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Spin-transfer tunnel junctions to miniaturize magnetic sensors

​​​​​​​​​A new type of magnetic sensor, patented by IRIG in 2020, is based on spin-transfer tunnel junctions. It operates over a wide magnetic field measurement range and enables signal detection independent of the measurement range. These are significant improvements over conventional magnetoresistive sensors.​

Published on 22 June 2023

​The principle of this electronic devices is of particular interest to the consumer market, the automotive industry and industry in general. Field measurement is based on the detection of periodic transitions between two resistor states, either by direct time measurement or pulse-width modulation conversion. It is an alternative to conventional hall-effect or magnetoresistive magnetic sensors. The magnetic sensor is based on a tunnel junction using the spin transfer effect, an elementary component developed for a non-volatile memory, but ingeniously put to good use here in another context. Its nanometric dimension is several thousand times smaller than that of other hall-effect or magnetoresistance sensors. In a first iteration, the sensor is already showing comparable performance: a detection range of 80 mT, a frequency bandwidth of 30 kHz, and a very low noise level. All the electronic components used are standard to facilitate integration into an integrated circuit.

A disadvantage of conventional magnetic sensors is that their operation involves a compromise between signal amplitude and the strength of the magnetic field to be measured, so high-sensitivity sensors have a limited measurement range. As a reminder, sensor sensitivity depends on the variation in resistance per unit of magnetic field; and sensor range is the resistance range where the magnetic field measurement remains linear.
Researchers at IRIG [collaboration] have developed a sensor based on a magnetic tunnel junction with spin transfer torque. Thanks to its physical principle, detection is no longer limited by the accuracy of resistance measurement. Resistance changes between the junction's high and low values are measured by applying a periodic sinusoidal or triangular voltage. The voltage at which the junction changes state therefore varies linearly with the applied field.

Two modes of magnetic field detection are possible: time conversion to find switching voltages, or modulation of a signal by resistance transitions. The modulation mode results in lower noise and better measurement resolution, thanks to more powerful detection electronics. The main advantages of the proposed sensor are its small size, hence low power consumption, and reduced packaging electronics footprint. The detection range can be wider than that of other magnetic sensors, as detectivity is no longer limited by field range. What's more, this sensor is insensitive to the intense fields that cause irreversible damage to magnetoresistive sensors. The design of this sensor is similar to STT MRAM magnetic tunnel junction memory cells, whose technology is already mature, enabling the sensor to be integrated into CMOS circuits. As a result, mass production can be carried out in MRAM foundries with few changes.

Sensor performance has not yet reached the level of commercially available magnetic sensors. But simple improvements to the sensing element and conditioning electronics are underway to further reduce noise levels. This type of sensor could find applications in a variety of fields, from industrial applications to the medical field, integrated on a chip, or in an array of detectors to produce 2D images.

Electronic microscopy of 50 nm diameter magnetic tunnel junction

University of Applied Sciences Northwestern Switzerland, ICube (University of Strasbourg) 
Swiss Nanoscience Institute (No. A16.10), ERC-2020-PoC (MAGALIGN No. 963895)

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