When an external magnetic field is applied to certain materials, a change in electrical resistance, also known as magnetoresistance, occurs as the electrons are deflected. The discovery of magnetoresistance paved the way for magnetic field sensors used in hard disk drives and other devices, revolutionising how data is stored and read.
In the search for an ideal magnetoresistance sensor, researchers have prized the properties of high sensitivity to low and high magnetic fields, tunability, and very small resistance variations due to temperature.
The hybrid sensor developed by the team, led by Associate Professor Yang Hyunsoo of the Department of Electrical and Computer Engineering at NUS’ Faculty of Engineering, may finally meet these requirements.
The sensor, made of graphene and boron nitride, includes layers of carrier-moving channels, each of which can be controlled by the magnetic field.The researchers characterised the sensor by testing it at various temperatures, angles of magnetic field, and with a different pairing material.
Dr Kalon Gopinadhan, of the NUS Nanoscience and Nanotechnology Institute and the Centre for Advanced 2D Materials, said: “We started by trying to understand how graphene responds under the magnetic field. We found that a bilayer structure of graphene and boron nitride displays an extremely large response with magnetic fields. This combination can be utilised for magnetic field sensing applications.”
Compared to other existing sensors, which are commonly made of silicon and indium antimonide, the group’s hybrid sensor displayed higher sensitivity to magnetic fields. In particular, when measured at 127°C, the researchers observed a gain in sensitivity of more than eight-fold over previously reported laboratory results and more than 200 times that of most commercially available sensors.
Another breakthrough in this research was the discovery that mobility of the graphene multilayers can be partially adjusted by tuning the voltage across the sensor, enabling the sensor’s characteristics to be optimised. In addition, the sensor showed very little temperature dependence over room temperature to 127°C range.
Graphene-based magnetoresistance sensors hold immense promise over existing sensors due to their stable performance over temperature variation, eliminating the necessity for expensive wafers or temperature correction circuitry. Production cost for graphene is also much lower than silicon and indium antimonide.
Potential applications for the new sensor include the automotive industry, where sensors in cars, located in devices like flow meters, position sensors and interlocks, are currently made of silicon or indium antimonide. For instance, when there is a change in temperature due to the car’s air-conditioner or heat from the sun, properties of the conventional sensors in the car change as well. To counter this, a temperature correction mechanism is required, incurring additional production cost. However, with the team’s new hybrid sensor, the need for expensive wafers to manufacture the sensors, and additional temperature correction circuitries can be eliminated.
Assoc Prof Yang said: “Our sensor is perfectly poised to pose a serious challenge in the magnetoresistance market by filling the performance gaps of existing sensors, and finding applications as thermal switches, hard drives and magnetic field sensors.”
Following this proof-of-concept study, the researchers plan to scale up their studies and manufacture industry-size wafers for industrial use.