High- $T_{\rm c}$ DC SQUIDs for Magnetoencephalography

We have investigated the microstructural and electron transport properties of 45° step-edge Josephson junctions grown on MgO substrates and used them for the preparation of superconducting quantum interference device (SQUID) magnetometers intended for magnetoencephalography (MEG) measurement systems. The high-T_c SQUID magnetometers also incorporate 16 mm multilayer superconducting flux transformers on the MgO substrates and demonstrate a magnetic field resolution of ~ 4 fT/√Hz at 77 K. Results are illustrated for the detection of auditory evoked magnetic responses of the human cortex and compared between high-T_c SQUIDs and a commercial low-T_c MEG system. Our results demonstrate that MEG systems can be upgraded using high-T_c SQUIDs to make them independent of helium and more user-friendly, saving operating costs and leading to the widespread utilization of MEG systems in clinical practice and at universities.