For investigations on small spin systems such as molecules or cold atom clouds, nano-particles, atoms or even on single electrons, it is becoming increasingly important to measure the properties of magnetic structures with nanometric dimensions. SQUIDs (Superconducting Quantum Interference Devices), as the most sensitive magnetic field sensors, are predestinated for this task. Their mode of operation is based on effects of superconductivity in Josephson tunnel junctions.

To measure magnetic fields, the nano- SQUIDs developed within the scope of a cooperation project between the University of Tübingen and PTB can be used with a lateral spatial resolution below 100 nm. To manufacture them, a technological process has been developed and optimized at PTB which allows extremely small Josephson junctions to be manufactured from a complex series of layers of niobium (superconductor) and HfTi (normal conductor). This involves the most modern thin-film procedures, electron- beam lithography and chemical-mechanical polishing. The contact surfaces of the first generation of nano- SQUIDs which were produced this way were 200 nm × 200 nm and the lateral dimension of the detector loop 500 nm only. First test measurements in the DC gradiometer mode have already shown excellent electrical properties. The magnetic flux noise of the nano-SQUIDs was around 250 nΦ₀/√Hz. In addition, the detectors could be used at relatively high magnetic fields of up to 500 mT.

Further optimization of the process has permitted further miniaturization of the Josephson contact surfaces down to 90 nm × 90 nm and of the SQUID detector loop down to 250 nm. The flux noise has hereby been reduced to 200 nΦ₀/√Hz, and the magnetic coupling to structures to be investigated has been clearly improved, so that an extremely low spin sensitivity of only 23 μ_B/√Hz could be achieved. In addition, with this latest generation of SQUIDs, detector loops can be manufactured which are vertical to the chip surface of the sensors, so that further miniaturization – and thus an increase in the spatial resolution – is possible. The nano- SQUIDs were integrated into a prototype of a low-temperature scanning SQUID microscope at the University of Basel. This microscope has already allowed nickel nanotubes to be measured magnetometrically.