A new type of magnetic-field sensor that is tough, easy to make and works with high precision has been developed by physicists in Australia, Germany and the US. The device can detect fields of 1–340 mT and, although it is not the most sensitive sensor in this range, a big plus is that the device does not need to be cooled to ultracold temperatures nor be recalibrated.
The device is the brainchild of a team led by Christoph Boehme of the University of Utah, along with colleagues from the universities of Sydney and Regensburg. At its heart is a thin layer of organic semiconductor sandwiched between two metal contacts. When a voltage is applied across the contacts, electrons and holes are injected into the semiconductor. Being charged particles, the holes and electrons each induce a small local electric polarization of the semiconductor that follows the particles as they pass through the material.
This combination of particle and polarization is called a “polaron”; when a hole polaron and an electron polaron interact, they can decay to create an “exciton” – a bound state of an electron and a hole. It turns out that the resistance of the semiconductor is a function of the rate at which the hole and electron polarons interact, which in turn depends on the relative orientation of the electron and hole spins.
The clue to how the device operates lies in the fact that applying an external magnetic field creates an energy gap between spins pointing parallel and antiparallel to the field. So, if a radio-frequency (RF) signal at this precise energy is applied to the film, the spins are flipped up and down, causing a sharp change in the resistance of the semiconductor. As this resonant energy is a linear function of the magnetic-field strength, the field can therefore be measured by changing the RF energy until resonance is achieved.
Slow measurement
One drawback of the device associated with the need to scan radio frequencies is that it can take a few seconds to measure a magnetic field. Boehme told physicsworld.com that this should not, however, be a problem for applications where the user already has an approximate idea of the field strength – sensing Earth’s magnetic field, for example. But when speed is of the essence, the team has thought of two ways that the device could be improved.
One would be to apply RF pulses that incorporate the full range of frequencies, and the other would be to combine the technology with an organic magnetic-resistance sensor – which operates at high speeds but needs to be recalibrated regularly. This recalibration could be done using the new sensor, as it itself does not need to be recalibrated given that its output is related to an intrinsic property of the electron – the gyromagnetic ratio.
While the device operates in the 1–340 mT range, Boehme says that the upper limit could be extended by simply using a higher-energy RF signal. For extremely high fields, however, a terahertz signal would be needed and these are difficult to generate. As for extending the range below 1 mT, this could be done by applying a small offset field of about 1 mT , claims Boehme. The ultimate sensitivity of the device is in the low nanotesla range – a limit that is related to the random magnetic fields produced by the hydrogen nuclei in the organic semiconductor.
Best of both worlds
J T Janssen of the UK’s National Physical Laboratory says that the new device fits nicely between the much more-accurate superconducting interference device (SQUID) – which requires liquid helium to operate – and the less-accurate Hall sensor, which is robust and only costs a few pence to make. “This new organic sensor seems to have the best of both worlds: it’s reasonably sensitive, it’s accurate and it’s cheap,” says Janssen.
He adds that the device is particularly suited to “fit-and-forget” applications, in which it is not possible to service or calibrate the sensor once installed. Examples include sensors in nuclear power plants, wind turbines and satellites.
The sensor is described in Nature Communications.
