A Magnetic Compass the Size of a Dust Speck express
Magnetic sensors smaller than a match head can already detect weak fields — from a beating heart or a distant planet. But making them even more sensitive involves a trade-off: improve one part, and you lose performance in another. Inside the sensor are tiny tunnel junctions (bridges through which current flows) and magnetic flux concentrators — metal plates that gather the field like a funnel gathers water.
The problem is that the more junctions you add, the wider the gap between the plates, and the funnel gathers less field. The authors figured out how to calculate this trade-off in advance. They simulated the sensor's behavior on a computer and derived a simple formula linking component dimensions to sensitivity. The formula is based on magnetic reluctance — the magnetic analog of electrical resistance.
Now engineers can quickly tailor sensor parameters to a specific task without manually testing hundreds of designs. Such ultra-sensitive, compact magnetometers are needed for space probes measuring planetary magnetic fields and for wearable devices tracking heart or brain activity.
Original abstract
as written by the authors arXiv:2604.21500
Miniaturized, ultra-sensitive and easily integrable magnetometers are needed for many applications, like space exploration or health monitoring. Achieving this goal requires a magnetic sensor with high sensitivity and low noise. High sensitivity (>1000 %/mT) can be obtained by integrating high gain permalloy flux concentrators (FC). And reducing the magnetic 1/f noise can be realized by increasing the number of magnetic tunnel junctions (MTJs) in the air-gap of the FC. However, this is obtained at the expense of a wider air-gap and consequently a decrease of the magnetic gain and thus of the sensitivity. In this paper, we explore a design optimization scheme in order to find the best trade-off between high FC gain and low magnetic noise. To model the gain of the flux concentrator, we propose two complementary approaches; one is based on finite elements simulations of the FC gain where the influence of geometrical parameters of the air-gap is investigated. Then, in a second step, we propose an analytical formula consistent with all our simulations results and based on magnetic reluctance. Finally, we derive an analytical model of the sensor detectivity from which we can extract the optimal sensor design which allows an improvement by three orders of magnitude of the performances compared to a single junction.
Earth's magnetic field is about a hundred times weaker than a refrigerator magnet, yet the new sensor can detect even its faintest fluctuations.