Superconducting quantum interferometers
Quantum interferometers for magnetic microscopy of microstructures and electric circuits at room temperature
The focus of our recent work has been the development of a scanning magnetic microscope based on high-Tc superconducting quantum interferometers (SQUIDs) with a ferromagnetic antenna. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid nitrogen-cooled SQUID-sensor and back again. The flux guide passes through the pick-up loop of the high-Tc SQUID providing an improved coupling of the object’s magnetic flux to the SQUID.
The device measures the z-component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in x-y direction by a motorized computer controlled scanning stage. A lateral resolution of better than 10 µm was achieved. Fields of wires carrying currents of less than 1 µA could be detected with a time constant of 1 sec. An increase in the flux noise of the SQUID due to the presence of the soft magnetic foil was not essential.
- M. I. Faley et al., IEEE Transactions on Appl. Supercond., 27, No.4, 1600905 (2017).
High-Tc quantum interferometers for biomagnetic measurements
The expected increase in the price of helium is set to seriously impact on operation costs associated with magnetoencephalography (MEG) which currently relies on low-Tc SQUIDs that are cooled exclusively by liquid helium. High-Tc SQUIDs are cooled by cheap, easy-to-handle and readily available liquid nitrogen, and can be placed much nearer to magnetic sources than low-Tc SQUIDs. The problem lies in producing HTc SQUIDs with a magnetic field resolution better than 10 fT/sqrt(Hz) at 77 K, required for their application in MEG systems.
We have developed 16-mm high-Tc SQUID magnetometers possessing a magnetic field resolution ≈ 4 fT/sqrt(Hz) at 77 K and, together with INM-4 FZJ, have demonstrated MEG measurements with high-Tc SQUIDs compared to those obtained using a commercial low-Tc MEG system. The low intrinsic noise and closer positioning of the high-Tc SQUIDs to the cortex contributed to the high signal-to-noise ratio of the MEG data obtained using the high-Tc system. These results open up new ways to upgrade MEG systems using low-Tc SQUIDs, which would make these systems independent of helium, more user-friendly and would save on a large part of the operating costs leading, in turn, to the widespread utilization of MEG systems.
High-Tc quantum interferometers for geomagnetic surveys
The most often used high-Tc flip-chip magnetometers have a pick-up loop size of about 8 mm x 8 mm, a magnetic field sensitivity 1 nT/Φ0 and magnetic field resolution down to about 15 fT/sqrt(Hz) at 77 K.
These magnetometers are able to operate outside magnetic shielding, which is essential for geomagnetic surveys.
Due to their relatively small size they can be integrated into a compact mobile vector magnetometer or tensor systems.
Cooling with liquid nitrogen has significant advantages compared to the low-Tc systems; the system’s signal-to-noise ratio and slew rates are significantly lower than those of room temperature systems.
Another advantage is that the supply of liquid helium is a serious problem, especially in remote areas, whereas liquid nitrogen can be readily obtained simply from the air.
Nanofabrication and Quantum Sensing
Phone: +49 2461 61-4366
Fax: +49 2461 61-6444