A micro-SQUID with dispersive readout for magnetic scanning microscopy

Abstract

We have designed and characterized a micro-SQUID with dispersive readout for use in low temperature scanning probe microscopy systems. The design features a capacitively shunted RF SQUID with a tunable resonance frequency from 5 to 12 GHz, micrometer spatial resolution and integrated superconducting coils for local application of magnetic fields. The SQUID is operated as a nonlinear oscillator with a flux- and power-dependent resonance frequency. Measurements for device characterization and noise benchmarking were carried out at 4 K. The measured flux noise above 10 kHz at 4 K is $80\mathrm{n}{\Phi }_0{\mathrm{Hz}}^{-1/2}$ at a bandwidth of 200 MHz. Estimations suggest that one can benefit from parametric gain based on inherent nonlinearity of the Josephson junction and reduce the flux noise to $30\mathrm{n}{\Phi }_0{\mathrm{Hz}}^{-1/2}$ at 100 mK, which corresponds to $7.5{\mu }_{\mathrm{B}}\mathrm{H}{\mathrm{z}}^{-1/2}$ for a magnetic moment located at the center of the pickup loop.