Wednesday, 17 February 2016

Iron nano-particle magnetic sensor – a new device to measure magnetic fields

Magnetic field measurements are found in many applications around us. This ranges from the compass system in our smartphones, vehicle detection for traffic lights, metal detection at airport security to non-contact measurements in power supplies. While many existing technologies are available, our industrial partners have identified gaps where existing technologies are insufficient in their current form. For instance, there is a need for sensors that can measure low magnetic fields reliably even after exposure to large magnetic fields. This means overcoming the magnetic remanence naturally occurring in most magnetic materials used in sensors. Another challenge that was presented to us was to be able to measure both low and large magnetic fields with a single compact system. Our approach, considering the use of nanostructured materials, is aimed at solving these challenges and provide our partners with useable and more efficient alternatives to existing solutions.

Jérôme Leveneur, John Futter and John Kennedy 
(from left) setting up the ion implanter.
Dr John Kennedy and his team of scientists at GNS Science developed a new process using ion implantation and thermal processing in order to create a new material in which electrical resistance varies with a magnetic field. Currently magnetometer devices measuring large magnetic fields are based on the Hall effect. However, they exhibit a large temperature dependency which is not favorable for easy measurements. The materials developed by the team use a different mechanism which is more resilient to temperature changes. It relies on having iron nanoparticles, with dimensions of the order of a thousand times smaller than the diameter of an average human hair, on the surface of an insulator 1.

Ion implantation is a technique used to modify the composition and properties of the  surface of materials. It uses a particle accelerator which first strips electrons from some atoms to create ions which are then accelerated and funneled towards the sample to modify it. The bombarding of ions through this method progressively modifies the material’s surface. This technique is commonly used in microelectronics to precisely tune the properties of electronic components on a chip (electronic circuit on a small plate of silicon). In fact, most consumer electronics have now had components treated with such a technique!

Schematics of magnetic sensor
Sensor are made by implanting iron atoms into the first 30 nanometres of a 400 nanometre thick substrate of SiO2 [Silicon dioxide], (1 nanometre = 0.000000001m). The ion implanted target is then heat treated for one hour at 1000 °C to form iron nano-particles.

Uniformly formed iron nano-particles measured by AFM.

Uniformity of the produced iron nano-particle is determined by measuring the surface with an Atomic Force Microscope (AFM). The AFM moves over the sample surface line-by-line and the up and down movements get recorded and stitched together to give a 3D topography image of the surface at very small scale.

1  United States Patent number: US 8,872,615 B2

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