by parallel developments in instrumentation. For example, the development of microchips, which are now so common in our everyday lives, could not have taken place without the availability of electron microscopy to image cross-sections through prototype devices. Scanning Hall microscopy is a so-called "scanning probe" imaging technique where a tiny sensor is rastered across the surface of a sample to create a map of the magnetic fields. In this case the sensors rely on the Hall effect which arises when the electron flow in a conducting sample is bent by a magnetic field creating a Hall voltage at right angles to the main current direction. At present scanning Hall microscopy is a relatively niche technique that is mainly confined to making measurements of magnetic materials at low temperatures (typically less than -170C). This is due to the fact that although existing Hall effect sensors have high sensitivity at low temperatures, this becomes very much worse at room temperature when other scanning probe imaging methods, for example magnetic force microscopy, are preferred. Recent developments in graphene technology mean that this situation is about to change. Graphene is a single atomic layer of carbon that was first isolated by scientists in Manchester in 2004, leading to the award of the physics Nobel Prize in 2010. It is remarkable for its very high conductivity and mechanical strength, and the electrical carriers in graphene are able to move very much more freely than electrons in copper. Recently scientists have shown that still higher conductivities can be obtained if the graphene is sandwiched between thin layers of an insulator called boron nitride. In this way an improvement in Hall sensor performance of more than a hundred times is possible at room temperature, rivalling the other available magnetic imaging techniques. We also plan to develop new "susceptibility" imaging modes when the Hall probe measures the response of a sample to a small oscillating magnetic field generated by a tiny coil integrated into the sensor. This will allow new types of samples to be studied, and different types of problems can be addressed. Our new sensors target applications in three important technological areas. We will use Hall microscopy to map the nanoscale current distribution in second generation high temperature superconducting tapes that have enormous potential for applications in lossless power transmission and energy storage. Hall susceptometry will be used for the non-invasive detection of defects in "3D printed" materials (for example steel) which are known to play a critical role in structural failure. Finally we will explore how Hall susceptometry can be used for routine process control of the uniformity of the magnetic properties of thin film ferromagnetic materials for applications in data storage.