Magnetic nanosensors for embedded temperature measurements

Magnetic nanosensors for embedded temperature measurements

Technology News |
The Thermal Magnetic Imaging and Control (Thermal MagIC) project at NIST in the US is developing nanoscale temperature sensors
By Nick Flaherty


Researchers at the National Institute of Standards and Technology (NIST) in Maryland are developing a nanoscale array of ultra-sensitive temperature sensors that can be embedded in all kinds of materials.

The system will be the first to make real-time measurements of temperature on the microscopic scale in an opaque 3D volume, which could include medical implants, refrigerators, electronics and potentially the human body.

The Thermal Magnetic Imaging and Control (Thermal MagIC) project could revolutionize temperature measurements in many fields: biology, medicine, chemical synthesis, refrigeration, the automotive industry, plastic production, says the team.  “Pretty much anywhere temperature plays a critical role,” said NIST physicist Cindi Dennis. “And that’s everywhere.”

Thermal MagIC will work by using nanometer-sized objects whose magnetic signals change with temperature. These sensors would be incorporated into the liquids or solids being studied, for example in melted plastic that might be used as part of an artificial joint replacement, or the liquid coolant being recirculated through a refrigerator. A remote sensing system would then pick up these magnetic signals, meaning the system being studied would be free from wires or other bulky external objects.

The final product could make temperature measurements that are 10 times more precise than state-of-the-art techniques, accurate to within 25 millikelvin in 100ms. The measurements would be traceable to the International System of Units (SI); in other words, its readings could be accurately related to the fundamental definition of the kelvin, the world’s basic unit of temperature.

The system aims to measure temperatures over the range from 200 to 400 kelvin (K), -73 to 126C. There is potential for a much larger temperature range, stretching from 4K to 600K, but that is not a part of current development plans.

“This is a big enough sea change that we expect that if we can develop it — and we have confidence that we can — other people will take it and really run with it and do things that we currently can’t imagine,” Dennis said.

The first step is creating the nanoscale magnets that will give off strong magnetic signals in response to temperature changes. These need to be 10 times more sensitive to temperature changes than any objects that currently exist and so will use multiple magnetic materials.

Next: Modelling the nanoparticle materials

To find the right combination of materials and structure, the team is using modelling software developed at NIST. The Object Oriented MicroMagnetic Framework (OOMMF) will be used to identify new materials to synthesize, which will then the characterised. The results will be fed back into OOMMF to make predictions about what combinations of materials they should try next.

“We have some very promising results from the magnetic nano-objects side of things, but we’re not quite there yet,” said Dennis.

The group has already found and tested a promising nanoparticle material made of iron and cobalt, with temperature sensitivities that varied in a controllable way depending on how the team prepared the material. Adding an appropriate shell material to encase this nanoparticle core would bring the team closer to creating a working temperature-sensitive nanoparticle for Thermal MagIC.

The signals from the magnets will be picked by a variant of a magnetic particle imager (MPI), which surrounds the sample and measures a magnetic signal coming off the nanoparticles.

MPI systems similar to this exist but are not sensitive enough to measure the kind of tiny magnetic signal that would come from a small change in temperature. The challenge for the NIST team is to boost the signal significantly.

“Our instrumentation is very similar to MPI, but since we have to measure temperature, not just measure the presence of a nano-object, we essentially need to boost our signal-to-noise ratio over MPI by a thousand or 10,000 times,” said NIST physicist Solomon Woods.

They plan to boost the signal using technologies such as superconducting quantum interference devices (SQUIDs), cryogenic sensors that measure extremely subtle changes in magnetic fields, or atomic magnetometers, which detect how energy levels of atoms are changed by an external magnetic field.

The final part of the project is making sure the measurements are traceable to the SI, a project led by NIST physicist Wes Tew. That will involve measuring the nano-thermometers’ magnetic signals at different temperatures that are simultaneously being measured by standard instruments.

“Despite the challenge of working during the pandemic, we have had some successes in our new labs,” Woods said. “These achievements include our first syntheses of multi-layer nanomagnetic systems for thermometry, and ultra-stable magnetic temperature measurements using techniques borrowed from atomic clock research.”

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