Field Stability of a Directly Coupled High- $T_{\rm c}$ SQUID Design Regarding MRX Measurements

Magnetic nanoparticles (MNPs) are of great interest for industrial and medical applications. Therefore, the properties of the particles must be known. In order to determine parameters as particle size and particle size distribution, several magnetic measurement schemes have been developed. In this paper, we present our results on the investigation of directly coupled high-T_c YBa₂Cu₃O₇ superconducting quantum interference devices (SQUIDs) for use as sensors in magnetorelaxometry (MRX) measurements. We choose a SQUID design with a slotted pickup loop, which is known to work well in static magnetic fields. The employed SQUID has a low noise of 50 fT/√Hz in a magnetically shielded environment and a modulation depth parameter β_L ≈ 1 . In order to proof the applicability for MRX measurements, the noise performance of the SQUID design was tested in static and switched magnetic fields, which were aligned in parallel to the surface of the sensor. For these measurements, a system was designed for MRX measurements on MNP. As it is shown in this paper, basic measurements on MNP can be performed with this system.