Nano-sized SQUID-on-tip for scanning probe microscopy

Abstract

We present a SQUID of novel design, which is fabricated on the tip of a pulled quartz tube in a simple 3-step evaporation process without need for any additional processing, patterning, or lithography. The resulting devices have SQUID loops with typical diameters in the range 75–300 nm. They operate in magnetic fields up to 0.6 T and have flux sensitivity of 1.8 μΦ0/Hz1/2 and magnetic field sensitivity of 10−7 T/Hz1/2, which corresponds to a spin sensitivity of 65 μB/Hz1/2 for aluminum SQUIDs. The shape of the tip and the small area of the SQUID loop, together with its high sensitivity, make our device an excellent tool for scanning SQUID microscopy: With the SQUID-on-tip glued to a tine of a quartz tuning fork, we have succeeded in obtaining magnetic images of a patterned niobium film and of vortices in a superconducting film in a magnetic field.