The set-up of a high temperature superconductor radio-frequency SQUID microscope formagnetic nanoparticle detection

Abstract

SQUID (superconducting quantum interference device) microscopes are versatileinstruments for biosensing applications, in particular for magnetic nanoparticle detection inimmunoassay experiments. We are developing a SQUID microscope based on an HTS rfSQUID magnetometer sensor with a substrate resonator. For the cryogenic set-up, aconfiguration was realized in which the cryostat is continuously refilled and kept at aconstant liquid nitrogen level by an isolated tube connection to a large liquid nitrogenreservoir. The SQUID is mounted on top of a sapphire finger, connected to theinner vessel of the stainless steel cryostat. The vacuum gap between the coldSQUID and room temperature sample is adjusted by the precise approach of a50 µm thin sapphire window using a single fine thread wheel. We investigated possible sensing tipconfigurations and different sensor integration techniques in order to achieve anoptimized design. A new scheme of coupling the rf SQUID from its back to aSrTiO3 substrate resonator was adopted for the purpose of minimization of the sensor-to-sample spacing.By SQUID substrate thinning and washer size reduction, the optimum coupling conditionsfor back coupling were determined for different rf SQUID magnetometers prepared onLaAlO3 and SrTiO3 substrates. The SQUID microscope system is characterized with respect to its spatialresolution and its magnetic field noise. The SQUID microscope instrument will be used formagnetic nanoparticle marker detection.