Frontiers of graphene-based Hall-effect sensors.

Hall sensors have become one of the most used magnetic sensors in recent decades, performing the vital function of providing a magnetic sense that is naturally absent in humans. Various electronic applications have evolved from circuit-integrated Hall sensors due to their low cost, simple linear magnetic field response, ability to operate in a large magnetic field range, high magnetic sensitivity and low electronic noise, in addition to many other advantages. Recent developments in the fabrication and performance of graphene Hall devices promise to open up the realm of Hall sensor applications by not only widening the horizon of current uses through performance improvements, but also driving Hall sensor electronics into entirely new areas. In this review paper we describe the evolution from the traditional selection of Hall device materials to graphene Hall devices, and explore the various applications enabled by them. This includes a summary of the selection of materials and architectures for contemporary micro-to nanoscale Hall sensors. We then turn our attention to introducing graphene and its remarkable physical properties and explore how this impacts the magnetic sensitivity and electronic noise of graphene-based Hall sensors. We summarise the current state-of-the art of research into graphene Hall probes, demonstrating their record-breaking performance. Building on this, we explore the various new application areas graphene Hall sensors are pioneering such as magnetic imaging and non-destructive testing. Finally, we look at recent encouraging results showing that graphene Hall sensors have plenty of room to improve, before then discussing future prospects for industry-level scalable fabrication.

Imaging of Strong Nanoscale Vortex Pinning in GdBaCuO High-Temperature Superconducting Tapes.

The high critical current density of second-generation high-temperature superconducting (2G-HTS) tapes is the result of the systematic optimisation of the pinning landscape for superconducting vortices through careful engineering of the size and density of defects and non-superconducting second phases. Here, we use scanning Hall probe microscopy to conduct a vortex-resolved study of commercial GdBaCuO tapes in low fields for the first time and complement this work with "local" magnetisation and transport measurements. Magnetic imaging reveals highly disordered vortex patterns reflecting the presence of strong pinning from a dense distribution of nanoscale Gd2O3 second-phase inclusions in the superconducting film. However, we find that the measured vortex profiles are unexpectedly broad, with full-width-half-maxima typically of 6 µm, and exhibit almost no temperature dependence in the range 10-85 K. Since the lateral displacements of pinned vortex cores are not expected to exceed the superconducting layer thickness, this suggests that the observed broadening is caused by the disruption of the circulating supercurrents due to the high density of nanoscale pinning sites. Deviations of our local magnetisation data from an accepted 2D Bean critical state model also indicate that critical state profiles relax quite rapidly by flux creep. Our measurements provide important information about the role second-phase defects play in enhancing the critical current in these tapes and demonstrate the power of magnetic imaging as a complementary tool in the optimisation of vortex pinning phenomena in 2G-HTS tapes.

Nanoscale graphene Hall sensors for high-resolution ambient magnetic imaging.

A major challenge to routine non-invasive, nanoscale magnetic imaging is the development of Hall sensors that are stable under ambient conditions and retain low minimum detectable fields down to nanoscale dimensions. To address these issues we have fabricated and characterised chemical vapour deposition (CVD) graphene Hall sensors with wire widths between 50 nm and 1500 nm, in order to exploit the high carrier mobility and tuneability of this material. The measured Hall voltage noise is in good agreement with theoretical models and we demonstrate that minimum detectable fields at fixed drive current are lowest in the vicinity of the charge neutrality point. Our best performing deep sub-micron sensors, based on a wire width of 85 nm, display the excellent room temperature resolution of 59 µT/√Hz at a dc drive current of 12 µA and measurement frequency of 531 Hz. We observe a weak increase in minimum detectable field as the active sensor area is reduced while the Hall offset field is largely independent of size. These figures-of-merit significantly surpass prior results on larger probes in competing materials systems, with considerable scope for further optimisation. Our results clearly demonstrate the feasibility of using CVD graphene to realise very high spatial resolution nanosensors for quantitative room temperature magnetic imaging.