High-temperature superconducting magnetometers for on-scalp MEG
Doctoral thesis, 2020

In the growing field of on-scalp magnetoencephalography (MEG), brain activity is studied by non-invasively mapping the magnetic fields generated by neuronal currents with sensors that are flexibly placed in close proximity to the subject's head. This thesis focuses on high-temperature superconducting magnetometers made from YBa2Cu3Ox-7 (YBCO), which enables a reduction in the sensor-to-room temperature standoff distance from roughly 2 cm (for conventional MEG systems) down to 1 mm. Because of the higher neuromagnetic signal magnitudes available to on-scalp sensors, simulations predict that even a relatively low-sensitivity (higher noise) full-head on-scalp MEG system can extract more information about brain activity than conventional systems.

In the first part of this thesis, the development of high critical temperature (high-Tc) superconducting quantum interference device (SQUID) magnetometers for a 7-channel on-scalp MEG system is described. The sensors are single layer magnetometers with a directly coupled pickup loop made on 10 mm × 10 mm substrates using bicrystal grain boundary Josephson junctions. We found that the kinetic inductance strongly varies with film quality and temperature. Determination of all SQUID parameters by combining measurements and inductance simulations led to excellent agreement between experimental results and theoretical predictions. This allowed us to perform an in-depth magnetometer optimization. The best magnetometers achieve a magnetic field noise level of 44 fT/√Hz at 78 K. Fabricated test SQUIDs provide evidence that noise levels below 30 fT/√Hz are possible for high quality junctions with fairly low critical currents and in combination with the optimized pickup loop design. Different feedback methods for operation in a densely-packed on-scalp MEG system were also investigated. Direct injection of current into the SQUID loop was identified as the best on-chip feedback method with feedback flux crosstalk below 0.5%. By reducing the operation temperature, the noise level can be further reduced, however, the effective area also decreases because of the decreasing kinetic inductance contribution. We present a method that allows for one-time sensor calibration independent of temperature.

In the second part, the design, operation, and performance of the constructed 7-channel on-scalp MEG system based on the fabricated magnetometers is presented. With a dense (2 mm edge-to-edge) hexagonal head-aligned array, the system achieves a small sensor-to-head standoff distance of 1-3 mm and dense spatial sampling. The magnetic field noise levels are 50-130 fT/√Hz and the sensor-to-sensor feedback flux crosstalk is below 0.6%. MEG measurements with the system demonstrate the feasibility of the approach and indicate that our on-scalp MEG system allows retrieval of information unavailable to conventional MEG.

In the third part, two alternative magnetometer types are studied for the next generation system. The first alternative is magnetometers based on Dayem bridge junctions instead of bicrystal grain boundary junctions. With a magnetometer based on the novel grooved Dayem bridge junctions, a magnetic field noise level of 63 fT/√Hz could be achieved, which shows that Dayem bridge junctions are starting to become a viable option for single layer magnetometers. The second alternative are high-Tc SQUID magnetometers with an inductively coupled flux transformer. The best device with bicrystal grain boundary junctions reaches a magnetic field noise level below 11 fT/√Hz and outperforms the best single layer device for frequencies above 20 Hz.

In the last part, the potential of kinetic inductance magnetometers (KIMs) is investigated. We demonstrate the first high-Tc KIMs, which can be operated in fields of 9-28 µT and achieve a noise level of 4 pT/√Hz at 10 kHz.

on-scalp MEG

multi-channel system

SQUID optimization

kinetic inductance magnetometer




SQUID magnetometer calibration

high-Tc SQUID

kinetic inductance

Kollektorn, Kemivägen 9, Chalmers
Opponent: Prof. Carmine Granata, Institute of Applied Sciences and Intelligent Systems of the National Research Council (CNR), Naples, Italy


Silvia Ruffieux

Chalmers, Microtechnology and Nanoscience (MC2), Quantum Device Physics

Feedback solutions for low crosstalk in dense arrays of high-T-c SQUIDs for on-scalp MEG

Superconductor Science and Technology,; Vol. 30(2017)p. art. nr 054006-

Journal article

The role of kinetic inductance on the performance of YBCO SQUID magnetometers

Superconductor Science and Technology,; Vol. 33(2020)

Journal article

A 7-Channel High-T-c SQUID-Based On-Scalp MEG System

IEEE Transactions on Biomedical Engineering,; Vol. 67(2020)p. 1483-1489

Journal article

Improved coupling of nanowire-based high-T-c SQUID magnetometers-simulations and experiments

Superconductor Science and Technology,; Vol. 30(2017)

Journal article

The magnetic field generated by tens of thousands of neurons firing at the same time is large enough to be measured outside of the head using highly sensitive magnetometers and special shielding. Magnetoencephalography (MEG) is a non-invasive and passive neuroimaging method that maps these extremely weak magnetic fields with an array of magnetometers surrounding the head. MEG has a good spatial resolution and an excellent temporal resolution (in the millisecond range), which allows for real-time tracking of brain activity. In neuroscience research, MEG has contributed significantly towards the understanding of brain dynamics, and the method is used clinically for the diagnosis of epilepsy, to localize epileptic foci, and for pre-surgical evaluation.

State-of-the-art MEG magnetometers are based on superconducting sensors that need to be cooled to -269° C. The sensors are therefore placed inside a rigid helmet for cooling, and a roughly 2 cm thick layer of thermal insulation is required to operate the sensors near a subject's head. This strongly limits the available magnetic field signal magnitude, which decays rapidly with distance from the brain. The advent of novel magnetic sensor technologies operating at higher temperatures has led to the growing field of on-scalp MEG, where the sensors are flexibly placed in close proximity to the scalp.

This thesis describes the design, fabrication, characterization, and optimization of high-temperature superconducting magnetometers for on-scalp MEG. Both established and novel sensor technologies are investigated. These alternative sensors operate at a higher temperature, -196° C, which allows the thermal insulation to be reduced to 1 mm. We found that the inductance varies strongly with temperature and film quality. Through careful determination of all relevant parameters using measurements and simulations, excellent agreement between experimental results and theoretical predictions could be found. This allowed for in-depth sensor optimization and near record noise levels. Using these sensors, a 7-channel on-scalp MEG system is constructed. MEG measurements with the system demonstrate the feasibility of the approach and indicate that our on-scalp MEG system allows retrieval of information unavailable to conventional MEG. The development of a 21-channel system with improved sensors is ongoing and may enable new neuroscience discoveries and improved treatments for brain diseases.

Nanoscale superconducting devices for a closer look at brain activity (NeuroSQUID)

Knut and Alice Wallenberg Foundation, 2015-01-01 -- 2020-12-31.

Areas of Advance

Nanoscience and Nanotechnology (SO 2010-2017, EI 2018-)

Subject Categories


Medical Laboratory and Measurements Technologies

Physical Sciences

Medical Engineering


Nano Technology

Condensed Matter Physics


Basic sciences

Driving Forces

Innovation and entrepreneurship


Nanofabrication Laboratory



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4823


Chalmers University of Technology

Kollektorn, Kemivägen 9, Chalmers


Opponent: Prof. Carmine Granata, Institute of Applied Sciences and Intelligent Systems of the National Research Council (CNR), Naples, Italy

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