Optimization of direct-coupled high-Tc SQUID magnetometers for operation in magnetically unshielded environment

To further improve the design of our direct-coupled SQUID magnetometers, we performed simulations of the current distribution and thus of the effective area and inductance for various distributions of the 16 parallel 50-mum-wide loops forming the pickup structure. It is shown that an optimization of the line distribution yields an increase of the effective area of only 2.2% but could be advantageous for the operation in time-varying magnetic fields. Based on the devices which were optimized with regard to their operation in magnetically unshielded environment, we have developed a single-channel second-order gradiometer system that allowed us to record real-time magnetocardiograms in a typical laboratory environment.