An integrated superconductive magnetic nanosensor for high-sensitivity nanoscaleapplications

Abstract

An integrated magnetic nanosensor based on a niobium dc SQUID (superconductingquantum interference device) for nanoscale applications is presented. The sensor, having awasher shape with a hole of 200 nm and two Josephson–Dayem nanobridges of80 nm × 100 nm, consists of a Nb(30 nm)/Al(30 nm) bilayer patterned by electron beam lithography (EBL)and shaped by lift-off and reactive ion etch (RIE) processes. The presence of the niobium coils,integrated on-chip and tightly coupled to the SQUID, allows us to easily excite the sensorin order to get the voltage–flux characteristics and to flux bias the SQUID at its optimalpoint. The measurements were performed at liquid helium temperature. A voltage swing of75 µV and a maximum voltage–flux transfer coefficient (responsivity) as high as1 mV/Φ0 were directly measured from the voltage–flux characteristic. The noise measurementswere performed in open loop mode, biasing the SQUID with a dc magneticflux at its maximum responsivity point and using direct-coupled low-noisereadout electronics. A white magnetic flux noise spectral density as low as2.5 μΦ0 Hz−1/2 was achieved, corresponding to a magnetization or spin sensitivity in units of the Bohr magneton of100 spin Hz−1/2. Possible applications of this nanosensor can be envisaged in magnetic detection ofnanoparticles and small clusters of atoms and molecules, in the measurement of nanoobjectmagnetization, and in quantum computing.