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Novel Sensor that Can Measure Small Magnetic Fields

Researchers of Aalto University and Scuola Normale Superiore develop a novel sensor to measure small magnetic fields.

The results of the research conducted at the Low Temperature Laboratory of the Aalto University School of Science and Technology and Scuola Normal Superiore were published in the Nature Physics online edition on 28th February.

The advantages of this novel and very sensitive magnetic field sensor developed by the researchers are small power dissipation, simple measurement and modifiability for different measurement ranges.

The new measuring transducer is closely related to the conventional SQUID (Superconducting Quantum Interference Device) magnetometer used, for example, in brain imaging. The core of the sensor is a superconducting loop, a part of which has been replaced with a normal metal conductor. Due to a so-called proximity phenomenon, normal metal becomes weakly superconducting when in contact with a superconductor.

– The original idea was to investigate the effect of the superconducting proximity phenomenon on the heat flow between electrons and lattice vibrations of normal metal. However, it was discovered that the structure could also function as a sensitive and tunable magnetic field sensor, Professor Jukka Pekola explains.

The structure referred to as SQUIPT (Superconducting Quantum Interference Proximity Transistor) combines the well-known electric transport properties of tunnel junctions and the adjustability made possible by the superconducting proximity effect. For the core loop, different length metal wires and materials can be used, which extends the operational range of the device.

Because the measurements are made using very weak direct current, the power consumption of the detecting device is small. The new sensor structure is well suited for low temperatures, where the noise level is lower and the device's performance and sensitivity are good.

The theoretical analysis and the first experiments conducted with the device seem promising.

– The sensor's distinguishing mark is its simplicity. A direct current input is fed to the device, the loop is placed in an external magnetic field, and the voltage is then monitored. The voltage periodically alternates as a function of the external magnetic field. With a direct voltage measurement we can measure even very small magnetic fields, Pekola clarifies.

The experiments so far have been mainly conducted at about one tenth of a degree above the absolute zero point, but the device also works in at a higher temperature of liquid helium (about 4 Kelvin or -269 °C), if the superconducting material chosen is suitable. This is important for practical applications.

The next task that the group is facing is optimization of parameters, which will clarify the measurement sensitivity that the device can achieve and will help to determine how small the fields it can measure are.

At this stage, Pekola's estimations about future practical applications for the sensor are cautious.

– The road from a prototype to a real product is long and can take several years. None the less, we have had preliminary discussions with VTT about the future development and production of the sensor. In the best of cases, this could replace the conventional SQUID magnetometer in some special applications, says Pekola.

The sensor structure was made at Micronova Centre for Micro and Nanotechnology, which is jointly run by the Aalto University School of Science and Technology and VTT.

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