Flakes of hexagonal boron nitride extracted with Scotch tape. Photo: Sasaki et al., 2023.
Flakes of hexagonal boron nitride extracted with Scotch tape. Photo: Sasaki et al., 2023.

Researchers at the University of Tokyo in Japan have achieved the delicate task of arranging quantum sensors at the nanoscale, allowing them to detect extremely small variations in magnetic fields. These high-resolution quantum sensors have potential uses in quantum materials and next-generation electronic devices. For example, the sensors could help develop hard disks that use nanomagnetic materials as storage elements. This study, reported in a paper in Applied Physics Letters, represents the first successful high-resolution imaging of a magnetic field using a nanoscale arrangement of quantum sensors.

Quantum sensors sense the environment around them using the properties of an atom. For example, a magnetic field can alter the spin of an atom, which can adopt two values, like the poles of a magnet. Magnetic field sensors based on this effect have many applications in biomedical and quantum materials research.

“Using such an unprecedented sensor, we want to observe a microscopic world that no one has ever seen,” said Kento Sasaki, an assistant professor at the University of Tokyo.

Sasaki and his team wanted to develop stable quantum sensors that could be placed near targets such as wires and disks. But until now, it has proven challenging to precisely arrange atoms so they can sense minute variations in a magnetic field.

“Although individual quantum sensors are small, their spatial resolution is restricted by the distance between the sensor and the measurement target,” explains Sasaki. To solve this problem, the researchers established a technique for creating nano-sized quantum sensors directly on the surface of the measurement target.

For their quantum sensors, the team used boron vacancies or lattice defects in the two-dimensional material known as hexagonal boron nitride. The boron vacancy defect is the new kid on the block, following its discovery as a quantum spin sensor in 2020.

The researchers used Scotch tape to pull individual layers from a crystal of hexagonal boron nitride and attached the layers to a gold wire. Then they bombarded them with a high-speed helium ion beam, which caused boron atoms to pop out of the layers, forming boron vacancy spots with a size of 100nm2.

Each spot contained many atom-sized vacancies that each behave like tiny magnetic needles. The closer the spots are to each other, the better the spatial resolution of the sensors. As current flowed through the wire, the team measured the magnetic field at each spot based on the intensity of light emitted from the spots in the presence of microwaves. The researchers were amazed when the measured values of the magnetic field matched closely with simulated values, proving the efficacy of these high-resolution quantum sensors.

The change in the spin state of the sensor in the presence of a magnetic field could be identified even at room temperature, thus allowing easy detection of the local magnetic fields and currents. Moreover, the boron nitride layers attach to objects by weak van der Waals forces, which means the quantum sensors can easily stick to different materials.

Sasaki and his team plan to use this technique for research into condensed matter physics and quantum materials. “It will enable direct detection of the magnetic field from, for example, peculiar states at edges of graphene and microscopic quantum dots,” says Sasaki.

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.