Quantum Sensor Directly Images Magnetotransport at Graphene Interfaces with 0.5 Tesla Resolution

Magnetotransport, the interplay between magnetism and electrical conduction, governs many fundamental phenomena in materials science, yet detailed understanding of current flow at the nanoscale remains elusive. Now, C. Ding, M. L. Palm, K. Kohli, and colleagues at institutions including the University of Cambridge and the National Institute for Materials Science demonstrate a breakthrough in visualising this process. They achieve direct imaging of magnetotransport at the interface between graphene and a metal using a novel scanning single-spin quantum magnetometer, operating at room temperature. This technique reveals the Lorentz deflection of current, a key signature of magnetotransport, and provides unprecedented insight into the complex factors governing electron flow in these hybrid devices, opening new avenues for characterising and optimising nanoscale electronics.

Magnetotransport underlies many important phenomena in condensed matter physics, such as the Hall effect and magnetoresistance. Current studies typically measure bulk resistance, lacking detailed information about spatial current patterns. This research presents nanoscale imaging of magnetotransport using a scanning single-spin quantum magnetometer, successfully demonstrated in a graphene-metal hybrid device at room temperature. By visualizing current flow at elevated magnetic fields, researchers directly observe the Lorentz deflection of charge carriers in the graphene channel, revealing the intricate spatial distribution of current density at the nanoscale. This technique provides a new pathway to investigate fundamental magnetotransport phenomena and explore novel spintronic devices.

Graphene Magnetometry with Nitrogen-Vacancy Diamonds

This work focuses on using scanning nitrogen-vacancy (NV) magnetometry to study the electronic properties of two-dimensional (2D) materials, particularly graphene. NV magnetometry measures magnetic fields with nanoscale resolution by exploiting the spin state of nitrogen-vacancy defects in diamond. Researchers are investigating fundamental phenomena like electron hydrodynamics, topological effects, and ballistic transport in graphene and other 2D materials, aiming to harness these unique properties to create new and improved electronic devices. Researchers directly observed the Lorentz deflection of current near the metal-graphene interface, a key signature of magnetotransport, by applying magnetic fields of approximately 0. 5 Tesla. Combining these local current measurements with global resistance data, the study reveals that the device’s transport properties are governed by a complex interplay of intrinsic magnetoresistance, carrier hydrodynamics, interface resistance, and the device’s nanoscale geometry.

A key finding is the ability to quantitatively map spatial variations in contact resistance across the interface, a notoriously difficult measurement for devices incorporating two-dimensional materials. The team discovered significant variations in contact resistance, with some areas exhibiting high resistance and others displaying low resistance. These variations, even without visible discontinuities at the interface, play a decisive role in the device’s overall transport behavior. The team achieved excellent agreement between experimental current density maps and finite element simulations by incorporating these interface resistance values into a comprehensive conductance model.

Further investigation into magnetotransport revealed that applying a 0. 5 Tesla magnetic field induces current deflection, a direct consequence of the Lorentz force. Detailed analysis of the current deflection near the injection contact demonstrates a spatial measurement of carrier mobility. Measurements show that approximately 40% of the current flows through the graphene ring, a value confirmed through simulations incorporating both diffusive and hydrodynamic regimes. By visualizing current flow at elevated magnetic fields, the team directly observed Lorentz deflection of current near the interface between materials, a key characteristic of magnetotransport. Combining this local current mapping with global resistance measurements, the study reveals that the device’s transport properties arise from a complex interplay of intrinsic magnetoresistance, carrier hydrodynamics, interface resistance, and the device’s specific geometry. Furthermore, this technique allows for quantitative mapping of spatial variations in contact resistance, a crucial parameter in electronic devices constructed from two-dimensional materials that is often difficult to characterize using conventional methods. This achievement establishes the potential of nanoscale current imaging as a powerful tool for investigating electronic transport in ways that are inaccessible to traditional resistance-based measurements.