Dipole-Sensitive Homogeneous-Field Compensated High- DC SQUID

Magnetic nanoparticles (MNPs) are of great interest for industrial and medical applications. Therefore, the properties of the particles must be well known. For the measurement of small amounts of particles, a sensor design with high dipole sensitivity is required, whereas homogeneous excitation fields must be shielded or compensated. The spatial dimensions need to be tuned to the sample size in order to maximize the coupling efficiency. In this paper, we present a new sensor design employing a high-T_cYBa₂Cu₃O₇ superconducting quantum interference device (SQUID), which is inductively coupled to a surrounding superconducting compensation loop. We choose a square SQUID design that is positioned in the axis of symmetry of the compensation loop in order to compensate spatial homogeneous and first-order gradient magnetic fields. The SQUID is operated in a flux-locked loop (FLL) with bias reversal. We investigate the dependence of the performance on the compensation loop layout. Design limitations are demonstrated, and the SQUID noise is characterized with and without an applied magnetic field.